The Project Gutenberg EBook of Outlines of Dairy Bacteriology, 8th edition, by
H. L. Russell
This eBook is for the use of anyone anywhere at no cost and with
almost no restrictions whatsoever. You may copy it, give it away or
re-use it under the terms of the Project Gutenberg License included
with this eBook or online at www.gutenberg.org
Title: Outlines of Dairy Bacteriology, 8th edition
A Concise Manual for the Use of Students in Dairying
Author: H. L. Russell
Release Date: January 11, 2009 [EBook #27778]
Language: English
*** START OF THIS PROJECT GUTENBERG EBOOK DAIRY BACTERIOLOGY, 8TH EDITION ***
Produced by Mark C. Orton, Linda McKeown, Josephine Paolucci
and the Online Distributed Proofreading Team at
https://www.pgdp.net.
OUTLINES
OF
DAIRY BACTERIOLOGY
A CONCISE MANUAL FOR THE USE OF STUDENTS IN DAIRYING
BY
H. L. RUSSELL
DEAN OF THE COLLEGE OF AGRICULTURE, UNIVERSITY OF WISCONSIN
EIGHTH EDITION
THOROUGHLY REVISED
MADISON, WISCONSIN
H. L. RUSSELL
1907
Copyrighted 1905
BY
H. L. RUSSELL
STATE JOURNAL PRINTING COMPANY,
Printers And Stereotypers,
Madison, Wis.
Transcriber's note:
For Text: A word surrounded by a cedilla such as ~this~ signifies that
the word is bolded in the text. A word surrounded by underscores like
_this_ signifies the word is italics in the text. For numbers and
equations, underscores before bracketed numbers in equations denote a
subscript.
Minor typos have been corrected.
PREFACE.
Knowledge in dairying, like all other technical industries, has grown
mainly out of experience. Many facts have been learned by observation,
but the _why_ of each is frequently shrouded in mystery.
Modern dairying is attempting to build its more accurate knowledge upon
a broader and surer foundation, and in doing this is seeking to
ascertain the cause of well-established processes. In this, bacteriology
is playing an important rôle. Indeed, it may be safely predicted that
future progress in dairying will, to a large extent, depend upon
bacteriological research. As Fleischmann, the eminent German dairy
scientist, says: "The gradual abolition of uncertainty surrounding dairy
manufacture is the present important duty which lies before us, and its
solution can only be effected by bacteriology."
It is therefore natural that the subject of Dairy Bacteriology has come
to occupy an important place in the curriculum of almost every Dairy
School. An exposition of its principles is now recognized as an integral
part of dairy science, for modern dairy practice is rapidly adopting the
methods that have been developed as the result of bacteriological study.
The rapid development of the subject has necessitated a frequent
revision of this work, and it is gratifying to the writer that the
attempt which has been made to keep these Outlines abreast of
bacteriological advance has been appreciated by students of dairying.
While the text is prepared more especially for the practical dairy
operator who wishes to understand the principles and reasons underlying
his art, numerous references to original investigations have been added
to aid the dairy investigator who wishes to work up the subject more
thoroughly.
My acknowledgments are due to the following for the loan of
illustrations: Wisconsin Agricultural Experiment Station; Creamery
Package Mfg. Co., Chicago, Ill.; and A. H. Reid, Philadelphia, Pa.
H. L. Russell.
University of Wisconsin.
CONTENTS.
CHAPTER I. Structure of the bacteria and conditions governing
their development and distribution 1
CHAPTER II. Methods of studying bacteria 13
CHAPTER III. Contamination of milk 19
CHAPTER IV. Fermentations in milk and their treatment 62
CHAPTER V. Relation of disease-bacteria to milk 82
Diseases transmissible from animal to man
through diseased milk 84
Diseases transmissible to man through infection
of milk after withdrawal 94
CHAPTER VI. Preservation of milk for commercial purposes 102
CHAPTER VII. Bacteria and butter making 134
Bacterial defects in butter 156
CHAPTER VIII. Bacteria in cheese 160
Influence Of bacteria in normal cheese processes 160
Influence of bacteria in abnormal cheese processes 182
CHAPTER I.
STRUCTURE OF THE BACTERIA AND CONDITIONS GOVERNING THEIR DEVELOPMENT AND
DISTRIBUTION.
Before one can gain any intelligent conception of the manner in which
bacteria affect dairying, it is first necessary to know something of the
life history of these organisms in general, how they live, move and
react toward their environment.
~Nature of Bacteria.~ Toadstools, smuts, rusts and mildews are known to
even the casual observer, because they are of evident size. Their
plant-like nature can be more readily understood from their general
structure and habits of life. The bacteria, however, are so small, that
under ordinary conditions, they only become evident to our unaided
senses by the by-products of their activity.
When Leeuwenhoek (pronounced Lave-en-hake) in 1675 first discovered
these tiny, rapidly-moving organisms he thought they were animals.
Indeed, under a microscope, many of them bear a close resemblance to
those minute worms found in vinegar that are known as "vinegar-eels."
The idea that they belonged to the animal kingdom continued to hold
ground until after the middle of the nineteenth century; but with the
improvement in microscopes, a more thorough study of these tiny
structures was made possible, and their vegetable nature demonstrated.
The bacteria as a class are separated from the fungi mainly by their
method of growth; from the lower algae by the absence of chlorophyll,
the green coloring matter of vegetable organisms.
~Structure of bacteria.~ So far as structure is concerned the bacteria
stand on the lowest plane of vegetable life. The single individual is
composed of but a single cell, the structure of which does not differ
essentially from that of many of the higher types of plant life. It is
composed of a protoplasmic body which is surrounded by a thin membrane
that separates it from neighboring cells that are alike in form and
size.
~Form and size.~ When a plant is composed of a single cell but little
difference in form is to be expected. While there are intermediate
stages that grade insensibly into each other, the bacteria may be
grouped into three main types, so far as form is concerned. These are
spherical, elongated, and spiral, and to these different types are given
the names, respectively, _coccus_, _bacillus_ and _spirillum_ (plural,
_cocci_, _bacilli_, _spirilla_) (fig. 1). A ball, a short rod, and a
corkscrew serve as convenient models to illustrate these different
forms.
[Illustration: FIG. 1. Different forms of bacteria. _a_, _b_, _c_,
represent different types as to form: _a_, coccus, _b_, bacillus, _c_,
spirillum; _d_, diplococcus or twin coccus; _e_, staphylococcus or
cluster coccus; _f_ and _g_, different forms of bacilli, _g_ shows
internal endospores within cell; _h_ and _i_, bacilli with motile organs
(cilia).]
In size, the bacteria are the smallest organisms that are known to
exist. Relatively there is considerable difference in size between the
different species, yet in absolute amount this is so slight as to
require the highest powers of the microscope to detect it. As an average
diameter, one thirty-thousandth of an inch may be taken. It is difficult
to comprehend such minute measurements, but if a hundred individual
germs could be placed side by side, their total thickness would not
equal that of a single sheet of paper upon which this page is printed.
~Manner of Growth.~ As the cell increases in size as a result of growth,
it elongates in one direction, and finally a new cell wall is formed,
dividing the so-called mother-cell into two, equal-sized daughter-cells.
This process of cell division, known as _fission_, is continued until
growth ceases and is especially characteristic of bacteria.
~Cell Arrangement.~ If fission goes on in the same plane continually, it
results in the formation of a cell-row. A coccus forming such a chain of
cells is called _strepto-coccus_ (chain-coccus). If only two cells
cohere, it is called a _diplo-coccus_ (twin-coccus). If the second cell
division plane is formed at right angles to the first, a _cell surface_
or _tetrad_ is formed. If growth takes place in three dimensions of
space, a _cell mass_ or _sarcina_ is produced. Frequently, these cell
aggregates cohere so tenaciously that this arrangement is of value in
distinguishing different species.
~Spores.~ Some bacteria possess the property of forming _spores_ within
the mother cell (called _endospores_, fig. 1g) that are analogous in
function to the seeds of higher plants and spores of fungi. By means of
these structures which are endowed with greater powers of resistance
than the vegetating cell, the organism is able to protect itself from
the effect of an unfavorable environment. Many of the bacilli form
endospores but the cocci do not. It is these spore forms that make it
so difficult to thoroughly sterilize milk.
~Movement.~ Many bacteria are unable to move from place to place. They
have, however, a vibrating movement known as the _Brownian_ motion that
is purely physical. Many other kinds are endowed with powers of
locomotion. Motion is produced by means of fine thread-like processes of
protoplasm known as _cilia_ (sing. _cilium_) that are developed on the
outer surface of the cell. By means of the rapid vibration of these
organs, the cell is propelled through the medium. Nearly all cocci are
immotile, while the bacilli may or may not be. These cilia are so
delicate that it requires special treatment to demonstrate their
presence.
~Classification.~ In classifying or arranging the different members of any
group of living objects, certain similarities and dissimilarities must
be considered. These are usually those that pertain to the structure and
form, as such are regarded as most constant. With the bacteria these
differences are so slight that they alone do not suffice to distinguish
distinctly one species from another. As far as these characters can be
used, they are taken, but in addition, many characteristics of a
physiological nature are added. The way that the organism grows in
different kinds of cultures, the by-products produced in different
media, and effect on the animal body when injected into the same are
also used as data in distinguishing one species from another.
~Conditions favoring bacterial growth.~ The bacteria, in common with all
other living organisms are affected by external conditions, either
favorably or unfavorably. Certain conditions must prevail before
development can occur. Thus, the organism must be supplied with an
adequate and suitable food supply and with moisture. The temperature
must also range between certain limits, and finally, the oxygen
requirements of the organism must be considered.
~Food supply.~ Most bacteria are capable of living on dead, inert, organic
matter, such as meats, milk and vegetable material, in which case, they
are known as _saprophytes_. In contradistinction to this class is a
smaller group known as _parasites_, which derive their nourishment from
the living tissues of animals or plants. The first group comprise by far
the larger number of known organisms which are concerned for the most
part in the decomposition of organic matter. The parasitic group
includes those which are the cause of various communicable diseases.
Between these two groups there is no sharp line of division, and in some
cases, certain species possess the faculty of growing either as
parasites or saprophytes, in which case they are known as _facultative_
parasites or saprophytes.
The great majority of bacteria of interest in dairying belong to the
saprophytic class; only those species capable of infecting milk through
the development of disease in the animal are parasites in the strict
sense of the term. Most disease-producing species, as diphtheria or
typhoid fever, while parasitic in man lead a saprophytic method of life
so far as their relation to milk is concerned.
Bacteria require for their growth, nitrogen, hydrogen, carbon, oxygen,
together with a limited amount of mineral matter. The nitrogen and
carbon are most available in the form of organic compounds, such as
albuminous material. Carbon in the form of carbohydrates, as sugar or
starch, is most readily attacked by bacteria.
Inasmuch as the bacteria are plant-cells, they must imbibe their food
from material in solution. They are capable of living on solid
substances, but in such cases, the food elements must be rendered
soluble, before they can be appropriated. If nutritive liquids are too
highly concentrated, as in the case of syrups and condensed milk,
bacteria cannot grow therein, although all the necessary ingredients may
be present. Generally, bacteria prefer a neutral or slightly alkaline
medium, rather than one of acid reaction; but there are numerous
exceptions to this general rule, especially among the bacteria found in
milk.
~Temperature.~ Growth of bacteria can only occur within certain
temperature limits, the extremes of which are designated as the
_minimum_ and _maximum_. Below and above these respective limits, life
may be retained in the cell for a time, but actual cell-multiplication
is stopped. Somewhere between these two cardinal temperature points, and
generally nearer the maximum limit is the most favorable temperature for
growth, known as the _optimum_. The temperature zone of most dairy
bacteria in which growth occurs ranges from 40°-45° F. to somewhat above
blood-heat, 105°-110° F., the optimum being from 80°-95° F. Many
parasitic species, because of their adaptation to the bodies of
warm-blooded animals, generally have a narrower range, and a higher
optimum, usually approximating the blood heat (98°-99° F). The broader
growth limits of bacteria in comparison with other kinds of life explain
why these organisms are so widely distributed in nature.
~Air supply.~ Most bacteria require as do the green plants and animal
life, the free oxygen of the air for their respiration. These are called
_aerobic_. Some species, however, and some yeasts as well possess the
peculiar property of taking the oxygen which they need from organic
compounds such as sugar, etc., and are therefore able to live and grow
under conditions where the atmospheric air is excluded. These are known
as _anaerobic_. While some species grow strictly under one condition or
the other, and hence are _obligate_ aerobes or anaerobes, others possess
the ability of growing under either condition and are known as
_facultative_ or optional forms. The great majority of milk bacteria are
either obligate or facultative aerobes.
~Rate of growth.~ The rate of bacterial development is naturally very much
affected by external conditions, food supply and temperature exerting
the most influence. In the neighborhood of the freezing point but little
growth occurs. The rate increases with a rise in temperature until at
the _optimum_ point, which is generally near the blood heat or slightly
below (90°-98° F.), a single cell will form two cells in 20 to 30
minutes. If temperature rises much above blood heat rate of growth is
lessened and finally ceases. Under ideal conditions, rapidity of growth
is astounding, but this initially rapid rate of development cannot be
maintained indefinitely, for growth is soon limited by the accumulation
of by-products of cell activity. Thus, milk sours rapidly at ordinary
temperatures until the accumulation of acid checks its development.
~Detrimental effect of external conditions.~ Environmental influences of a
detrimental character are constantly at work on bacteria, tending to
repress their development or destroy them. These act much more readily
on the vegetating cell than on the more resistant spore. A thorough
knowledge of the effect of these antagonistic forces is essential, for
it is often by their means that undesirable bacteria may be killed out.
~Effect of cold.~ While it is true that chilling largely prevents
fermentative action, and actual freezing stops all growth processes,
still it does not follow that exposure to low temperatures will
effectually destroy the vitality of bacteria, even in the vegetative
condition. Numerous non-spore-bearing species remain alive in ice for a
prolonged period, and recent experiments with liquid air show that even
a temperature of -310° F. for hours does not effectually kill all
exposed cells.
~Effect of heat.~ High temperatures, on the other hand, will destroy any
form of life, whether in the vegetative or latent stage. The temperature
at which the vitality of the cell is lost is known as the _thermal death
point_. This limit is not only dependent upon the nature of the
organism, but varies with the time of exposure and the condition in
which the heat is applied. In a moist atmosphere the penetrating power
of heat is great; consequently cell-death occurs at a lower temperature
than in a dry atmosphere. An increase in time of exposure lowers the
temperature point at which death occurs.
For vegetating forms the thermal death point of most bacteria ranges
from 130°-140° F. where the exposure is made for ten minutes which is
the standard arbitrarily selected. In the spore stage resistance is
greatly increased, some forms being able to withstand steam at 210°-212°
F. from one to three hours. If dry heat is employed, 260°-300° F. for an
hour is necessary to kill spores. Where steam is confined under
pressure, a temperature of 230°-240° F. for 15-20 minutes suffices to
kill all spores.
~Drying.~ Spore-bearing bacteria like anthrax withstand drying with
impunity; even tuberculous material, although not possessing spores
retains its infectious properties for many months. Most of the dairy
bacteria do not produce spores, and yet in a dry condition, they retain
their vitality unimpaired for considerable periods, if they are not
subjected to other detrimental influences.
~Light.~ Bright sunlight exerts on many species a powerful disinfecting
action, a few hours being sufficient to destroy all cells that are
reached by the sun's rays. Even diffused light has a similar effect,
although naturally less marked. The active rays in this disinfecting
action are those of the chemical or violet end of the spectrum, and not
the heat or red rays.
~Influence of chemical substances.~ A great many chemical substances exert
a more or less powerful toxic action of various kinds of life. Many of
these are of great service in destroying or holding bacterial growth in
check. Those that are toxic and result in the death of the cell are
known as _disinfectants_; those that merely inhibit, or retard growth
are known as _antiseptics_. All disinfectants must of necessity be
antiseptic in their action, but not all antiseptics are disinfectants
even when used in strong doses. Disinfectants have no place in dairy
work, except to destroy disease bacteria, or preserve milk for
analytical purposes. Corrosive sublimate or potassium bichromate are
most frequently used for these purposes. The so-called chemical
preservatives used to "keep" milk depend for their effect on the
inhibition of bacterial growth. With a substance so violently toxic as
formaldehyde (known as formalin, freezene) antiseptic doses are likely
to be exceeded. In this country most states prohibit the use of these
substances in milk. Their only function in the dairy should be to check
fermentative or putrefactive processes outside of milk and so keep the
air free from taints.
~Products of growth.~ All bacteria in their development form certain more
or less characteristic by-products. With most dairy bacteria, these
products are formed from the decomposition of the medium in which the
bacteria may happen to live. Such changes are known, collectively, as
fermentations, and are characterised by the production of a large amount
of by-products, as a result of the development of a relatively small
amount of cell-life. The souring of milk, the formation of butyric acid,
the making of vinegar from cider, are all examples of fermentative
changes.
With many bacteria, especially those that affect proteid matter,
foul-smelling gases are formed. These are known as putrefactive changes.
All organic matter, under the action of various organisms, sooner or
later undergoes decay, and in different stages of these processes,
acids, alkalies, gases and numerous other products are formed. Many of
these changes in organic matter occur only when such material is brought
in direct contact with the living bacterial cell.
In other instances, soluble, non-vital ferments known as _enzyms_ are
produced by the living cell, which are able to act on organic matter, in
a medium free from live cells, or under conditions where the activity of
the cell is wholly suspended. These enzyms are not confined to bacteria
but are found throughout the animal and plant world, especially in those
processes that are concerned in digestion. Among the better known of
these non-vital ferments are rennet, the milk-curdling enzym; diastase
or ptyalin of the saliva, the starch-converting enzym; pepsin and
trypsin, the digestive ferments of the animal body.
Enzyms of these types are frequently found among the bacteria and yeasts
and it is by virtue of this characteristic that these organisms are
able to break down such enormous quantities of organic matter. Most of
these enzyms react toward heat, cold and chemical poisons in a manner
quite similar to the living cells. In one respect they are readily
differentiated, and that is, that practically all of them are capable of
producing their characteristic chemical transformations under
anaesthetic conditions, as in a saturated ether or chloroform
atmosphere.
~Distribution of bacteria.~ As bacteria possess greater powers of
resistance than most other forms of life, they are to be found more
widely distributed than any other type. At the surface of the earth,
where conditions permit of their growth, they are found everywhere,
except in the healthy tissues of animals and plants. In the superficial
soil layers, they exist in myriads, as here they have abundance of
nourishment. At the depth of several feet however, they diminish rapidly
in numbers, and in the deeper soil layers, from six to ten feet or more,
they are not present, because of the unsuitable growth conditions.
The bacteria are found in the air because of their development in the
soil below. They are unable to grow even in a moist atmosphere, but are
so readily dislodged by wind currents that over land areas the lower
strata of the air always contain them. They are more numerous in summer
than in winter; city air contains larger numbers than country air.
Wherever dried fecal matter is present, as in barns, the air contains
many forms.
Water contains generally enough organic matter in solution, so that
certain types of bacterial life find favorable growth conditions. Water
in contact with the soil surface takes up many impurities, and is of
necessity rich in microbes. As the rain water percolates into the soil,
it loses its germ content, so that the normal ground water, like the
deeper soil layers, contains practically no bacterial life. Springs
therefore are relatively deficient in germ life, except as they become
infected with soil organisms, as the water issues from the soil. Water
may serve to disseminate certain infectious diseases as typhoid fever
and cholera among human beings, and a number of animal maladies.
While the inner tissues of healthy animals are free from bacteria, the
natural passages as the respiratory and digestive tracts, being in more
direct contact with the exterior, become more readily infected. This is
particularly true with reference to the intestinal tract, for in the
undigested residue, bacterial activity is at a maximum. The result is
that fecal matter contains enormous numbers of organisms so that the
possibility of pollution of any food medium such as milk with such
material is sure to introduce elements that seriously affect the quality
of the product.
CHAPTER II.
METHODS OF STUDYING BACTERIA.
~Necessity of bacterial masses for study.~ The bacteria are so extremely
small that it is impossible to study individual germs separately without
the aid of first-class microscopes. For this reason, but little advance
was made in the knowledge of these lower forms of plant life, until the
introduction of culture methods, whereby a single organism could be
cultivated and the progeny of this cell increased to such an extent in a
short course of time, that they would be visible to the unaided eye.
This is done by growing the bacteria in masses on various kinds of food
media that are prepared for the purpose, but inasmuch as bacteria are so
universally distributed, it becomes an impossibility to cultivate any
special form, unless the medium in which they are grown is first freed
from all pre-existing forms of germ life. To accomplish this, it is
necessary to subject the nutrient medium to some method of
sterilization, such as heat or filtration, whereby all life is
completely destroyed or eliminated. Such material after it has been
rendered germ-free is kept in sterilized glass tubes and flasks, and is
protected from infection by cotton stoppers.
~Culture media.~ For culture media, many different substances are
employed. In fact, bacteria will grow on almost any organic substance
whether it is solid or fluid, provided the other essential conditions of
growth are furnished. The food substances that are used for culture
purposes are divided into two classes; solids and liquids.
Solid media may be either permanently solid like potatoes, or they may
retain their solid properties only at certain temperatures like gelatin
or agar. The latter two are of utmost importance in bacteriological
research, for their use, which was introduced by Koch, permits the
separation of the different forms that may happen to be in any mixture.
Gelatin is used advantageously because the majority of bacteria present
wider differences due to growth upon this medium than upon any other. It
remains solid at ordinary temperatures, becoming liquid at about 70° F.
Agar, a gelatinous product derived from a Japanese sea-weed, has a much
higher melting point, and can be successfully used, especially with
those organisms whose optimum growth point is above the melting point of
gelatin.
Besides these solid media, different liquid substances are extensively
used, such as beef broth, milk, and infusions of various vegetable and
animal tissues. Skim-milk is of especial value in studying the milk
bacteria and may be used in its natural condition, or a few drops of
litmus solution may be added in order to detect any change in its
chemical reaction due to the bacteria.
[Illustration: FIG. 2. A gelatin plate culture showing appearance of
different organisms in a sample of milk. Each mass represents a
bacterial growth (colony) derived from a single cell. Different forms
react differently toward the gelatin, some liquefying the same, others
growing in a restricted mass. _a_, represents a colony of the ordinary
bread mold; _b_, a liquefying bacterium; _c_, and _d_, solid forms.]
~Methods of isolation.~ Suppose for instance one wishes to isolate the
different varieties of bacteria found in milk. The method of procedure
is as follows: Sterile gelatin in glass tubes is melted and cooled down
so as to be barely warm. To this gelatin which is germ-free a drop of
milk is added. The gelatin is then gently shaken so as to thoroughly
distribute the milk particles, and poured out into a sterile flat glass
dish and quickly covered. This is allowed to stand on a cool surface
until the gelatin hardens. After the culture plate has been left for
twenty-four to thirty-six hours at the proper temperature, tiny spots
will begin to appear on the surface, or in the depth of the culture
medium. These patches are called _colonies_ and are composed of an
almost infinite number of individual germs, the result of the continued
growth of a single organism that was in the drop of milk which was
firmly held in place when the gelatin solidified. The number of these
colonies represents approximately the number of germs that were present
in the milk drop. If the plate is not too thickly sown with these germs,
the colonies will continue to grow and increase in size, and as they do,
minute differences will begin to appear. These differences may be in the
color, the contour and the texture of the colony, or the manner in
which it acts toward gelatin. In order to make sure that the seeding in
not too copious so as to interfere with continued study, an
_attenuation_ is usually made. This consists in taking a drop of the
infected gelatin in the first tube, and transferring it to another tube
of sterile media. Usually this operation is repeated again so that these
culture plates are made with different amounts of seed with the
expectation that in at least one plate the seeding will not be so thick
as to prevent further study. For transferring the culture a loop made of
platinum wire is used. By passing this through a gas flame, it can be
sufficiently sterilized.
[Illustration: FIG. 3. Profile view of gelatin plate culture; _b_, a
liquefying form that dissolves the gelatin; _c_ and _d_, surface
colonies that do not liquefy the gelatin.]
To further study the peculiarities of different germs, the separate
colonies are transferred to other sterile tubes of culture material and
thus _pure cultures_ of the various germs are secured. These cultures
then serve as a basis for continued study and must be planted and grown
upon all the different kinds of media that are obtainable. In this way
the slight variations in the growth of different forms are detected and
the peculiar characteristics are determined, so that the student is able
to recognize this form when he meets it again.
These culture methods are of essential importance in bacteriology, as it
is the only way in which it is possible to secure a quantity of germs of
the same kind.
~The microscope in bacterial investigation.~ In order to verify the purity
of the cultures, the microscope is in constant demand throughout all the
different stages of the isolating process. For this purpose, it is
essential that the instrument used shall be one of strong magnifying
powers (600-800 diameters), combined with sharp definition.
[Illustration: FIG. 4. Pure cultures of different kinds of bacteria in
gelatin tubes. _a_, growth slight in this medium; _b_, growth copious at
and near surface. Fine parallel filaments growing out into medium
liquefying at surface; _c_, a rapid liquefying form; _d_, a
gas-producing form that grows equally well in lower part of tube as at
surface (facultative anaerobe); _e_, an obligate anaerobe, that develops
only in absence of air.]
The microscopical examination of any germ is quite as essential as the
determination of culture characteristics; in fact, the two must go hand
in hand. The examination reveals not only the form and size of the
individual germs, but the manner in which they are united with each
other, as well as any peculiarities of movement that they may possess.
In carrying out the microscopical part of the work, not only is the
organism examined in a living condition, but preparations are made by
using solutions of anilin dyes as staining agents. These are of great
service in bringing out almost imperceptible differences. The art of
staining has been carried to the highest degree of perfection in
bacteriology, especially in the detection of germs that are found in
diseased tissues in the animal or human body.
In studying the peculiarities of any special organism, not only is it
necessary that these cultural and microscopical characters should be
closely observed, but special experiments must be carried out along
different lines, in order to determine any special properties that the
germ may possess. Thus, the ability of any form to act as a fermentative
organism can be tested by fermentation experiments; the property of
causing disease, studied by the inoculation of pure cultures into
animals. A great many different methods have been devised for the
purpose of studying special characteristics of different bacteria, but a
full description of these would necessarily be so lengthy that in a work
of this character they must be omitted. For details of this nature
consult standard reference books on bacteriological technique.
CHAPTER III.
CONTAMINATION OF MILK.
No more important lesson is to be learned than that which relates to the
ways in which milk is contaminated with germ life of various kinds; for
if these sources of infection are thoroughly recognized they can in
large measure be prevented, and so the troubles which they engender
overcome. Various organisms find in milk a congenial field for
development. Yeasts and some fungi are capable of growth, but more
particularly the bacteria.
~Milk a suitable bacterial food.~ The readiness with which milk undergoes
fermentative changes indicates that it is well adapted to nourish
bacterial life. Not only does it contain all the necessary nutritive
substances but they are diluted in proper proportions so as to render
them available for bacterial as well as mammalian life.
Of the nitrogenous compounds, the albumen is in readily assimilable
form. The casein, being insoluble, is not directly available, until it
is acted upon by proteid-dissolving enzyms like trypsin which may be
secreted by bacteria. The fat is relatively resistant to change,
although a few forms are capable of decomposing it. Milk sugar, however,
is an admirable food for many species, acids and sometimes gases being
generally produced.
~Condition when secreted.~ When examined under normal conditions milk
always reveals bacterial life, yet in the secreting cells of the udder
of a healthy cow germ life is not found. Only when the gland is diseased
are bacteria found in any abundance. In the passage of the milk from
the secreting cells to the outside it receives its first infection, so
that when drawn from the animal it generally contains a considerable
number of organisms.
[Illustration: FIG. 5. Microscopic appearance of milk showing relative
size of fat globules and bacteria.]
~Contamination of milk.~ From this time until it is consumed in one form
or another, it is continually subjected to contamination. The major part
of this infection occurs while the milk is on the farm and the degree of
care which is exercised while the product is in the hands of the milk
producer is the determining factor in the course of bacterial changes
involved. This of course does not exclude the possibility of
contamination in the factory, but usually milk is so thoroughly seeded
by the time it reaches the factory that the infection which occurs here
plays a relatively minor rôle to that which happens earlier. The great
majority of the organisms in milk are in no wise dangerous to health,
but many species are capable of producing various fermentative changes
that injure the quality of the product for butter or cheese. To be able
to control abnormal changes of an undesirable character one must know
the sources of infection which permit of the introduction of these
unwelcome intruders.
~Sources of infection.~ The bacterial life that finds its way into milk
while it is yet on the farm may be traced to several sources, which may
be grouped under the following heads: unclean dairy utensils, fore milk,
coat of animal, and general atmospheric surroundings. The relative
importance of these various factors fluctuates in each individual
instance.
~Dairy utensils.~ Of first importance are the vessels that are used during
milking, and also all storage cans and other dairy utensils that come in
contact with the milk after it is drawn. By unclean utensils, actually
_visible_ dirt need not always be considered, although such material is
often present in cracks and angles of pails and cans. Unless cleansed
with especial care, these are apt to be filled with foul and decomposing
material that suffices to seed thoroughly the milk. Tin utensils are
best. Where made with joints, they should be well flushed with solder so
as to be easily cleaned (see Fig. 6). In much of the cheaper tin ware on
the market, the soldering of joints and seams is very imperfect,
affording a place of refuge for bacteria and dirt.
Cans are often used when they are in a condition wholly unsuitable for
sanitary handling of milk. When the tin coating becomes broken and the
can is rusty, the quality of the milk is often profoundly affected.
Olson[1] of the Wisconsin Station has shown that the action of rennet is
greatly impaired where milk comes in contact with a rusty iron surface.
[Illustration: FIG. 6.]
With the introduction of the form or hand separator a new milk utensil
has been added to those previously in use and one which is very
frequently not well cleaned. Where water is run through the machine to
rinse out the milk particles, gross bacterial contamination occurs and
the use of the machine much increases the germ content of the milk.
Every time the separator is used it should be taken apart and thoroughly
cleaned and dried before reassembling.[2]
~Use of milk-cans for transporting factory by-products.~ The general
custom of using the milk-cans to carry back to the farm the factory
by-products (skim-milk or whey) has much in it that is to be deprecated.
These by-products are generally rich in bacterial life, more especially
where the closest scrutiny is not given to the daily cleaning of the
vats and tanks. Too frequently the cans are not cleaned immediately upon
arrival at the farm, so that the conditions are favorable for rapid
fermentation. Many of the taints that bother factories are directly
traceable to such a cause. A few dirty patrons will thus seriously
infect the whole supply. The responsibility for this defect should,
however, not be laid entirely upon the shoulders of the producer. The
factory operator should see that the refuse material does not accumulate
in the waste vats from day to day and is not transformed into a more or
less putrid mass. A dirty whey tank is not an especially good object
lesson to the patron to keep his cans clean.
It is possible that abnormal fermentations or even contagious diseases
may thus be disseminated.
Suppose there appears in a dairy an infectious milk trouble, such as
bitter milk. This milk is taken to the factory and passes unnoticed into
the general milk-supply. The skim-milk from the separator is of course
infected with the germ, and if conditions favor its growth, the whole
lot soon becomes tainted. If this waste product is returned to the
different patrons in the same cans that are used for the fresh milk, the
probabilities are strongly in favor of some of the cans being
contaminated and thus infecting the milk supply of the patrons. If the
organism is endowed with spores so that it can withstand unfavorable
conditions, this taint may be spread from patron to patron simply
through the infection of the vessels that are used in the transportation
of the by-products. Connell has reported just such a case in a Canadian
cheese factory where an outbreak of slimy milk was traced to infected
whey vats. Typhoid fever among people, foot and mouth disease and
tuberculosis among stock are not infrequently spread in this way. In
Denmark, portions of Germany and some states in America, compulsory
heating of factory by-products is practiced to eliminate this danger.[3]
~Pollution of cans from whey tanks.~ The danger is greater in cheese
factories than in creameries, for whey usually represents a more
advanced stage of fermentation than skim-milk. The higher temperature at
which it is drawn facilitates more rapid bacterial growth, and the
conditions under which it is stored in many factories contribute to the
ease with which fermentative changes can go on in it. Often this
by-product is stored in wooden cisterns or tanks, situated below ground,
where it becomes impossible to clean them out thoroughly. A custom that
is almost universally followed in the Swiss cheese factories, here in
this country, as in Switzerland, is fully as reprehensible as any dairy
custom could well be. In Fig. 7 the arrangement in vogue for the
disposal of the whey is shown. The hot whey is run out through the
trough from the factory into the large trough that is placed over the
row of barrels, as seen in the foreground. Each patron thus has allotted
to him in his individual barrel his portion of the whey, which he is
supposed to remove day by day. No attempt is made to clean out these
receptacles, and the inevitable result is that they become filled with a
foul, malodorous liquid, especially in summer. When such material is
taken home in the same set of cans that is used to bring the fresh milk
(twice a day as is the usual custom in Swiss factories), it is no wonder
that this industry is seriously handicapped by "gassy" fermentations
that injure materially the quality of the product. Not only is the above
danger a very considerable one, but the quality of the factory
by-product for feeding purposes, whether it is skim-milk or whey, is
impaired through the development of fermentative changes.
[Illustration: FIG. 7. Swiss cheese factory (Wisconsin), showing
careless way in which whey is handled. Each patron's share is placed in
a barrel, from which it is removed by him. No attempt is made to cleanse
these receptacles.]
~Improved methods of disposal of by-products.~ The difficulties which
attend the distribution of these factory by-products have led to
different methods of solution. One is to use another separate set of
receptacles to carry back these products to the farm. This method has
been tried, and while it is deemed impracticable by many to handle two
sets of vessels, yet some of the most progressive factories report
excellent results where this method is in use.
Large barrels could be used for this purpose to economize in wagon
space.
Another method that has met with wider acceptance, especially in
creameries, is the custom of pasteurizing or scalding the skim-milk
immediately after it is separated, so that it is returned to the farmer
in a hot condition. In factories where the whole milk is pasteurized,
further treatment of the by-product is not necessary. In most factories
steam, generally exhaust, is used directly in the milk, and experience
has shown that such milk, without any cooling, will keep sweet for a
considerable number of hours longer than the untreated product. It is
noteworthy that the most advanced and progressive factories are the ones
that appreciate the value of this work, and although it involves some
time and expense, experience has shown the utility of the process in
that a better grade of milk is furnished by the patrons of factories
which follow this practice.[4] The exclusion of all danger of animal or
human disease is also possible in this way.
~Cleaning dairy utensils.~ The thorough cleaning of all dairy apparatus
that in any way comes in contact with the milk is one of the most
fundamental and important problems in dairying. All such apparatus
should be so constructed as to permit of easy cleaning. Tinware,
preferably of the pressed variety, gives the best surface for this
purpose and is best suited for the handling of milk.
Milk vessels should never be allowed to become dry when dirty, for dried
particles of milk residue are extremely difficult to remove. In cleaning
dairy utensils they should first be rinsed in lukewarm instead of hot
water, so as to remove organic matter without coagulating the milk. Then
wash thoroughly in hot water, using a good washing powder. The best
washing powders possess considerable disinfecting action.[5] Strong
alkalies should not be used. After washing rinse thoroughly in clean hot
water. If steam is available, as it always is in creameries, cans and
pails should be turned over jet for a few moments. While a momentary
exposure will not suffice to completely sterilize such a vessel, yet
many bacteria are killed in even a short exposure, and the cans dry more
thoroughly and quickly when heated by steam.
Not only should the greatest care be paid to the condition of the cans
and milk-pails, but all dippers, strainers, and other utensils that come
in contact with the milk must be kept equally clean. Cloth strainers,
unless attended to, are objectionable, for the fine mesh of the cloth
retains so much moisture that they become a veritable hot-bed of
bacterial life, unless they are daily boiled or steamed.
The inability to thoroughly render vessels bacteria-free with the
conveniences which are generally to be found on the farm has led in some
cases to the custom of washing and sterilizing the milk cans at the
factory.
~Germ content of milk utensils.~ Naturally the number of bacteria found in
different milk utensils after they have received their regular cleaning
will be subject to great fluctuations; but, nevertheless, such
determinations are of value as giving a scientific foundation for
practical methods of improvement. The following studies may serve to
indicate the relative importance of the utensils as a factor in milk
contamination.
Two cans were taken, one of which had been cleaned in the ordinary way,
while the other was sterilized by steaming. Before milking, the udder
was thoroughly cleaned and special precautions taken to avoid raising of
dust; the fore milk was rejected. Milk drawn into these two cans showed
the following germ content:
No. bacteria Hours before
per cc. souring.
Steamed pail 165 28-1/2
Ordinary pail 4265 23
Harrison[6] has shown how great a variation is in the bacterial content
in milk cans. The utensils were rinsed with 100 cc. of sterile water and
numerical determinations of this rinsing water made. In poorly cleaned
cans, the average germ content was 442,000; in cans washed in tepid
water and then scalded--the best farm practice--54,000, and in cans
carefully washed and then steamed for five minutes, 880.
Another method used by the writer is to wash the utensil with 100 cc.
sterile wash water, using a sterile swab to remove dirt. Then repeat the
process twice or more with fresh rinsing waters, making plate cultures
from each. The following data were obtained from three such
determinations:
No. bacteria in different washings. Total No.
I. II. III. bacteria.
7,800,000 1,450,000 49,000 9,299,000
283,000 43,400 35,000 361,400
1,685,000 105,000 61,400 1,851,400
~Infection of milk in udder cavity.~ A frequently neglected but
considerable factor of infection is that which is attributable to the
bacteria which are present in the udder and which are removed in large
numbers during the milking process. An examination of the fore milk, i.
e., the first few streams from each teat, and that which is subsequently
withdrawn, generally reveals a very much larger number of organisms in
the fore milk.[7] Not infrequently will this part of the milk when drawn
under as careful conditions as possible, contain several score thousand
organisms per cc. If successive bacterial determinations are made at
different periods of the milking, as shown in the following experiment,
a marked diminution is to be noted after the first portion of the milk
is removed:
_Bacterial content at different periods of milking._
Fore 200th 2000th 4300th 6500th Strippings.
milk. cc. cc. cc. cc.
Expt. 1 6,500 1,700 475 220 75 5
Expt. 2 8,100 1,650 400 240 50 10
By some observers it has been claimed that it is possible to secure
absolutely sterile milk in the strippings but this is rarely so. It is
quite probable that such reported results are due to the fact that too
small quantities of milk were used in the examinations and so erroneous
conclusions were drawn. This marked diminution in numbers indicates that
the larger proportion of the organisms found in the fore milk are
present in the lower portion of the udder and milk ducts. When
consideration is given to the structure of the udder, it is readily
apparent that infection will be greater here than above.
[Illustration: FIG. 8. Sectional view of udder showing teat with milk
duct connecting exterior with the milk cistern. Milk sinuses are mostly
shown in cross section interspersed and below the secreting tissue
(Moore and Ward).]
The udder is composed of secreting tissue (_gland cells_) held in place
by fibrous connective tissue. Ramifying throughout this glandular
structure are numerous channels (_milk sinuses_) that serve to convey
the milk from the cells where it is produced into the _milk cistern_, a
common receptacle just above the teats. This cavity is connected with
the exterior through the milk duct in the teat, which is more or less
tightly closed by the circular sphincter muscles, thus preventing the
milk from flowing out. The mucous membranes of the milk duct and cistern
are naturally moist. The habits of the animal render it impossible to
prevent infection of the external opening at the end of the teat and
there is no mechanical reason why bacteria cannot readily find their way
along the moist lining membrane for some distance. If organisms are
adapted to this kind of an environment, ideal conditions exist for their
multiplication, as moisture, warmth and suitable food supply are
present. The question arises how far up into this organ is penetration
possible? Within late years numerous observations have been made on the
presence of organisms in the upper portion of the udder in contact with
the secreting tissue.[8]
These investigations prove that bacteria are distributed throughout the
whole of the udder, although numerically they are much less abundant in
the region of the milk-secreting tissue than in the lower portion.
Ward's conclusions are "that milk when secreted by the glands of a
healthy udder is sterile. It may however, immediately become
contaminated by the bacteria which are normally present in the smaller
milk ducts of the udder."
~Nature of bacteria in fore milk.~ Generally speaking the number of
different species found in the fore milk is not large, and of those
which do appear many occur at only occasional intervals. Moore[9] in the
examination of 9 udders found 20 different forms, and of these only 3
species predominated, all of which proved to be micrococci. Streptococci
have also been quite frequently reported. Freudenreich[10] found the
most common types to be cocci, belonging to both the liquefying and
non-liquefying class.
Peptonizing[11] and spore-bearing[12] species have also been reported as
well as gas-producing[13] forms allied to the colon bacillus. Such
findings are, however, due in all probability to accidental invasion.
Most investigators report the absence of the distinctively lactic-acid
group of organisms.[14]
~Origin of bacteria in udder.~ There is no question but that many of the
types of bacteria found in the udder gain access from the outside. Those
belonging to the spore-bearing, digesting and intestinal types have such
a favorable opportunity for introduction from outside and are so
unlikely to have come directly from the body of the animal, that the
external source of infection is much more probable. Whether this
explanation answers the origin of the cocci that are so generally found
in the upper portion of the udder is questionable. The statement is
ordinarily made that the inner tissues of healthy organs are
bacteria-free, but the studies of Ford[15] seem to indicate that 70 per
cent. of such organs, removed under aseptic conditions from guinea pigs,
rabbits, dogs and cats contained living organisms. Others have reported
similar results in which cocci have been found[16] very similar to those
occurring in the udder. These findings increase the probability that the
origin of this type is from the blood. The persistence of certain
species in the udder for months as noted by Ward indicates possibility
of growth of some forms at least. Stocking[17] has shown where cows are
not milked clean that the germ content of succeeding milkings is greatly
increased.
~Artificial introduction of bacteria into udder.~ If bacteria are capable
of actually developing in the udder proper, it ought to be possible to
easily demonstrate this by the artificial introduction of cultures. In a
number of cases[18] such experiments have been made with various
saprophytic forms, such as _B. prodigiosus_, lactic acid bacilli and
others. In no case has it appeared evident that actual growth has
occurred, although the introduced organism has been demonstrated in
diminishing numbers for 5-6 days. Even the common lactic acid germ and a
yellow liquefying coccus isolated from the fore milk failed to persist
for more than a few days when thus artificially introduced. This failure
to colonize is indeed curious and needs explanation. Is it due to
unsuitable environmental conditions or attributable to the germicidal
influence of the milk?
Various body fluids are known to possess the property of destroying
bacteria and it is claimed by Fokker[19] that this same property was
found in freshly drawn milk. This peculiarity has also been investigated
by Freudenreich,[20] and Hunziker[21] who find a similar property.
No material increase in germ content takes place in milk for several
hours when chilled to 40°-70° F.; on the other hand an actual, but
usually not a marked decrease is observed for about 6 hours. This
phenomenon varies with the milk of different cows. Nothing is known as
to the cause of this apparent germicidal action. The question is yet by
no means satisfactorily settled, although the facts on which the
hypothesis is based are not in controversy. If such a peculiarity
belongs to milk, it is not at all improbable that it may serve to keep
down the germ content in the udder. Freudenreich[22] found that udders
which were not examined for some time after death showed abundant
growth, which fact he attributed to the loss of this germicidal
property.
The infection of the whole milk can be materially reduced by rejecting
the fore milk, but it is questionable whether such rejection is worth
while, except in the case of "sanitary" dairies where milk is produced
with as low a germ content as possible. The intrinsic loss in butter fat
in the fore milk is inconsiderable as the first few streams contain only
about one-fifth the normal fat content.
~Infection of milk after withdrawal from animal.~ The germ content of the
milk, when it is being drawn from the animal is immediately increased
upon contact with the atmosphere. These organisms are derived from the
surrounding air and the utensils in which the milk is received and
stored. The number of organisms which find their way into the milk
depends largely upon the character of the surroundings. Bacteria are so
intimately associated with dirt, dust and filth of all kinds that
wherever the latter are found, the former are sure to be present in
abundance.
The most important factors in the infection of the milk after withdrawal
are the pollution which is directly traceable to the animal herself and
the condition of the milk utensils. Fortunately both of these sources of
contamination are capable of being greatly minimized by more careful
methods of handling.
~Infection directly from the cow.~ It is a popular belief that the
organisms found in milk are derived from the feed and water which the
cow consumes, the same passing directly from the intestinal tract to the
milk by the way of the blood circulation. Such a view has no foundation
in fact as bacteria absorbed into the circulation are practically all
destroyed in the tissues by the action of the body fluids and cells.[23]
While organisms cannot pass readily from the intestine to the udder, yet
this must not be interpreted as indicating that no attention should be
given to the bacterial character of the material consumed. The water
supply given should be pure and wholesome and no decomposed or spoiled
food should be used.
The infection traceable directly to the cow is modified materially by
the conditions under which the animal is kept and the character of the
feed consumed. The nature of the fecal matter is in part dependent upon
the character of the food. The more nitrogenous rations with which
animals are now fed leads to the production of softer fecal discharges
which is more likely to soil the coat of the animal unless care is
taken. The same is true with animals kept on pasture in comparison with
those fed dry fodder.
Stall-fed animals, however, are more likely to have their flanks fouled,
unless special attention is paid to the removal of the manure. All dairy
stalls should be provided with a manure drop which should be cleaned as
frequently as circumstances will permit.
[Illustration: FIG. 9. Showing the bacterial contamination arising from
hair. These hairs were allowed to fall on a sterile gelatin surface. The
adherent bacteria developed readily in this medium, and the number of
bacteria thus introduced into the milk from these hairs can be estimated
by the number of developing colonies.]
The animal herself contributes materially to the quota of germ life
finding its way into the milk through the dislodgment of dust and filth
particles adhering to its hairy coat. The nature of this coat is such as
to favor the retention of these particles. Unless care is taken the
flanks and udder become polluted with fecal matter, which upon drying is
displaced with every movement of the animal. Every hair or dirt particle
so dislodged and finding its way into the milk-pail adds its quota of
organisms to the liquid. This can be readily demonstrated by placing
cow's hairs collected with care on the moist surface of gelatin culture
plates. Almost invariably, bacteria will be found in considerable
numbers adhering to such hairs as is indicated in Fig. 9. Dirt particles
are even richer in germ life. Not only is there the dislodgment of
hairs, epithelial scales and masses of dirt and filth, but during the
milking process, as at all other times, every motion of the animal is
accompanied by a shower of _invisible_ particles more or less teeming
with bacterial life.
The amount of actual impurities found in milk is often considerable and
when it is remembered that about one-half of fresh manure dissolves in
milk,[24] and thus does not appear as sediment, the presence of this
undissolved residue bespeaks filthy conditions as to milking. From
actual tests made, it is computed that the city of Berlin, Germany
consumes about 300 pounds of such dirt and filth daily. Renk has laid
down the following rule with reference to this insoluble dirt: If a
sample of milk shows any evidence of impurity settling on a transparent
bottom within two hours, it should be regarded as too dirty for use.
While the number of organisms here introduced is at all times large, the
character of the species is of even greater import. Derived primarily
from dirt and fecal matter, it is no wonder that such forms are able to
produce very undesirable fermentative changes.
~Influence of milker.~ The condition of the milker is not to be ignored in
determining all possible factors of infection, for when clothed in the
dust-laden garments that have been worn all day, a favorable opportunity
for direct contamination is possible. The filthy practice of wetting the
hands with milk just before milking is to be condemned. The milker's
hands should be washed immediately before milking in clean water and
dried. A pinch of vaseline on hands is sometimes used to obtain a firmer
grasp and prevents the ready dislodgment of scales.[25] It must also be
borne in mind that the milker may spread disease through the milk. In
typhoid fever and diphtheria, the germs often remain in the system for
weeks and thus make infection possible. Stocking[26] has shown that the
individual milker exerts a potent influence on the total germ content of
milk, even where the procedure is quite the same. In sanitary dairies
milkers are usually clad in white duck suits.
~Milking by machinery.~ Several mechanical devices have been invented for
milking, some of which have been tested bacteriologically as to their
efficiency. Harrison[27] has examined the "Thistle" machine but found a
much higher germ content than with hand-drawn milk. The recent
introduction of the Burrel-Lawrence-Kennedy machine has led to numerous
tests in which very satisfactory results have been obtained. If the
rubber parts of the milker are thoroughly cleaned and kept in lime water
solution, they remain nearly sterile. When milk is properly handled, the
germ content may be greatly reduced.
~Reduction in dirt and adherent bacteria.~ No factor of contamination is
so susceptible of improvement as that which relates to the reduction in
filth and dirt which gains access during and immediately subsequent to
the milking. The care which is taken to keep the coat of the animal
clean by grooming lessens very much the grosser portion of such
contamination, but with a dry, hairy coat, fine scales and dust
particles must of necessity be dislodged.[28] Ordinarily the patron
thinks all evidence of such dirt is removed if the milk is strained, but
this process only lessens the difficulty; it does not overcome it.
Various methods are in use, the effectiveness of which is subject to
considerable variation. Some of these look to the elimination of the
bacteria after they are once introduced into the milk; others to the
prevention of infection in the first place.
_1. Straining the milk._ Most of the visible, solid particles of filth,
such as hairs, dirt particles, etc., can be removed by simple straining,
the time-honored process of purification. As ordinarily carried out,
this process often contributes to instead of diminishing the germ life
in milk. The strainer cloths unless washed and thoroughly sterilized by
boiling harbor multitudes of organisms from day to day and may thus
actually add to the organisms present. Various methods have been
suggested for this simple process, but the most practical and efficient
strainer is that made of fine wire gauze to which is added 3-4 layers of
cheese cloth, the whole to set over the storage milk can.
_2. Filtration._ In Europe especially, the system of cleaning milk by
filtration through sand, gravel and other substances has been quite
extensively used. These filters are built in sections and the milk
passes from below upward. The filtering substance is washed in hot water
immediately after use and then steamed and finally baked. While it is
possible to remove the solid impurities in this way, the germ content
cannot be greatly reduced.[29] Cellulose filters have also been
suggested[30] as an improvement over the sand filters. Methods of
filtration of this character have not been used under commercial
conditions here in this country.
_3. Clarification in separator._ Within recent years the custom has
grown of clarifying milk or removing the visible dirt by passing the
milk through a centrifugal separator the cream and skim milk being
remixed after separation. This process naturally removes the solid
impurities as dirt, hairs, epithelial scales and cells, also some of the
casein, making what is known as centrifuge slime. This conglomerate mass
is incomparably rich in germ life and the natural inference would be
that the bacterial content of the milk would be greatly reduced by this
procedure. Eckles and Barnes[31] noted a reduction of 37 to 56 per cent.
of the bacteria but others have failed to observe such reductions.[32]
This condition is explained by the more thorough breaking up of the
bacterial masses in the process, thus apparently not reducing them in
numbers.
It is somewhat surprising that in spite of the elimination of much
organic matter and bacteria, such clarified milk sours as rapidly as the
untreated product.[33]
The mechanical shock of separation ruptures the clusters of fat globules
and so delays creaming and also lessens the consistency of cream derived
from such milk. This practical disadvantage together with the increased
expense of the operation and the failure to materially enhance the
keeping quality of the product outweigh the advantage which might come
from removal of solid impurities which can be largely accomplished on
the farm by efficient straining.
_4. Washing the udder._ If a surface is moist, bacteria adherent to it
cannot be dislodged by ordinary movements. Thus the air over
snow-covered mountains or oceans is relatively devoid of germ life. The
method of moistening the udder is applied with success to the hairy coat
of the animal thus subserving the double purpose of cleaning the animal
and preventing in large measure the continual dislodgment of dust
particles. After these parts have been well carded to remove loose hairs
and dirt particles, the skin should be thoroughly moistened with clean
water and then wiped. It has been urged that this procedure lessens the
yield of milk but Eckles[34] concludes from experiments that when the
animal becomes accustomed to this treatment, no noticeable change in
amount of milk or butter-fat is produced.
The effectiveness of this method in reducing the actual amount of dirt
and filth introduced into the milk as well as the great diminution in
germ life is shown by the instructive experiments of Fraser[35] who
found that the actual amount of dirt dislodged from udders of apparently
clean animals during the milking process was three and one-half times as
much as when the cow's udders were washed. From udders visibly polluted
one ounce of such filth was removed in 275 pounds of milk, while from
cows whose udders had been washed, the same amount of dirt was
distributed through 24,030 pounds.
Fraser observed as a result of 420 examinations that the average germ
content of 4-inch culture dishes under clean but unwashed udders was
578, while under washed animals it was reduced to 192. From numerous
tests made in the writer's laboratory, it is evident that the germ
content of the milk in the pail is increased from 20,000-40,000 bacteria
_per minute_ during the milking period. By far the larger part of this
pollution can be easily prevented by cleaning and dampening the udder.
_5. Diminishing exposed surface of pail._ The entrance of organisms into
the milk can be greatly reduced by lessening the area of the milk pail
directly exposed to the dust shower. A number of so-called sanitary or
hygienic milk pails have been devised for this purpose. In one case the
pail is smaller at the top than bottom, but in most of them the common
form is kept and the exposed area is lessened by means of a cover, the
milk being received through a narrower opening. In some cases, strainers
are also interposed so as to remove more effectually the coarse
particles. It is necessary to have these covers and strainers
constructed in such a way so they can be easily removed and cleaned.
[Illustration: FIG. 10. Sanitary milk pails designed to diminish the
introduction of hairs, scales, dirt, etc., into milk.]
Stocking tested one of these pails (A, Fig. 10) and found that 63 per
cent of the dirt and 29 per cent. of the bacteria were prevented from
passing into the milk. Eckles examined one in which the germ content was
found to be 3200 per cc. as against 43200 per cc. in a common open
pail. This milk did not sour until it was 64 hours old in the first case
while in the latter it curdled in 43 hours.
~Air in barn.~ The atmosphere of the barn where the milking is done may
frequently contribute considerable infection. Germ life is incapable of
development in the air, but in a dried condition, organisms may retain
their vitality for long periods. Anything which contributes to the
production of dust in the stable and aids in the stirring up of the same
increases the number of organisms to be found in the air (Fig. 11).
Thus, the feeding of dry fodder and the bedding of animals with straw
adds greatly to the germ life floating in the air. Dust may vary much in
its germ content depending upon its origin. Fraser found the dust from
corn meal to contain only about one-sixth to one-eighth as much germ
life as that from hay or bran.[36] In time most of these dust particles
settle to the floor, but where the herd is kept in the barn, the
constant movement of the animals keeps these particles more or less in
motion. Much can be done by forethought to lessen the germ content of
stables. Feeding dry feed should not be done until after milking.[37] In
some of the better sanitary dairies, it is customary to have a special
milking room that is arranged with special reference to the elimination
of all dust. In this way this source of infection may be quite obviated
as the air of a clean, still room is relatively free from bacteria,
especially if the floor is moistened. It has often been noted that the
milk of stall-fed animals does not keep as well as that milked out of
doors, a condition in part attributable to the lessened contamination.
[Illustration: FIG. 11. Effect of contaminated air. The number of spots
indicate the colonies that have developed from the bacteria which fell
in 30 seconds on the surface of the gelatin plate (3 inches in
diameter). This exposure was made at time the cows were fed.]
~Relative importance of different sources of infection.~ It is impossible
to measure accurately the influence of the different sources of
infection as these are continually subject to modification in each and
every case. As a general rule, however, where milk is drawn and handled
without any special care, the utensils and the animal contribute the
larger proportion of dirt and bacteria that find their way into milk.
Where the manner of milking and handling is designed to exclude the
largest number of organisms possible, the bacteria appearing in the fore
milk make up the major portion remaining. By putting into practice the
various suggestions that have been made with reference to diminishing
the bacterial content of milk, it is possible to greatly reduce the
number of organisms found therein, and at the same time materially
improve the keeping quality of the milk. Backhaus[38] estimates that
the germ life in milk can be easily reduced to one-two thousandth of its
original number by using care in milking. He reports a series of
experiments covering two years in which milk was secured that averaged
less than 10,000 bacteria per cc., while that secured under ordinary
conditions averaged over 500,000.
[Illustration: FIG. 12. Bacterial content of milk handled in ordinary
way. Each spot represents a colony growing on gelatin plate. Compare
with Fig. 13, where same quantity of milk is used in making culture.
Over 15,000 bacteria per cc. in this milk.]
Fig. 13 gives an illustration as to what care in milking will do in the
way of eliminating bacteria. Fig. 12 shows a gelatin plate seeded with
the same quantity of milk that was used in making the culture indicated
by Fig. 13. The first plate was inoculated with milk drawn under good
conditions, the germ content of which was found to be 15,500 bacteria
per cc., while the sample secured under as nearly aseptic conditions as
possible (Fig. 13) contained only 330 organisms in the same volume.
[Illustration: FIG. 13. Bacterial content of milk drawn with care.
Diminished germ content is shown by smaller number of colonies (330
bacteria per cc.). Compare this culture with that shown in Fig. 12.]
~"Sanitary" or "certified" milk.~ Within recent years there has been more
or less generally introduced into many cities, the custom of supplying
high grade milk that has been handled in a way so as to diminish its
germ content as much as possible. Milk of this character is frequently
known as "sanitary," "hygienic" or "certified," the last term being used
in connection with a certification from veterinary authorities or boards
of health as to the freedom of animals from contagious disease.
Frequently a numerical bacterial standard is exacted as a pre-requisite
to the recommendation of the board of examining physicians. Thus, the
Pediatric Society of Philadelphia requires all children's milk that
receives its recommendation to have not more than 10,000 bacteria per
cc. Such a standard has its value in the scrupulous cleanliness that
must prevail in order to secure these results. This in itself is
practically a guarantee of the absence of those bacteria liable to
produce trouble in children. The number of organisms found in such milks
is surprisingly low when compared with ordinary milk. Naturally, there
is considerable fluctuation from day to day, and occasionally the germ
content is increased to a high figure without any apparent reason. The
average results though, show a greatly reduced number of organisms. De
Schweinitz[39] found in a Washington dairy in 113 examinations extending
throughout a year, an average of 6,485 bacteria per cc. The daily
analyses made of the Walker-Gordon supply sold in Philadelphia for an
entire year, showed that the milk almost always contained less than
5,000 bacteria per cc. and on 120 days out of the year the germ content
was 1,000 organisms per cc. or less.
From a practical point of view, the improvement in quality of sanitary
milk, in comparison with the ordinary product is seen in the enhanced
keeping quality. During the Paris Exposition in 1900, milk and cream
from several such dairies in the United States were shipped to Paris,
arriving in good condition after 15 to 18 days transit. When milk has
been handled in such a way, it is evident that it is much better suited
to serve as a food supply than where it has undergone the fermentative
changes incident to the development of myriads of organisms.
~Application of foregoing precautions to all milk producers.~ Milk is so
susceptible to bacterial changes that it is necessary to protect it from
invasion, if its original purity is to be maintained, and yet, from a
practical point of view, the use to which it is destined has much to do
with the care necessary to take in handling. The effect of the bacterial
contamination of milk depends largely upon the way in which the product
is used. To the milk-man engaged in the distribution of milk for direct
consumption, all bacterial life is more or less of a detriment, while to
the butter-maker and cheese-maker some forms are a direct necessity. It
is unnecessary and impracticable to require the same degree of care in
handling milk destined to be worked up into factory products as is done,
for instance, in sanitary milk supplies, but this fact should not be
interpreted to mean that the care of milk for factories is a matter of
small consequence. In fact no more important dairy problem exists, and
the purer and better the quality of the raw material the better the
product will be. Particularly is this true with reference to
cheese-making.
Dairymen have learned many lessons in the severe school of experience,
but it is earnestly to be hoped that future conditions will not be
summed up in the words of the eminent German dairy scientist, Prof.
Fleischmann, when he says that "all the results of scientific
investigation which have found such great practical application in the
treatment of disease, in disinfection, and in the preservation of
various products, are almost entirely ignored in milking."
~Growth of bacteria in milk.~ Milk is so well suited as a medium for the
development of germ life that it might be expected that all
microörganisms would develop rapidly therein, and yet, as a matter of
fact, growth does not begin at once, even though the milk may be richly
seeded. At ordinary temperatures, such as 70° F., no appreciable
increase is to be noted for a period of 6-9 hours; at lower temperatures
(54° F.) this period is prolonged to 30-40 hours or even longer. After
this period has elapsed, active growth begins and continues more or less
rapidly until after curdling.
The cause of this suspended development is attributed to the germicidal
properties inherent to the milk.[40]
Milk is of course seeded with a considerable variety of organisms at
first. The liquefying and inert species are the most abundant, the
distinctively lactic acid class occurring sparsely, if at all. As milk
increases in age, germ growth begins to occur. More or less development
of all types go on, but soon the lactic species gain the ascendency,
owing to their being better suited to this environment; they soon
outstrip all other species, with the result that normal curdling
generally supervenes. The growth of this type is largely conditioned by
the presence of the milk sugar. If the sugar is removed from milk by
dialysis, the liquid undergoes putrefactive changes due to the fact that
the putrefactive bacteria are able to grow if no acid is produced.
~Relation of temperature to growth.~ When growth does once begin in milk,
the temperature at which it is stored exerts the most profound effect on
the rate of development. When milk is not artificially cooled, it
retains its heat for some hours, and consequently the conditions become
very favorable for the rapid multiplication of the contained organisms,
as is shown in following results obtained by Freudenreich[41]:
_No. of bacteria per cc. in milk kept at different temperatures._
77° F. 95° F.
5 hrs. after milking 10,000 30,000
8 " " " 25,000 12,000,000
12 " " " 46,000 35,280,000
26 " " " 5,700,000 50,000,000
[Illustration: FIG. 14. Effect of cooling milk on the growth of
bacteria.]
Conn[42] is inclined to regard temperature of more significance in
determining the keeping quality than the original infection of the milk
itself. Milk which curdled in 18 hours at 98° F., did not curdle in 48
hours at 70°, and often did not change in two weeks, if the temperature
was kept at 50° F.
Where kept for a considerable period at this low temperature, the milk
becomes filled with bacteria of the undesirable putrefactive type, the
lactic group being unable to form acid in any appreciable amounts.
Running well water can be used for cooling, if it is possible to secure
it at a temperature of 48°-50° F. The use of ice, of course, gives
better results, and in summer is greatly to be desired. The influence of
these lowered temperatures makes it possible to ship milk long
distances[43] by rail for city supplies, if the temperature is kept low
during transit.
~Mixing night and morning milk.~ Not infrequently it happens when old milk
is mixed with new, that the course of the fermentative changes is more
rapid than would have been the case if the two milks had been kept
apart. Thus, adding the cooled night milk to the warm morning milk
sometimes produces more rapid changes in both. The explanation for this
often imperfectly understood phenomenon is that germ growth may have
gone on in the cooled milk, and when this material is added to the
warmer, but bacteria-poor, fresh milk, the temperature of the whole mass
is raised to a point suitable for the more rapid growth of all bacteria
than would have occurred if the older milk had been kept chilled.
~Number of bacteria in milk.~ The number of organisms found in milk
depends upon (1) the original amount of contamination, (2) the age of
the milk, and (3) the temperature at which it has been held. These
factors all fluctuate greatly in different cases; consequently, the germ
life is subject to exceedingly wide variations. Here in America, milk
reaches the consumer with less bacteria than in Europe, although it may
often be older. This is due largely to the more wide-spread use of ice
for chilling the milk _en route_ to market. Examinations have been made
of various supplies with the following results: Sedgwick and Batchelder
found in 57 tests of Boston milk from 30,000-4,220,000 per cc. Jordan
and Heineman found 30% of samples of Chicago milk to range from 100,000
to 1,000,000 while nearly one half were from 1-20,000,000 per cc. The
germ content of city milks increase rapidly in the summer months.
Park[44] found 250,000 organisms per cc. in winter, about 1,000,000 in
cool weather and 5,000,000 per cc. in hot summer weather. Knox and
Bassett in Baltimore report 1,500,000 in spring and nearly 4,500,000 in
summer. Eckles[45] studied milk under factory conditions. He finds from
1,000,000 to 5,000,000 per cc. in winter, and in summer from 15-30
millions.
~Bacterial standards for city supplies.~ It would be very desirable to
have a hygienic standard for city milk supplies, as there is a butter
fat and milk-solid test, but the wide spread variation in germ content
and the impracticability of utilizing ordinary bacterial determinations
(on account of time required) makes the selection of such a standard
difficult. Some hold, as Park, that such a standard is feasible. The New
York City Milk commission has set a standard of 30,000 bacteria per cc.
for their certified milk and 100,000 per cc. for inspected milk.
Rochester, N. Y. has attempted the enforcement of such a standard
(limit, 100,000 per cc.) with good results it is claimed while Boston
has placed the legal limit at 500,000 per cc. Quantitative standards
would seem more applicable to "certified" or sanitary supplies than to
general city supplies, where the wide range in conditions lead to such
enormous variations that the bacterial standard seems too refined a
method for practical routine inspection.
~Other tests.~ Any test to be of much service must be capable of being
quickly applied. The writer believes for city milk inspectors that the
acid test would serve a very useful purpose. This test measures the
acidity of the milk. There is, of course, no close and direct
relationship between the development of acidity and the growth of
bacteria, yet in a general way one follows the other at normal
temperatures. Where the temperature is kept rather low, bacterial growth
might go on without much acid development, but in the great majority of
cases a high degree of acidity means either old milk, in which there has
been a long period of incubation, or high temperature, where rapid
bacterial growth has been possible. Either of these conditions
encourages germ growth and thus impairs the quality of the milk.
The rapid determination of acidity may be made in an approximate manner
so as to serve as a test at the weigh-can or intake. The test is best
made by the use of the well known alkaline tablet which is composed of a
solid alkali, and the indicator, phenolphthalein. The tablets are
dissolved in water, one to each ounce used. A number of white cups are
filled with the proper quantity of the solution necessary to neutralize
say, 0.2 per cent. lactic acid. Then, as the milk is delivered, the
proper quantity is taken from each can to which is added the tablet
solution. A retention of the pink color shows that there is not
sufficient acid in the milk to neutralize the alkali used; a
disappearance of color indicates an excess of acid. The standard
selected is of course arbitrarily chosen. In our experience, 0.2 per
cent. acidity (figured as lactic), has proven a satisfactory point. With
carefully handled milk the acidity ought to be reduced to about 0.15 per
cent. The acidity of the milk may be abnormally reduced if milk is kept
in rusty cans, owing to action of acid on the metal.
[Illustration: FIG. 15. Apparatus used in making rapid acid test. A
definite quantity of the alkali solution and indicator is placed in the
white tea cup. To this is added the quantity of milk by means of the
cartridge measure which would just be neutralized if the acidity was 0.2
per cent. A retention of the pink color shows a low acid milk; its
disappearance, a high acid milk.]
~Kinds of bacteria in milk.~ The number of bacteria in milk is not of so
much consequence as the kinds present. With reference to the number of
different species, the more dirt and foreign matter the milk contains,
the larger the number of varieties found in the same. While milk may
contain forms that are injurious to man, still the great majority of
them have no apparent effect on human health. In their effect on milk,
the case is much different. Depending upon their action in milk, they
may be grouped into three classes:
1. Inert group, those producing no visible change in the milk.
2. Sour milk forms, those breaking up the milk sugar with or without the
formation of gas.
3. Digesting or peptonizing group, those capable of rendering the casein
of milk soluble and more or less completely digested.
A surprisingly large number of bacteria that are found in milk belong to
the first class. Undoubtedly they affect the chemical characteristics of
the milk somewhat, but not to the extent that it becomes physically
perceptible. Eckles[46] reports in a creamery supply from 20 to 55 per
cent. of entire flora as included in this class.
By far the most important group is that embraced under the second head.
It includes not only the true lactic acid types in which no gas is
formed, but those species capable of producing gases and various kinds
of acids. These organisms are the distinctively milk bacteria, although
they do not predominate when the milk is first drawn. Their adaptation
to this medium is normally shown, however, by this extremely rapid
growth, in which they soon gain the ascendency over all other species
present. It is to this lactic acid class that the favorable
flavor-producing organisms belong which are concerned in butter-making.
They are also indispensable in cheese-making.
The third class represents those capable of producing a liquefied or
digested condition on gelatin or in milk. They are usually the
spore-bearing species which gain access from filth and dirt. Their high
powers of resistance due to spores makes it difficult to eradicate this
type, although they are materially held in subjection by the lactic
bacteria. The number of different kinds that have been found in milk is
quite considerable, something over 200 species having been described
more or less thoroughly. In all probability, however, many of these
forms will be found to be identical when they are subjected to a more
critical study.
~Direct absorption of taints.~ A tainted condition in milk may result from
the development of bacteria, acting upon various constituents of the
milk, and transforming these in such a way as to produce by-products
that impair the flavor or appearance of the liquid; or it may be
produced by the milk being brought in contact with any odoriferous or
aromatic substance, under conditions that permit of the direct
absorption of such odors.
This latter class of taints is entirely independent of bacterial action,
and is largely attributable to the physical property which milk
possesses of being able to absorb volatile odors, the fat in particular,
having a great affinity for many of these substances. This direct
absorption may occur before the milk is withdrawn from the animal, or
afterwards if exposed to strong odors.
It is not uncommon for the milk of animals advanced in lactation to have
a more or less strongly marked odor and taste; sometimes this is apt to
be bitter, at other times salty to the taste. It is a defect that is
peculiar to individual animals and is liable to recur at approximately
the same period in lactation.
The peculiar "cowy" or "animal odor" of fresh milk is an inherent
peculiarity that is due to the direct absorption of volatile elements
from the animal herself. This condition is very much exaggerated when
the animal consumes strong-flavored substances as garlic, leeks, turnips
and cabbage. The volatile substances that give to these vegetables their
characteristic odor are quickly diffused through the system, and if such
foods are consumed some few hours before milking, the odor in the milk
will be most pronounced. The intensity of such taints is diminished
greatly and often wholly disappears, if the milking is not done for some
hours (8-12) after such foods are consumed.
This same principle applies in lesser degree to many green fodders that
are more suitable as feed for animals, as silage, green rye, rape, etc.
Not infrequently, such fodders as these produce so strong a taint in
milk as to render it useless for human use. Troubles from such sources
could be entirely obviated by feeding limited quantities of such
material immediately after milking. Under such conditions the taint
produced is usually eliminated before the next milking. The milk of
swill-fed cows is said to possess a peculiar taste, and the urine of
animals fed on this food is said to be abnormally acid. Brewers' grains
and distillery slops when fed in excess also induce a similar condition
in the milk.
Milk may also acquire other than volatile substances directly from the
animal, as in cases where drugs, as belladonna, castor oil, sulfur,
turpentine, jalap, croton oil, and many others have been used as
medicine. Such mineral poisons as arsenic have been known to appear
eight hours after ingestion, and persist for a period of three weeks
before being eliminated.
~Absorption of odors after milking.~ If milk is brought in contact with
strong odors after being drawn from the animal, it will absorb them
readily, as in the barn, where frequently it is exposed to the odor of
manure and other fermenting organic matter.
It has long been a popular belief that milk evolves odors and cannot
absorb them so long as it is warmer than the surrounding air, but from
experimental evidence, the writer[47] has definitely shown that the
direct absorption of odors takes place much more rapidly when the milk
is warm than when cold, although under either condition, it absorbs
volatile substances with considerable avidity. In this test fresh milk
was exposed to an atmosphere impregnated with odors of various essential
oils and other odor-bearing substances. Under these conditions, the
cooler milk was tainted very much less than the milk at body temperature
even where the period of exposure was brief. It is therefore evident
that an exposure in the cow barn where the volatile emanations from the
animals themselves and their excreta taint the air will often result in
the absorption of these odors by the milk to such an extent as to
seriously affect the flavor.
The custom of straining the milk in the barn has long been deprecated as
inconsistent with proper dairy practice, and in the light of the above
experiments, an additional reason is evident why this should not be
done.
Even after milk is thoroughly cooled, it may absorb odors as seen where
the same is stored in a refrigerator with certain fruits, meats, fish,
etc.
~Distinguishing bacterial from non-bacterial taints.~ In perfectly fresh
milk, it is relatively easy to distinguish between taints caused by the
growth of bacteria and those attributable to direct absorption.
If the taint is evident at time of milking, it is in all probability due
to character of feed consumed, or possibly to medicines. If, however,
the intensity of the taint grows more pronounced as the milk becomes
older, then it is probably due to living organisms, which require a
certain period of incubation before their fermentative properties are
most evident.
Moreover, if the difficulty is of bacterial origin, it can be frequently
transferred to another lot of milk (heated or sterilized is preferable)
by inoculating same with some of the original milk. Not all abnormal
fermentations are able though to compete with the lactic acid bacteria,
and hence outbreaks of this sort soon die out by the re-establishment of
more normal conditions.
~Treatment of directly absorbed taints.~ Much can be done to overcome
taints of this nature by exercising greater care in regard to the feed
of animals, and especially as to the time of feeding and milking. But
with milk already tainted, it is often possible to materially improve
its condition. Thorough aeration has been frequently recommended, but
most satisfactory results have been obtained where a combined process of
aeration and pasteurization was resorted to. Where the milk is used in
making butter, the difficulty has been successfully met by washing the
cream with twice its volume of hot water in which a little saltpeter has
been dissolved (one teaspoonful per gallon), and then separating it
again.[48]
The treatment of abnormal conditions due to bacteria has been given
already under the respective sources of infection, and is also still
further amplified in following chapter.
~Aeration.~ It is a common belief that aeration is of great aid in
improving the quality of milk, yet, when closely studied, no material
improvement can be determined, either where the milk is made into butter
or sold as milk. Dean in Canada and Storch in Denmark have both
experimented on the influence of aeration in butter making, but with
negative results. Marshall and Doane failed to observe any material
improvement in keeping quality, but it is true that odors are eliminated
from the milk during aeration. The infection of the milk during aeration
often more than counterbalances the reputed advantage. Especially is
this so if the aeration is carried out in an atmosphere that is not
perfectly clean and pure.
In practice aeration differs greatly. In some cases, air is forced into
the milk; in others, the milk is allowed to distribute itself in a thin
sheet over a broad surface and fall some distance so that it is brought
intimately in contact with the air. This latter process is certainly
much more effective if carried out under conditions which preclude
infection. It must be remembered that aeration is frequently combined
with cooling, in which case the reputed advantages may not be entirely
attributable to the process of aeration.
~Infection of milk in the factory.~ The problem of proper handling of milk
is not entirely solved when the milk is delivered to the factory or
creamery, although it might be said that the danger of infection is much
greater while the milk is on the farm.
In the factory, infection can be minimized because effective measures of
cleanliness can be more easily applied. Steam is available in most
cases, so that all vats, cans, churns and pails can be thoroughly
scalded. Special emphasis should be given to the matter of cleaning
pumps and pipes. The difficulty of keeping these utensils clean often
leads to neglect and subsequent infection. In Swiss cheese factories,
the custom of using home-made rennet solutions is responsible for
considerable factory infection. Natural rennets are soaked in whey which
is kept warm in order to extract the rennet ferment. This solution when
used for curdling the milk often adds undesirable yeasts and other
gas-generating organisms, which are later the cause of abnormal ferment
action in the cheese (See page 186).
The influence of the air on the germ content of the milk is, as a rule,
overestimated. If the air is quiet, and free from dust, the amount of
germ life in the same is not relatively large. In a creamery or factory,
infection from this source ought to be much reduced, for the reason that
the floors and wall are, as a rule, quite damp, and hence germ life
cannot easily be dislodged. The majority of organisms found under such
conditions come from the person of the operators and attendants. Any
infection can easily be prevented by having the ripening cream-vats
covered with a canvas cloth. The clothing of the operator should be
different from the ordinary wearing-apparel. If made of white duck, the
presence of dirt is more quickly recognized, and greater care will
therefore be taken than if ordinary clothes are worn.
The surroundings of the factory have much to do with the danger of germ
infection. Many factories are poorly constructed and the drainage is
poor, so that filth and slime collect about and especially under the
factory. The emanations from these give the peculiar "factory odor" that
indicates fermenting matter. Not only are these odors absorbed
directly, but germ life from the same is apt to find its way into the
milk. Connell[49] has recently reported a serious defect in cheese that
was traced to germ infection from defective factory drains.
The water supply of a factory is also a question of prime importance.
When taken from a shallow well, especially if surface drainage from the
factory is possible, the water may be contaminated to such an extent as
to introduce undesirable bacteria in such numbers that the normal course
of fermentation may be changed. The quality of the water, aside from
flavor, can be best determined by making a curd test (p. 76) which is
done by adding some of the water to boiled milk and incubating the same.
If "gassy" fermentations occur, it signifies an abnormal condition. In
deep wells, pumped as thoroughly as is generally the case with factory
wells, the germ content should be very low, ranging from a few score to
a few hundred bacteria per cc. at most.
Harrison[50] has recently traced an off-flavor in cheese in a Canadian
factory to an infection arising from the water-supply. He found the same
germ in both water and cheese and by inoculating a culture into
pasteurized milk succeeded in producing the undesirable flavor. The
danger from ice is much less, for the reason that good dairy practice
does not sanction using ice directly in contact with milk or cream.
Then, too, ice is largely purified in the process of freezing, although
if secured from a polluted source, reliance should not be placed in the
method of purification; for even freezing does not destroy all
vegetating bacteria.
FOOTNOTES:
[1] Olson. 24 Rept. Wis. Expt. Stat., 1907.
[2] Erf and Melick Bull. 131, Kan. Expt. Stat., Apr. 1905.
[3] Storch (40 Rept. Danish Expt. Stat., Copenhagen, 1898) has devised a
test whereby it can be determined whether this treatment has been
carried out or not: Milk contains a soluble enzym known as galactase
which has the property of decomposing hydrogen peroxid. If milk is
heated to 176° F. (80° C.) or above, this enzym is destroyed so that the
above reaction no longer takes place. If potassium iodid and starch are
added to unheated milk and the same treated with hydrogen peroxid, the
decomposition of the latter agent releases oxygen which acts on the
potassium salt, which in turn gives off free iodine that turns the
starch blue.
[4] McKay, N. Y. Prod. Rev., Mch. 22, 1899.
[5] Doane, Bull. 79, Md. Expt. Stat., Jan. 1902.
[6] Harrison, 22 Rept. Ont. Agr'l Coll., 1896, p. 113.
[7] Moore and Ward, Bull. 158, Cornell Expt. Stat., Jan. 1899; Ward,
Bull. 178, Cornell Expt. Stat., Jan. 1900.
[8] Harrison, 22 Rept. Ont. Agr. Coll., 1896, p. 108; Moore, 12 Rept.
Bur. Animal Ind., U. S. Dept. Ag., 1895-6, p. 261.
[9] Moore, Bacteria in Milk, N. Y. Dept. Ag., 1902.
[10] Freudenreich, Cent. f. Bakt., II Abt., 10: 418, 1903.
[11] Harrison, 22 Rept. Ont. Agr. Coll., 1896, p. 108.
[12] Marshall, Bull. 147, Mich. Expt. Stat., p. 42.
[13] Moore and Ward, Bull. 158, Cornell Expt. Stat., Jan. 1899.
[14] Burr, R. H. Cent. f. Bakt., II Abt., 8: 236, 1902. Freudenreich, l.
c. p. 418. Ward, Bull. 178, Cornell Expt. Stat., p. 277. Bolley (Cent.
f. Bakt., II Abt., 1: 795, 1895), in 30 experiments found 12 out of 16
species to belong to lactic class. Harrison (Trans. Can. Inst., 7: 474,
1902-3) records the lactic type as most commonly present.
[15] Ford, Journ. of Hyg., 1901, 1: 277.
[16] Freudenreich, l. c. p. 421.
[17] Stocking, Bull. 42, Storrs Expt. Stat., June, 1906.
[18] Dinwiddie, Bull, 45 Ark. Expt. Stat., p. 57. Ward, Journ. Appld.
Mic. 1: 205, 1898. Appel, Milch Zeit., No. 17, 1900. Harrison and
Cumming, Journ. Appld. Mic. 5: 2087. Russell and Hastings, 21 Rept. Wis.
Expt. Stat., 158, 1904.
[19] Fokker, Zeit. f. Hyg., 9: 41, 1890.
[20] Freudenreich, Ann. de Microg., 3: 118, 1891.
[21] Hunziker, Bull. 197, Cornell Expt. Stat., Dec. 1901.
[22] Freudenreich, Cent. f. Bakt., II Abt., 10: 417, 1903.
[23] This general statement is in the main correct, although Ford
(Journ. of Hyg., 1: 277, 1901) claims to have found organisms sparingly
present in healthy tissues.
[24] Backhaus, Milch Zeit., 26: 357, 1897.
[25] Freudenreich, Die Bakteriologie, p. 30.
[26] Stocking, Bull. 42, Storrs Expt. Stat., June 1906.
[27] Harrison, Cent. f. Bakt., II Abt., 5: 183, 1899.
[28] Drysdale, Trans. High. and Agr. Soc. Scotland. 5 Series, 10: 166,
1898.
[29] Schuppan, (Cent. f. Bakt., 13: 155, 1893) claims to have found a
reduction of 48 per cent. in the Copenhagen filters while in the more
extended work of Dunbar and Kister (Milch Zeit., pp. 753, 787, 1899) the
bacterial content was higher in the filtered milk in 17 cases out of 22.
[30] Backhaus and Cronheim, Journ. f. Landw., 45: 222, 1897.
[31] Eckles and Barnes, Bull. 159 Iowa Expt. Stat., Aug. 1901.
[32] Dunbar and Kister, Milch Zeit., p. 753, 1899. Harrison and Streit,
Trans. Can. Inst., 7: 488, 1902-3.
[33] Doane, Bull. 88 Md. Expt. Stat., May 1903.
[34] Eckles, Hoard's Dairyman, July 8, 1898.
[35] Fraser, Bull. 91, Ill. Expt. Stat.
[36] Fraser, Bull. 91, Ill. Expt. Stat., Dec. 1903.
[37] Stocking, Bull. 42, Storrs Expt. Stat., June, 1906.
[38] Backhaus. Ber. Landw. Inst. Univ. Königsberg 2: 12, 1897.
[39] De Schweinitz, Nat. Med. Rev., April, 1899.
[40] Conn, Proc. Soc. Amer. Bacteriologists, 1902.
[41] Freudenreich, Ann. de Microg., 2:115, 1890.
[42] Conn, Bull. 26, Storrs Expt. Stat.
[43] New York City is supplied with milk that is shipped 350 miles.
[44] Park, N. Y. Univ. Bull., 1: 85, 1901.
[45] Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.
[46] Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.
[47] Russell, 15 Rept. Wis. Expt. Stat. 1898, p. 104.
[48] Alvord, Circ. No. 9, U. S. Dept. Agric. (Div. of Bot.).
[49] Connell, Rept. of Commissioner of Agr., Canada, 1897, part XVI, p.
15.
[50] Harrison, Hoard's Dairyman, March 4, 1898.
CHAPTER IV.
FERMENTATIONS IN MILK AND THEIR TREATMENT.
Under the conditions in which milk is drawn, it is practically
impossible to secure the same without bacterial contamination. The
result of the introduction of these organisms often changes its
character materially as most bacteria cause the production of more or
less pronounced fermentative processes. Under normal conditions, milk
sours, i. e., develops lactic acid, but at times this more common
fermentation may be replaced by other changes which are marked by the
production of some other more or less undesirable flavor, odor or change
in appearance.
In referring to these changes, it is usually customary to designate them
after the most prominent by-product formed, but it must be kept in mind
that generally some other decomposition products are usually produced.
Whether the organisms producing this or that series of changes prevail
or not depends upon the initial seeding, and the conditions under which
the milk is kept. Ordinarily, the lactic acid organisms grow so
luxuriantly in the milk that they overpower all competitors and so
determine the nature of the fermentation; but occasionally the milk
becomes infected with other types of bacteria in relatively large
numbers and the conditions may be especially suitable to the development
of these forms, thereby modifying the course of the normal changes that
occur.
The kinds of bacteria that find it possible to develop in milk may be
included under two heads:
1. Those which cause no appreciable change in the milk, either in taste,
odor or appearance. While these are frequently designated as the inert
bacteria, it must not be supposed that they have absolutely no effect on
milk. It is probably true in most cases that slight changes of a
chemical nature are produced, but the nature of the changes do not
permit of ready recognition.
2. This class embraces all those organisms which, as a result of their
growth, are capable of producing evident changes. These transformations
may be such as to affect the taste, as in the sour milk or in the bitter
fermentations, or the odor, as in some of the fetid changes, or the
appearance of the milk, as in the slimy and color changes later
described.
~Souring of milk.~ Ordinarily if milk is allowed to stand for several days
at ordinary temperatures it turns sour. This is due to the formation of
lactic acid, which is produced by the decomposition of the milk-sugar.
While this change is well nigh universal, it does not occur without a
pre-existing cause, and that is the presence of certain living bacterial
forms. These organisms develop in milk with great rapidity, and the
decomposition changes that are noted in souring are due to the
by-products of their development.
The milk-sugar undergoes fermentation, the chief product being lactic
acid, although various other by-products, as other organic acids
(acetic, formic and succinic), different alcohols and gaseous products,
as CO_{2}, H, N and methane (CH_{4}) are produced in small amounts.
In this fermentation, the acidity begins to be evident to the taste when
it reaches about 0.3 per cent., calculated as lactic acid. As the
formation of acid goes on, the casein is precipitated and incipient
curdling or lobbering of the milk occurs. This begins to be apparent
when the acidity is about 0.4 per cent., but the curd becomes more solid
with increasing acidity. The rapidity of curdling is also dependent upon
the temperature of the milk. Thus milk which at ordinary temperatures
might remain fluid often curdles when heated. The growth of the bacteria
is continued until about 0.8 to 1.0 per cent. acid is formed, although
the maximum amount fluctuates considerably with different lactic acid
species. Further formation then ceases even though all of the milk-sugar
is not used up, because of the inability of the lactic bacteria to
continue their growth in such acid solutions.
As this acidity is really in the milk serum, cream never develops so
much acid as milk, because a larger proportion of its volume is made up
of butter-fat globules. This fact must be considered in the ripening of
cream in butter-making where the per cent. of fat is subject to wide
fluctuations.
The formation of lactic acid is a characteristic that is possessed by a
large number of bacteria, micrococci as well as bacilli being numerously
represented. Still the preponderance of evidence is in favor of the view
that a few types are responsible for most of these changes. The most
common type found in spontaneously soured milk changes the milk-sugar
into lactic acid without the production of any gas. This type has been
described by various workers on European as well as American milks, and
is designated by Conn as the _Bact. lactis acidi_ type.[51] It is
subject to considerable variation under different conditions.
Curiously enough if milk which has been drawn with special care is
examined immediately after milking, the lactic organisms are not usually
found. They are incapable of development in the udder itself, as shown
by injections into the milk cistern. They abound, however, on hay, in
dust, in the barn air, on the hairy coat of the animal, and from these
sources easily gain access to the milk. In this medium they find an
exceptionally favorable environment and soon begin a very rapid growth,
so that by the time milk is consumed, either in the form of milk or milk
products, they make up numerically the larger portion of the bacteria
present.
Another widely disseminated, although numerically less prevalent, type
is _B. lactis aerogenes_. This type forms gas in milk so that the soured
milk is torn by the presence of gas bubbles. It also grows more
luxuriantly in contact with the air.
Other types occur more or less sporadically, some of which are capable
of liquefying the casein of milk while at the same time they also
develop lactic acid. Conn and Aikman refer to the fact that over one
hundred species capable of producing variable quantities of lactic acid
are already known. It is fair to presume, however, that a careful
comparative study of these would show that simply racial differences
exist in many cases, and therefore, that they are not distinct species.
As a group these bacteria are characterized by their inability to
liquefy gelatin or develop spores. On account of this latter
characteristic they are easily destroyed when milk is pasteurized. They
live under aerobic or anaerobic conditions, many of them being able to
grow in either environment, although, according to McDonnell,[52] they
are more virulent when air is not excluded.
While growth of these lactic forms may go on in milk throughout a
relatively wide range in temperature, appreciable quantities of acid are
not produced except very slowly at temperatures below 50° F.[53]
From the standpoint of frequency the most common abnormal changes that
occur in milk are those in which gases of varying character are
developed in connection with acids, from the milk sugar. Other volatile
products imparting bad flavors usually accompany gas production. These
fermentations are of most serious import in the cheese industry, as they
are especially prone to develop in the manufacture of milk into certain
types of cheese. Not often is their development so rapid that they
appear in the milk while it is yet in the hands of the milk producer,
but almost invariably the introduction of the causal organisms takes
place while the milk is on the farm. Numerous varieties of bacteria
possess this property of producing gas (H and CO_{2} are most common
although N and methane (CH_{4}) are sometimes produced). The more common
forms are those represented by _B. lactis aerogenes_ and the common
fecal type, _B. coli commune_. The ordinary habitat of this type is dirt
and intestinal filth. Hence careless methods of milk handling invite
this type of abnormal change in milk.
It is a wide-spread belief that thunder storms cause milk to sour
prematurely, but this idea has no scientific foundation. Experiments[54]
with the electric spark, ozone and loud detonations show no effect on
acid development, but the atmospheric conditions usually incident to a
thunder storm are such as permit of a more rapid growth of organisms.
There is no reason to believe but that the phenomenon of souring is
wholly related to the development of bacteria. Sterile milks are never
affected by the action of electric storms.
~"Gassy" milks.~ Where these gas bacteria abound, the amount of lactic
acid is generally reduced, due to the splitting up of some of the sugar
into gaseous products. This type of germ life does not seem to be able
to develop well in the presence of the typical lactic acid non
gas-forming bacteria.
[Illustration: FIG. 16. Cheese made from "gassy" milk.]
~"Sweet curdling" and digesting fermentations.~ Not infrequently milk,
instead of undergoing spontaneous souring, curdles in a weakly acid or
neutral condition, in which state it is said to have undergone "sweet
curdling." The coagulation of the milk is caused by the action of enzyms
of a rennet type that are formed by the growth of various species of
bacteria. Later the whey separates more or less perfectly from the curd,
producing a "wheyed off" condition. Generally the coagulum in these
cases is soft and somewhat slimy. The curd usually diminishes in bulk,
due to the gradual digestion or peptonization of the casein by
proteid-dissolving enzyms (tryptic type) that are also produced by the
bacteria causing the change.
A large number of bacteria possess the property of affecting milk in
this way. So far as known they are able to liquefy gelatin (also a
peptonizing process) and form spores. The Tyrothrix type of bacteria (so
named by Duclaux on account of the supposed relation to cheese ripening)
belongs to this class. The hay and potato forms are also digesters.
Organisms of this type are generally associated with filth and manure,
and find their way into the milk from the accumulations on the coat of
the animal.
Conn[55] has separated the rennet enzym from bacterial cultures in a
relatively pure condition, while Fermi[56] has isolated the digestive
ferment from several species.
Duclaux[57] has given to this digesting enzym the name _casease_ or
cheese ferment. These isolated ferments when added to fresh milk possess
the power of causing the characteristic curdling and subsequent
digestion quite independent of cell development. The quantity of ferment
produced by different species differs materially in some cases. In these
digestive fermentations, the chemical transformations are profound, the
complex proteid molecule being broken down into albumoses, peptones,
amido-acids (tyrosin and leucin) and ammonia as well as fatty acids.
Not infrequently these fermentations gain the ascendency over the normal
souring change, but under ordinary conditions they are held in abeyance,
although this type of bacteria is always present to some extent in milk.
When the lactic acid bacteria are destroyed, as in boiled, sterilized
or pasteurized milk, these rennet-producing, digesting species develop.
~Butyric acid fermentations.~ The formation of butyric acid in milk which
may be recognized by the "rancid butter" odor is not infrequently seen
in old, sour milk, and for a long time was thought to be a continuation
of the lactic fermentation, but it is now believed that these organisms
find more favorable conditions for growth, not so much on account of the
lactic acid formed as in the absence of dissolved oxygen in the milk
which is consumed by the sour-milk organisms.
Most of the butyric class of bacteria are spore-bearing, and hence they
are frequently present in boiled or sterilized milk. The by-products
formed in this series of changes are quite numerous. In most cases,
butyric acid is prominent, but in addition to this, other organic acids,
as lactic, succinic, and acetic, are produced, likewise different
alcohols. Concerning the chemical origin of butyric acid there is yet
some doubt. Duclaux[58] affirms that the fat, sugar and casein are all
decomposed by various forms. In some cases, the reaction of the milk is
alkaline, with other species it may be neutral or acid. This type of
fermentation has not received the study it deserves.
In milk these organisms are not of great importance, as this
fermentation does not readily gain the ascendency over the lactic
bacteria.
~Ropy or slimy milk.~ The viscosity of milk is often markedly increased
over that which it normally possesses. The intensity of this abnormal
condition may vary much; in some cases the milk becoming viscous or
slimy; in others stringing out into long threads, several feet in
length, as in Fig. 17. Two sets of conditions are responsible for these
ropy or slimy milks. The most common is where the milk is clotted or
stringy when drawn, as in some forms of garget. This is generally due to
the presence of viscid pus, and is often accompanied by a bloody
discharge, such a condition representing an inflamed state of the udder.
Ropiness of this character is not usually communicable from one lot of
milk to another.
[Illustration: FIG. 17. Ropy milk.]
The communicable form of ropy milk only appears after the milk has been
drawn from the udder for a day or so, and is caused by the development
of various species of bacteria which find their way into the milk after
it is drawn. These defects are liable to occur at any season of the
year. Their presence in a dairy is a source of much trouble, as the
unsightly appearance of the milk precludes its use as food, although
there is no evidence that these ropy fermentations are dangerous to
health.
There are undoubtedly a number of different species of bacteria that are
capable of producing these viscid changes,[59] but it is quite probable
that they are not of equal importance in infecting milk under natural
conditions.
In the majority of cases studied in this country,[60] the causal
organism seems to be _B. lactis viscosus_, a form first found by Adametz
in surface waters.[61] This organism possesses the property of
developing at low temperatures (45°-50° F.), and consequently it is
often able in winter to supplant the lactic-acid forms. Ward has found
this germ repeatedly in water tanks where milk cans are cooled; and
under these conditions it is easy to see how infection of the milk might
occur. Marshall[62] reports an outbreak which he traced to an external
infection of the udder; in another case, the slime-forming organism was
abundant in the barn dust. A defect of this character is often
perpetuated in a dairy for some time, and may therefore become
exceedingly troublesome. In one instance in the writer's experience, a
milk dealer lost over $150 a month for several months from ropy cream.
Failure to properly sterilize cans, and particularly strainer cloths, is
frequently responsible for a continuance of trouble of this sort.
The slimy substance formed in milk comes from various constituents of
the milk, and the chemical character of the slime produced also varies
with different germs. In some cases the slimy material is merely the
swollen outer cell membrane of the bacteria themselves as in the case of
_B. lactis viscosus_; in others it is due to the decomposition of the
proteids, but often the chief decomposition product appears to come from
a viscous fermentation of the milk-sugar.
An interesting case of a fermentation of this class being utilized in
dairying is seen in the use of "lange wei" (long or stringy whey) which
is employed as a starter in Holland to control the gassy fermentations
in Edam cheese. This slimy change is due to the growth of
_Streptococcus Hollandicus_.[63]
~Alcoholic fermentations.~ Although glucose or cane-sugar solutions are
extremely prone to undergo alcoholic fermentation, milk sugar does not
readily undergo this change. Where such changes are produced it is due
to yeasts. Several outbreaks attributable to such a cause have been
reported.[64] Russell and Hastings[65] have found these milk-sugar
splitting yeasts particularly abundant in regions where Swiss cheese is
made, a condition made possible by the use of whey-soaked rennets in
making such cheese.
Kephir and Koumiss are liquors much used in the Orient which are made
from milk that has undergone alcoholic fermentation. Koumiss was
originally made from mare's milk but is now often made from cows' milk
by adding cane sugar and yeast. In addition to the CO_{2} developed,
alcohol, lactic acid, and casein-dissolving ferments are formed. Kephir
is made by adding to milk Kephir grains, which are a mass of yeast and
bacterial cells. The yeasts produce alcohol and CO_{2} while the
bacteria change the casein of milk, rendering it more digestible. These
beverages are frequently recommended to persons who seem to be unable to
digest raw milk readily. The exact nature of the changes produced are
not yet well understood.[66]
~Bitter milk.~ The presence of bitter substances in milk may be ascribed
to a variety of causes. A number of plants, such as lupines, ragweed and
chicory, possess the property of affecting milk when the same are
consumed by animals. At certain stages in lactation, a bitter salty
taste is occasionally to be noted that is peculiar to individual
animals.
A considerable number of cases of bitter milk have, however, been traced
to bacterial origin. For a number of years the bitter fermentation of
milk was thought to be associated with the butyric fermentation, but
Weigmann[67] showed that the two conditions were not dependent upon each
other. He found that the organism which produced the bitter taste acted
upon the casein.
Conn[68] observed a coccus form in bitter cream that was able to impart
a bitter flavor to milk. Sometimes a bitter condition does not develop
in the milk, but may appear later in the milk products, as in the case
of a micrococcus which Freudenreich[69] found in cheese.
Harrison[70] has traced a common bitter condition in Canadian milk to a
milk-sugar splitting yeast, _Torula amara_ which not only grows rapidly
in milk but produces an undesirable bitterness in cheddar cheese.
Cream ripened at low temperatures not infrequently develops a bitter
flavor, showing that the optimum temperature for this type of
fermentation is below the typical lactic acid change.
Milk that has been heated often develops a bitter condition. The
explanation of this is that the bacteria producing the bitter substances
usually possess endospores, and that while the boiling or sterilizing of
milk easily kills the lactic acid germs, these forms on account of their
greater resisting powers are not destroyed by the heat.
~Soapy milk:~ A soapy flavor in milk was traced by Weigmann and Zirn[71]
to a specific bacillus, _B. lactis saponacei_, that they found gained
access to the milk in one case from the bedding and in another instance
from hay. A similar outbreak has been reported in this country,[72] due
to a germ acting on the casein and albumen.
~Red milk.~ The most common trouble of this nature in milk is due to
presence of blood, which is most frequently caused by some wound in the
udder. The ingestion of certain plants as sedges and scouring rushes is
also said to cause a bloody condition; madders impart a reddish tinge
due to coloring matter absorbed. Defects of this class can be readily
distinguished from those due to germ growth because they are apparent at
time of milking. Where blood is actually present, the corpuscles settle
out in a short time if left undisturbed.
There are a number of chromogenic or color-producing bacteria that are
able to grow in milk, but their action is so slow that generally they
are not of much consequence. Moreover their development is usually
confined to the surface of the milk as it stands in a vessel. The most
important is the well-known _B. prodigiosus_. Another form found at
times in milk possessing low acidity[73] is _B. lactis erythrogenes_.
This species only develops the red color in the dark. In the light, it
forms a yellow pigment. Various other organisms have been reported at
different times.[74]
~Blue milk.~ Blue milk has been known for many years, its communicable
nature being established as long ago as 1838. It appears on the surface
of milk first as isolated particles of bluish or grey color, which
later become confluent, the blue color increasing in intensity as the
acidity increases. The causal organism, _B. cyanogenes_, is very
resistant toward drying,[75] thus accounting for its persistence. In
Mecklenberg an outbreak of this sort once continued for several years.
It has frequently been observed in Europe in the past, but is not now so
often reported. Occasional outbreaks have been reported in this country.
~Other kinds of colored milk.~ Two or three chromogenic forms producing
still other colors have occasionally been found in milk. Adametz[76]
discovered in a sample of cooked milk a peculiar form (_Bacillus
synxanthus_) that produced a citron-yellow appearance which precipitated
and finally rendered soluble the casein. Adametz, Conn, and List have
described other species that confer tints of yellow on milk. Some of
these are bright lemon, others orange, and some amber in color.
Still other color-producing bacteria, such as those that produce violet
or green changes in the milk, have been observed. In fact, almost any of
the chromogenic bacteria are able to produce their color changes in milk
as it is such an excellent food medium. Under ordinary conditions, these
do not gain access to milk in sufficient numbers so that they modify the
appearance of it except in occasional instances.
~Treatment of abnormal fermentations.~ If the taint is recognized as of
bacterial origin (see p. 57) and is found in the mixed milk of the herd,
it is necessary to ascertain, first, whether it is a general trouble, or
restricted to one or more animals. This can sometimes be done by
separating the milk of the different cows and noting whether any
abnormal condition develops in the respective samples.
~Fermentation tests.~ The most satisfactory way to detect the presence of
the taints more often present is to make a fermentation test of one kind
or another. These tests are most frequently used at the factory, to
enable the maker to detect the presence of milk that is likely to prove
unfit for use, especially in cheese making. They are based upon the
principle that if milk is held at a moderately high temperature, the
bacteria will develop rapidly. A number of different methods have been
devised for this purpose. In Walther's lacto-fermentator samples of milk
are simply allowed to stand in bottles or glass jars until they sour.
They are examined at intervals of several hours. If the curdled milk is
homogeneous and has a pure acid smell, the milk is regarded as all
right. If it floats in a turbid serum, is full of gas or ragged holes,
it is abnormal. As generally carried out, no attempt is made to have
these vessels sterile. Gerber's test is a similar test that has been
extensively employed in Switzerland. Sometimes a few drops of rennet are
added to the milk so as to curdle the same, and thus permit of the more
ready detection of the gas that is evolved.
~Wisconsin curd test.~ The method of testing milk described below was
devised at the Wisconsin Experiment Station in 1895 by Babcock, Russell
and Decker.[77] It was used first in connection with experimental work
on the influence of gas-generating bacteria in cheese making, but its
applicability to the detection of all taints in milk produced by
bacteria makes it a valuable test for abnormal fermentations in general.
In the curd test a small pat of curd is made in a glass jar from each
sample of milk. These tests may be made in any receptacle that has been
cleaned in boiling water, and to keep the temperature more nearly
uniform these jars should be immersed in warm water, as in a wash tub or
some other receptacle. When the milk is about 95° F., about ten drops of
rennet extract are added to each sample and mixed thoroughly with the
milk. The jars should then remain undisturbed until the milk is
completely curdled; then the curd is cut into small pieces with a case
knife and stirred to expel the whey. The whey should be poured off at
frequent intervals until the curd mats. If the sample be kept at blood
heat (98° F.) for six to eight hours, it will be ready to examine.
[Illustration: FIG. 18. Improved bottles for making curd test. _A_, test
bottle complete; _B_, bottle showing construction of cover; _S_, sieve
to hold back the curd when bottle is inverted; _C_, outer cover with _(D
H)_ drain holes to permit of removal of whey.]
More convenient types of this test than the improvised apparatus just
alluded to have been devised by different dairy manufacturers.
Generally, they consist of a special bottle having a full-sized top,
thus permitting the easy removal of the curd. The one shown in Fig. 18
is provided with a sieve of such construction that the bottles will
drain thoroughly if inclined in an inverted position.
~Interpretation of results of test.~ The curd from a good milk has a firm,
solid texture, and should contain at most only a few small pin holes. It
may have some large, irregular, "mechanical" holes where the curd
particles have failed to cement, as is seen in Fig. 19. If gas-producing
bacteria are very prevalent in the milk, the conditions under which the
test is made cause such a rapid growth of the same that the evidence of
the abnormal fermentation may be readily seen in the spongy texture of
the curd (Fig. 20). If the undesirable organisms are not very abundant
and the conditions not especially suited to their growth, the "pin
holes" will be less frequent.
[Illustration: FIG. 19. Curd from a good milk. The large irregular holes
are mechanical.]
Sometimes the curds show no evidence of gas, but their abnormal
condition can be recognized by the "mushy" texture and the presence of
"off" flavors that are rendered more apparent by keeping them in closed
bottles. This condition is abnormal and is apt to produce quite as
serious results as if gas was formed.
~Overcoming taints by use of starters.~ Another method of combatting
abnormal fermentations that is often fruitful, is that which rests upon
the inability of one kind of bacteria to grow in the same medium in
competition with certain other species.
Some of the undesirable taints in factories can be controlled in large
part by the introduction of starters made from certain organisms that
are able to obtain the ascendency over the taint-producing germ. Such a
method is commonly followed when a lactic ferment, either a commercial
pure culture, or a home-made starter, is added to milk to overcome the
effect of gas-generating bacteria.
[Illustration: FIG. 20. Curd from a badly tainted milk. Large ragged
holes are mechanical; numerous small holes due to gas. This curd was a
"floater."]
A similar illustration is seen in the case of the "lange wei" (slimy
whey), that is used in the manufacture of Edam cheese to control the
character of the fermentation of the milk.
This same method is sometimes applied in dealing with certain abnormal
fermentations that are apt to occur on the farm. It is particularly
useful with those tainted milks known as "sweet curdling." The ferment
organisms concerned in this change are unable to develop in the
presence of lactic acid bacteria, so the addition of a clean sour milk
as a starter restores the normal conditions by giving the ordinary milk
bacteria the ascendency.
~Chemical disinfection.~ In exceptional instances it may be necessary to
employ chemical disinfectants to restore the normal conditions. Of
course with such diseases as tuberculosis, very stringent measures are
required, as they are such a direct menace to human life, but with these
abnormal or taint-producing fermentations, care and cleanliness, well
directed, will usually overcome the trouble.
If it becomes necessary to employ chemical substances as disinfecting
agents, their use should always be preceded by a thorough cleansing with
hot water so that the germicide may come in direct contact with the
surface to be disinfected.
It must be borne in mind that many chemicals act as deodorants, _i.e._,
destroy the offensive odor, without destroying the cause of the trouble.
_Sulfur_ is often recommended as a disinfecting agent, but its use
should be carefully controlled, otherwise the vapors have but little
germicidal power. The common practice of burning a small quantity in a
room or any closed space for a few moments has little or no effect upon
germ life. The effect of sulfur vapor (SO_{2}) alone upon germ life is
relatively slight, but if this gas is produced in the presence of
moisture, sulfurous acid (H_{2}SO_{3}) is formed, which is much more
efficient. To use this agent effectively, it must be burned in large
quantities in a moist atmosphere (three lbs. to every 1,000 cubic feet
of space), for at least twelve hours. After this operation, the space
should be thoroughly aired.
_Formalin_, a watery solution of a gas known as formaldehyde, is a new
disinfectant that recent experience has demonstrated to be very useful.
It may be used as a gas where rooms are to be disinfected, or applied as
a liquid where desired. It is much more powerful in its action than
sulfur, and it has a great advantage over mercury and other strong
disinfectants, as it is not so poisonous to man as it is to the lower
forms of life.
_Bleaching powder or chloride of lime_ is often recommended where a
chemical can be advantageously used. This substance is a good
disinfectant as well as a deodorant, and if applied as a wash, in the
proportion of four to six ounces of the powder to one gallon of water,
it will destroy most forms of life. In many cases this agent is
inapplicable on account of its odor.
_Corrosive sublimate_ (HgCl_{2}) for most purposes is a good
disinfectant, but it is such an intense poison that its use is dangerous
in places that are at all accessible to stock.
For the disinfection of walls in stables and barns, common thin _white
wash_ Ca(OH)_{2} is admirably adapted if made from freshly-burned quick
lime. It possesses strong germicidal powers, increases the amount of
light in the barn, is a good absorbent of odors, and is exceedingly
cheap.
Carbolic acid, creosote, and such products, while excellent
disinfectants, cannot well be used on account of their odor, especially
in factories.
For gutters, drains, and waste pipes in factories, _vitriol salts_
(sulfates of copper, iron and zinc) are sometimes used. These are
deodorants as well as disinfectants, and are not so objectionable to use
on account of their odor.
These suggestions as to the use of chemicals, however, only apply to
extreme cases and should not be brought into requisition until a
thorough application of hot water, soap, a little soda, and the
scrubbing brush have failed to do their work.
FOOTNOTES:
[51] Günther and Thierfelder, Arch. f. Hyg., 25:164, 1895; Leichmann,
Cent. f. Bakt., 2:281, 1896; Esten, 9 Rept. Storrs Expt. Stat., p. 44,
1896; Dinwiddie, Bull. 45, Ark. Expt. Stat., May, 1897; Kozai, Zeit. f.
Hyg., 38:386, 1901; Weigmann, Hyg. Milk Congress, Hamburg, 1903, p. 375.
[52] McDonnell, Inaug. Diss., Kiel. 1899, p. 39.
[53] Kayser, Cent. f. Bakt. II. Abt. 1:436.
[54] Treadwell, Science, 1894, 17:178.
[55] Conn, 5 Rept. Storrs Expt. Stat., 1892, p. 396.
[56] Fermi, Arch. f. Hyg., 1892, 14:1.
[57] Duclaux, Le Lait, p. 121.
[58] Duclaux, Principes de Laiterie, p. 67.
[59] Guillebeau (Milch Zeit., 1892, p. 808) has studied over a dozen
different forms that possess this property.
[60] Ward, Bull. 165, Cornell Expt. Stat., Mch., 1899; also Bull. 195,
Ibid., Nov., 1901.
[61] Adametz, Landw. Jahr., 1891, p. 185.
[62] Marshall, Mich. Expt. Stat., Bull. 140.
[63] Milch Zeit., 1899, p. 982.
[64] Duclaux, Principes de Laiterie, p. 60. Heinze and Cohn, Zeit. f.
Hyg., 46: 286, 1904.
[65] Bull. 128, Wis. Expt. Stat., Sept. 1905.
[66] Freudenreich, Landw. Jahr. d. Schweiz, 1896, 10; 1.
[67] Weigmann, Milch Zeit., 1890, p. 881.
[68] Conn, 3 Rept. Storrs Expt. Stat., 1890, p. 158.
[69] Freudenreich, Fühl. Landw. Ztg. 43: 361.
[70] Harrison, Bull. 120 Ont. Agr'l. Coll., May, 1902.
[71] Milch Zeit. 22:569.
[72] Marshall, Bull. 146, Mich. Expt. Stat., p. 16.
[73] Grotenfelt, Milch Zeit., 1889, p. 263.
[74] Menge, Cent. f. Bakt., 6:596; Keferstein, Cent. f. Bakt., 21:177.
[75] Heim, Arb. a. d. Kais. Gesundheitsamte, 5:578.
[76] Adametz, Milch Zeit., 1890, p. 225.
[77] 12 Rept. Wis. Expt. Stat., 1895, p. 148; also Bull. 67, Ibid.,
June, 1898.
CHAPTER V.
RELATION OF DISEASE-BACTERIA TO MILK.
Practical experience with epidemic disease has abundantly demonstrated
the fact that milk not infrequently serves as a vehicle for the
dissemination of contagion. Attention has been prominently called to
this relation by Ernest Hart,[78] who in 1880 compiled statistical
evidence showing the numerous outbreaks of various contagious diseases
that had been associated with milk infection up to that time. Since
then, further compilations have been made by Freeman,[79] and also by
Busey and Kober,[80] who have collected the data with reference to
outbreaks from 1880 to 1899.
These statistics indicate the relative importance of milk as a factor in
the dissemination of disease.
The danger from this source is much intensified for the reason that
milk, generally speaking, is consumed in a raw state; and also because a
considerable number of disease-producing bacteria are able, not merely
to exist, but actually thrive and grow in milk, even though the normal
milk bacteria are also present. Moreover the recognition of the presence
of such pathogenic forms is complicated by the fact that often they do
not alter the appearance of the milk sufficiently so that their
presence can be detected by a physical examination. These facts which
have been experimentally determined, coupled with the numerous clinical
cases on record, make a strong case against milk serving as an agent in
the dissemination of disease.
~Origin of pathogenic bacteria in milk.~ Disease-producing bacteria may be
grouped with reference to their relation toward milk into two classes,
depending upon the manner in which infection occurs:
Class I. Disease-producing bacteria capable of being transmitted
directly from a diseased animal to man through the medium of infected
milk.
Class II. Bacteria pathogenic for man but not for cattle which are
capable of thriving in milk after it is drawn from the animal.
In the first group the disease produced by the specific organism must be
common to both cattle and man. The organism must live a parasitic life
in the animal, developing in the udder, and so infect the milk supply.
It may, of course, happen that diseases toward which domestic animals
alone are susceptible may be spread from one animal to another in this
way without affecting human beings.
In the second group, the bacterial species lives a saprophytic
existence, growing in milk, if it happens to find its way therein. In
such cases milk indirectly serves as an agent in the dissemination of
disease, by giving conditions favorable to the growth of the disease
germ.
By far the most important of diseases that may be transmitted directly
from animal to man through a diseased milk supply is tuberculosis, but
in addition to this, foot and mouth disease (aphthous fever in
children), anthrax and acute enteric troubles have also been traced to a
similar source of infection.
The most important specific diseases that have been disseminated through
subsequent pollution of the milk are typhoid fever, diphtheria, scarlet
fever and cholera, but, of course, the possibility exists that any
disease germ capable of living and thriving in milk may be spread in
this way. In addition to these diseases that are caused by the
introduction of specific organisms (the causal organism of scarlet fever
has not yet been definitely determined), there are a large number of
more or less illy-defined troubles of an intestinal character that occur
especially in infants and young children that are undoubtedly
attributable to the activity of microörganisms that gain access to milk
during and subsequent to the milking, and which produce changes in milk
before or after its ingestion that result in the formation of toxic
products.
DISEASES TRANSMISSIBLE FROM ANIMAL TO MAN THROUGH DISEASED MILK.
~Tuberculosis.~ In view of the wide-spread distribution of this disease in
both the human and the bovine race, the relation of the same to milk
supplies is a question of great importance. It is now generally admitted
that the different types of tubercular disease found in different kinds
of animals and man are attributable to the development of the same
organism, _Bacillus tuberculosis_, although there are varieties of this
organism found in different species of animals that are sufficiently
distinct to permit of recognition.
The question of prime importance is, whether the bovine type is
transmissible to the human or not. Artificial inoculation of cattle with
tuberculous human sputum as well as pure cultures of this variety show
that the human type is able to make but slight headway in cattle. This
would indicate that the danger of cattle acquiring the infection from
man would in all probability be very slight, but these experiments offer
no answer as to the possibility of transmission from the bovine to the
human. Manifestly it is impossible to solve this problem by direct
experiment upon man except by artificial inoculation, but comparative
experiments upon animals throw some light on the question.
Theo. Smith[81] and others[82] have made parallel experiments with
animals such as guinea pigs, rabbits and pigeons, inoculated with both
bovine and human cultures of this organism. The results obtained in the
case of all animals tested show that the virulence of the two types was
much different, but that the bovine cultures were much more severe.
While of course this does not prove that transmission from bovine to
human is possible, still the importance of the fact must not be
overlooked.
In a number of cases record of accidental infection from cattle to man
has been noted.[83] These have occurred with persons engaged in making
post-mortem examinations on tuberculous animals, and the tubercular
nature of the wound was proven in some cases by excision and
inoculation.
In addition to data of this sort that is practically experimental in
character, there are also strong clinical reasons for considering that
infection of human beings may occur through the medium of milk.
Naturally such infection should produce intestinal tuberculosis, and it
is noteworthy that this phase of the disease is quite common in
children especially between the ages of two and five.[84] It is
difficult to determine, though, whether primary infection occurred
through the intestine, for, usually, other organs also become involved.
In a considerable number of cases in which tubercular infection by the
most common channel, inhalation, seems to be excluded, the evidence is
strong that the disease was contracted through the medium of the milk,
but it is always very difficult to exclude the possibility of pulmonary
infection.
Tuberculosis as a bovine disease has increased rapidly during recent
decades throughout many portions of the world. This has been most marked
in dairy regions. Its extremely insidious nature does not permit of an
early recognition by physical means, and it was not until the
introduction of the tuberculin test[85] in 1892, as a diagnostic aid
that accurate knowledge of its distribution was possible. The quite
general introduction of this test in many regions has revealed an
alarmingly large percentage of animals as affected. In Denmark in 1894
over forty per cent were diagnosed as tubercular. In some parts of
Germany almost as bad a condition has been revealed. Slaughter-house
statistics also show that the disease has increased rapidly since 1890.
In this country the disease on the average is much less than in Europe
and is also very irregularly distributed. In herds where it gained a
foothold some years ago, often the majority of animals are frequently
infected; many herds, in fact the great majority, are wholly free from
all taint. The disease has undoubtedly been most frequently introduced
through the purchase of apparently healthy but incipiently affected
animals. Consequently in the older dairy regions where stock has been
improved the most by breeding, more of the disease exists than among the
western and southern cattle.
[Illustration FIG. 21: Front view of a tuberculous udder, showing extent
of swelling in single quarter.]
~Infectiousness of milk of reacting animals.~ Where the disease appears in
the udder the milk almost invariably contains the tubercle organism.
Under such conditions the appearance of the milk is not materially
altered at first, but as the disease progresses the percentage of fat
generally diminishes, and at times in the more advanced stages where the
physical condition of the udder is changed (Fig. 21), the milk may
become "watery"; but the percentage of animals showing such udder
lesions is not large, usually not more than a few per cent. (4 per cent.
according to Ostertag.)
On the other hand, in the earlier phases of the disease, where its
presence has been recognized solely by the aid of the tuberculin test,
before there are any recognizable physical symptoms in any part of the
animal, the milk is generally unaffected. Between these extremes,
however, is found a large proportion of cases, concerning which so
definite data are not available. The results of investigators on this
point are conflicting and further information is much desired. Some have
asserted so long as the udder itself shows no lesions that no tubercle
bacilli would be present,[86] but the findings of a considerable number
of investigators[87] indicate that even when the udder is apparently not
diseased the milk may contain the specific organism as revealed by
inoculation experiments upon animals. In some cases, however, it has
been demonstrated by post-mortem examination that discoverable udder
lesions existed that were not recognizable before autopsy was made. In
the experimental evidence collected, a varying percentage of reacting
animals were found that gave positive results; and this number was
generally sufficient to indicate that the danger of using milk from
reacting animals was considerable, even though apparently no disease
could be found in the udder.
The infectiousness of milk can also be proven by the frequent
contraction of the disease in other animals, such as calves and pigs
which may be fed on the skim milk. The very rapid increase of the
disease among the swine of Germany and Denmark,[88] and the frequently
reported cases of intestinal infection of young stock also attest the
presence of the organism in milk.
The tubercle bacillus is so markedly parasitic in its habits, that,
under ordinary conditions, it is incapable of growing at normal air
temperatures. There is, therefore, no danger of the germ developing in
milk after it is drawn from the animal, unless the same is kept at
practically blood heat.
Even though the milk of some reacting animals may not contain the
dangerous organism at the time of making the test, it is quite
impossible to foretell how long it will remain free. As the disease
becomes more generalized, or if tuberculous lesions should develop in
the udder, the milk may pass from a healthy to an infectious state.
This fact makes it advisable to exclude from milk supplies intended for
human use, all milk of animals that respond to the tuberculin test; or
at least to treat it in a manner so as to render it safe. Whether it is
necessary to do this or not if the milk is made into butter or cheese is
a somewhat different question. Exclusion or treatment is rendered more
imperative in milk supplies, because the danger is greater with children
with whom milk is often a prominent constituent of their diet, and also
for the reason that the child is more susceptible to intestinal
infection than the adult.
The danger of infection is much lessened in butter or cheese, because
the processes of manufacture tend to diminish the number of organisms
originally present in the milk, and inasmuch as no growth can ordinarily
take place in these products the danger is minimized. Moreover, the fact
that these foods are consumed by the individual in smaller amounts than
is generally the case where milk is used, and also to a greater extent
by adults, lessens still further the danger of infection.
Notwithstanding this, numerous observers[89] especially in Germany have
succeeded in finding the tubercle bacillus in market butter, but this
fact is not so surprising when it is remembered that a very large
fraction of their cattle show the presence of the disease as indicated
by the tuberculin test, a condition that does not obtain in any large
section in this country.
The observations on the presence of the tubercle bacillus in butter have
been questioned somewhat of late[2] by the determination of the fact
that butter may contain an organism that possesses the property of being
stained in the same way as the tubercle organism. Differentiation
between the two forms is rendered more difficult by the fact that this
tubercle-like organism is also capable of producing in animals lesions
that stimulate those of tuberculosis, although a careful examination
reveals definite differences. Petri[90] has recently determined that
both the true tubercle and the acid-resisting butter organism may be
readily found in market butter.
In the various milk products it has been experimentally determined that
the true tubercle bacillus is able to retain its vitality in butter for
a number of months and in cheese for nearly a year.
~Treatment of milk from tuberculosis cows.~ While it has been shown that
it is practically impossible to foretell whether the milk of any
reacting animal actually contains tubercle bacilli or not, still the
interests of public health demand that no milk from such stock be used
for human food until it has been rendered safe by some satisfactory
treatment.
_1. Heating._ By far the best treatment that can be given such milk is
to heat it. The temperature at which this should be done depends upon
the thermal death point of the tubercle bacillus, a question concerning
which there has been considerable difference of opinion until very
recently. According to the work of some of the earlier investigators,
the tubercle bacillus in its vegetative stage is endowed with powers of
resistance greater than those possessed by any other pathogenic
organism. This work has not been substantiated by the most recent
investigations on this subject. In determining the thermal death point
of this organism, as of any other, not only must the temperature be
considered, but the period of exposure as well, and where that exposure
is made in milk, another factor must be considered, viz., the presence
of conditions permitting of the formation of a "scalded layer," for as
Smith[91] first pointed out, the resistance of the tubercle organism
toward heat is greatly increased under these conditions. If tuberculous
milk is heated in a closed receptacle where this scalded membrane cannot
be produced, the tubercle bacillus is killed at 140° F. in 15 to 20
minutes. These results which were first determined by Smith, under
laboratory conditions, and confirmed by Russell and Hastings,[92] where
tuberculous milk was heated in commercial pasteurizers, have also been
verified by Hesse.[93] A great practical advantage which accrues from
the treatment of milk at 140° F. is that the natural creaming is
practically unaffected. Of course, where a higher temperature is
employed, the period of exposure may be materially lessened. If milk is
momentarily heated to 176° F., it is certainly sufficient to destroy the
tubercle bacillus. This is the plan practiced in Denmark where all skim
milk and whey must be heated to this temperature before it can be taken
back to the farm, a plan which is designed to prevent the dissemination
of tuberculosis and foot and mouth disease by means of the mixed
creamery by-products. This course renders it possible to utilize with
perfect safety, for milk supplies, the milk of herds reacting to the
tuberculin test, and as butter of the best quality can be made from
cream or milk heated to even high temperatures,[94] it thus becomes
possible to prevent with slight expense what would otherwise entail a
large loss.
_2. Dilution._ Another method that has been suggested for the treatment
of this suspected milk is dilution with a relatively large volume of
perfectly healthy milk. It is a well known fact that to produce
infection, it requires the simultaneous introduction of a number of
organisms, and in the case of tuberculosis, especially that produced by
ingestion, this number is thought to be considerable. Gebhardt[95] found
that the milk of tuberculous cows, which was virulent when injected by
itself into animals, was innocuous when diluted with 40 to 100 times its
volume of healthy milk. This fact is hardly to be relied upon in
practice, unless the proportion of reacting to healthy cows is
positively known.
It has also been claimed in the centrifugal separation of cream from
milk[96] that by far the larger number of tubercle bacilli were thrown
out with the separator slime. Moore[97] has shown that the tubercle
bacilli in an artificially infected milk might be reduced in this way,
so as to be no longer microscopically demonstrable, yet the purification
was not complete enough to prevent the infection of animals inoculated
with the milk.
Another way to exclude all possibility of tubercular infection in milk
supplies is to reject all milk from reacting animals. This method is
often followed where pasteurization or sterilization is not desired. In
dairies where the keeping quality is dependent upon the exclusion of
bacteria by stringent conditions as to milking and handling ("sanitary"
or "hygienic" milk), the tuberculin test is frequently used as a basis
to insure healthy milk.
~Foot and mouth disease.~ The wide-spread extension of this disease
throughout Europe in recent years has given abundant opportunity to show
that while it is distinctively an animal malady, it is also
transmissible to man, although the disease is rarely fatal. The causal
organism has not been determined with certainty, but it has been shown
that the milk of affected animals possesses infectious properties[98]
although appearing unchanged in earlier phases of the disease.
Hertwig showed the direct transmissibility of the disease to man by
experiments made on himself and others. By ingesting milk from an
affected animal, he was able to produce the symptoms of the disease, the
mucous membrane of the mouth being covered with the small vesicles that
characterize the malady. It has also been shown that the virus of the
disease may be conveyed in butter.[99] This disease is practically
unknown in this country, although widely spread in Europe.
There are a number of other bovine diseases such as anthrax,[100]
lockjaw,[101] and hydrophobia[102] in which it has been shown that the
virus of the disease is at times to be found in the milk supply, but
often the milk becomes visibly affected, so that the danger of using the
same is greatly minimized.
There are also a number of inflammatory udder troubles known as garget
or mammitis. In most of these, the physical appearance of the milk is so
changed, and often pus is present to such a degree as to give a very
disagreeable appearance to the milk. Pus-forming bacteria (staphylococci
and streptococci) are to be found associated with such troubles. A
number of cases of gastric and intestinal catarrh have been reported as
caused by such milks.[103]
DISEASES TRANSMISSIBLE TO MAN THROUGH INFECTION OF MILK AFTER
WITHDRAWAL.
Milk is so well adapted to the development of bacteria in general, that
it is not surprising to find it a suitable medium for the growth of many
pathogenic species even at ordinary temperatures. Not infrequently,
disease-producing bacteria are able to grow in raw milk in competition
with the normal milk bacteria, so that even a slight contamination may
suffice to produce infection.
The diseases that are most frequently disseminated in this way are
typhoid fever, diphtheria, scarlet fever and cholera, together with the
various illy-defined intestinal troubles of a toxic character that occur
in children, especially under the name of cholera infantum, summer
complaint, etc.
Diseases of this class are not derived directly from animals because
cattle are not susceptible to the same.
~Modes of infection.~ In a variety of ways, however, the milk may be
subject to contaminating influences after it is drawn from the animal,
and so give opportunity for the development of disease-producing
bacteria. The more important methods of infection are as follows:
_1. Infection directly from a pre-existing case of disease on premises._
Quite frequently a person in the early stage of a diseased condition may
continue at his usual vocation as helper in the barn or dairy, and so
give opportunity for direct infection to occur. In the so-called cases
of "walking typhoid," this danger is emphasized. It is noteworthy in
typhoid fever that the bacilli frequently persist in the urine and in
diphtheria they often remain in the throat until after convalescence. In
some cases infection has been traced to storage of the milk in rooms in
the house where it became polluted directly by the emanations of the
patient.[104] Among the dwellings of the lower classes where a single
room has to be used in common this source of infection has been most
frequently observed.
_2. Infection through the medium of another person._ Not infrequently
another individual may serve in the capacity of nurse or attendant to a
sick person, and also assist in the handling of the milk, either in
milking the animals or caring for the milk after it has been drawn.
Busey and Kober report twenty-one outbreaks of typhoid fever in which
dairy employees also acted in the capacity of nurses.
_3. Pollution of milk utensils._ The most frequent method of infection
of cans, pails, etc., is in cleaning them with water that may be
polluted with disease organisms. Often wells may be contaminated with
diseased matter of intestinal origin, as in typhoid fever, and the use
of water at normal temperatures, or even in a lukewarm condition, give
conditions permitting of infection. Intentional adulteration of milk
with water inadvertently taken from polluted sources has caused quite a
number of typhoid outbreaks.[105] Sedgwick and Chapin[106] found in the
Springfield, Mass., epidemic of typhoid that the milk cans were placed
in a well to cool the milk, and it was subsequently shown that the well
was polluted with typhoid fecal matter.
_4. Pollution of udder_ of animal _by wading in infected water_, or by
washing same with contaminated water. This method of infection would
only be likely to occur in case of typhoid. An outbreak at the
University of Virginia in 1893[107] was ascribed to the latter cause.
_5. Pollution of creamery by-products, skim-milk, etc._ Where the milk
supply of one patron becomes infected with pathogenic bacteria, it is
possible that disease may be disseminated through the medium of the
creamery, the infective agent remaining in the skim milk after
separation and so polluting the mixed supply. This condition is more
likely to prevail with typhoid because of the greater tolerance of this
organism for acids such as would be found in raw milk. The outbreaks at
Brandon,[108] England, in 1893, Castle Island,[109] Ireland, and
Marlboro,[110] Mass., in 1894, were traced to such an origin.
While most outbreaks of disease associated with a polluted milk supply
originate in the use of the milk itself, yet infected milk may serve to
cause disease even when used in other ways. Several outbreaks of typhoid
fever have been traced to the use of ice cream where there were strong
reasons for believing that the milk used in the manufacture of the
product was polluted.[111] Hankin[112] details a case of an Indian
confection made largely from milk that caused a typhoid outbreak in a
British regiment.
Although the evidence that milk may not infrequently serve as an agent
in spreading disease is conclusive enough to satisfactorily prove the
proposition, yet it should be borne in mind that the organism of any
specific disease in question has rarely ever been found. The reasons for
this are quite the same as those that govern the situation in the case
of polluted waters, except that the difficulties of the problem are much
greater in the case of milk than with water. The inability to readily
separate the typhoid germ, for instance, from the colon bacillus, an
organism frequently found in milk, presents technical difficulties not
easily overcome. The most potent reason of failure to find disease
bacteria is the fact that infection in any case must occur sometime
previous to the appearance of the outbreak. Not only is there the usual
period of incubation, but it rarely happens that an outbreak is
investigated until a number of cases have occurred. In this interim the
original cause of infection may have ceased to be operative.
~Typhoid fever.~ With reference to the diseases likely to to be
disseminated through the medium of milk, infected after being drawn from
the animal, typhoid fever is the most important. The reason for this is
due (1) to the wide spread distribution of the disease; (2) to the fact
that the typhoid bacillus is one that is capable of withstanding
considerable amounts of acid, and consequently finds even in raw milk
containing the normal lactic acid bacteria conditions favorable for its
growth.[113] Ability to grow under these conditions can be shown not
only experimentally, but there is abundant clinical evidence that even a
slight infection often causes extensive outbreaks, as in the Stamford,
Conn., outbreak in 1895 where 386 cases developed in a few weeks, 97 per
cent. of which occurred on the route of one milk-man. In this case the
milk cans were thoroughly and properly cleaned, but were rinsed out with
_cold_ water from a shallow well that was found to be polluted.
The most common mode of pollution of milk with typhoid organisms is
where the milk utensils are infected in one way or another.[114] Second
in importance is the carrying of infection by persons serving in the
dual capacity of nurse and dairy attendant.
~Cholera.~ This germ does not find milk so favorable a nutrient medium as
the typhoid organism, because it is much more sensitive toward the
action of acids. Kitasato[115] found, however, that it could live in
raw milk from one to four days, depending upon the amount of acid
present. In boiled or sterilized milk it grows more freely, as the
acid-producing forms are thereby eliminated. In butter it dies out in a
few days (4 to 5).
On account of the above relation not a large number of cholera outbreaks
have been traced to milk, but Simpson[116] records a very striking case
in India where a number of sailors, upon reaching port, secured a
quantity of milk. Of the crew which consumed this, every one was taken
ill, and four out of ten died, while those who did not partake escaped
without any disease. It was later shown that the milk was adulterated
with water taken from an open pool in a cholera infected district.
~Diphtheria.~ Milk occasionally, though not often, serves as a medium for
the dissemination of diphtheria. Swithinbank and Newman[117] cites four
cases in which the causal organism has been isolated from milk. It has
been observed that growth occurs more rapidly in raw than in sterilized
milk.[118]
Infection in this disease is more frequently attributable to direct
infection from patient on account of the long persistence of this germ
in the throat, or indirectly through the medium of an attendant.
~Scarlet fever.~ Although it is more difficult to study the relation of
this disease to contaminated milk supplies, because the causal germ of
scarlet fever is not yet known, yet the origin of a considerable number
of epidemics has been traced to polluted milk supplies. Milk doubtless
is infected most frequently from persons in the earlier stages of the
disease when the infectivity of the disease is greater.
~Diarrhoeal diseases.~ Milk not infrequently acquires the property of
producing diseases of the digestive tract by reason of the development
of various bacteria that form more or less poisonous by-products. These
troubles occur most frequently during the summer months, especially with
infants and children, as in cholera infantum and summer complaint. The
higher mortality of bottle-fed infants[119] in comparison with those
that are nursed directly is explicable on the theory that cows' milk is
the carrier of the infection, because in many cases it is not consumed
until there has been ample time for the development of organisms in it.
Where milk is pasteurized or boiled it is found that the mortality among
children is greatly reduced. As a cause of sickness and death these
diseases exceed in importance all other specific diseases previously
referred to. These troubles have generally been explained as produced by
bacteria of the putrefactive class which find their way into the milk
through the introduction of filth and dirt at time of milking.[120]
Flügge[121] has demonstrated that certain peptonizing species possess
toxic properties for animals. Recent experimental inquiry[122] has
demonstrated that the dysentery bacillus (Shiga) probably bears a causal
relation to some of these summer complaints.
~Ptomaine poisoning.~ Many cases of poisoning from food products are also
reported with adults. These are due to the formation of various toxic
products, generally ptomaines, that are produced as a result of
infection of foods by different bacteria. One of these substances,
_tyrotoxicon_, was isolated by Vaughan[123] from cheese and various
other products of milk, and found to possess the property of producing
symptoms of poisoning similar to those that are noted in such cases. He
attributes the production of this toxic effect to the decomposition of
the elements in the milk induced by putrefactive forms of bacteria that
develop where milk is improperly kept.[124] Often outbreaks of this
character[125] assume the proportions of an epidemic, where a large
number of persons use the tainted food.
FOOTNOTES:
[78] Hart, Trans. Int. Med. Cong., London, 1881, 4:491-544.
[79] Freeman, Med. Rec., March 28, 1896.
[80] Busey and Kober, Rept. Health Off. of Dist. of Col., Washington, D.
C., 1895, p. 299. These authors present in this report an elaborate
article on morbific and infectious milk, giving a very complete
bibliography of 180 numbers. They append to Hart's list (which is
published in full) additional outbreaks which have occurred since,
together with full data as to extent of epidemic, circumstances
governing the outbreak, as well as name of original reporter and
reference.
[81] Smith, Theo., Journ. of Expt. Med., 1898, 3:451.
[82] Dinwiddie, Bull. 57, Ark. Expt. Stat., June, 1899; Ravenel, Univ.
of Penn. Med. Bull., Sept. 1901.
[83] Ravenel, Journ. of Comp. Med. & Vet. Arch., Dec. 1897; Hartzell,
Journ. Amer. Med. Ass'n, April 16, 1898.
[84] Stille, Brit. Med. Journ., Aug. 19, 1899.
[85] This test is made by injecting into the animal a small quantity of
tuberculin, which is a sterilized glycerin extract of cultures of the
tubercle bacillus. In a tuberculous animal, even in the very earliest
phases of the disease, tuberculin causes a temporary fever that lasts
for a few hours. By taking the temperature a number of times before and
after injection it is possible to readily recognize any febrile
condition. A positive diagnosis is made where the temperature after
inoculation is at least 2.0° F. above the average normal, and where the
reaction fever is continued for a period of some hours.
[86] Martin, Brit. Med. Journ. 1895, 1:937; Nocard, Les Tuberculoses
animales, 1895.
[87] C. O. Jensen, Milch Kunde und Milch hygiene, p. 69.
[88] Ostertag, Milch Zeit., 22:672.
[89] Obermüller, Hyg. Rund., 1897, p. 712; Petri, Arb. a. d. Kais. Ges.
Amte, 1898, 14: 1; Hormann und Morgenroth, Hyg. Rund., 1898, p. 217.
[90] Rabinowitsch, Zeit. f. Hyg., 1897, 26: 90.
[91] Th. Smith. Journ. of Expt. Med., 1899, 4:217.
[92] Russell and Hastings, 18 Rept. Wis. Expt. Stat., 1901.
[93] Hesse, Zeit. f. Hyg., 1900, 34:346.
[94] Practically all of the finest butter made in Denmark is made from
cream that has been pasteurized at temperatures varying from 160°-185°
F.
[95] Gebhardt, Virch. Arch., 1890, 119:12.
[96] Scheurlen, Arb. a. d. k. Ges. Amte, 1891, 7:269; Bang, Milch Zeit.,
1893, p. 672.
[97] Moore, Year Book of U. S. Dept. Agr., 1895, p. 432.
[98] Weigel and Noack, Jahres. d. Ges. Med., 1890, p. 642; Weissenberg,
Allg. med. Cent. Zeit., 1890, p. 1; Baum, Arch. f. Thierheilkunde, 1892,
18:16.
[99] Schneider, Münch, med. Wochenschr., 1893, No. 27; Fröhner, Zeit f.
Fleisch u. Milchhygiene, 1891, p. 55.
[100] Feser, Deutsche Zeit. f. Thiermed., 1880, 6:166.
[101] Nocard, Bull. Gén., 1885, p. 54.
[102] Deutsche Viertelsjahr. f. offentl. Gesundheitspflege, 1890,
20:444.
[103] Zeit. f. Fleisch und Milch hygiene, 11:114.
[104] E. Roth, Deutsche Vierteljahresschr. f. offentl. Gesundheitspfl.,
1890, 22:238
[105] S. W. North, London Practitioner, 1889, 43:393.
[106] Sedgwick and Chapin, Boston Med. & Surg. Journ., 1893, 129:485.
[107] Dabney, Phila. Med. News, 1893, 63:630.
[108] Welphy, London Lancet, 1894, 2:1085.
[109] Brit. Med. Journ., 1894, 1:815.
[110] Mass. Bd. Health Rept., 1894, p. 765.
[111] Turner, London Practitioner, 1892, 49:141; Munro, Brit. Med.
Journ., 1894, 2:829.
[112] Hankin, Brit. Med. Journ., 1894, 2:613.
[113] Heim (Arb. a. d. Kais. Gesundheitsamte, 1889, 5:303) finds it
capable of living from 20-30 days in milk.
[114] Schüder (Zeit. f. Hyg., 1902, 38:34) examined the statistics of
638 typhoid epidemics. He found 71 per cent. due to infected drinking
water, 17 per cent. to infected milk, and 3.5 per cent. caused by other
forms of food.
[115] Kitasato. Arb. a. d. Kais. Gesundheitsamte, 1:470.
[116] Simpson, London Practitioner, 1887, 39:144.
[117] Swithinbank and Newman, Bacteriology of Milk, p. 341.
[118] Schottelius and Ellerhorst. Milch Zeit., 1897, pp. 40 and 73.
[119] Baginsky, Hyg. Rund., 1895, p. 176.
[120] Gaffky, Deutsch. med. Wochen., 18:14.
[121] Flügge. Zeit., f. Hyg., 17:272, 1894.
[122] Duval and Bassett, Studies from the Rockefeller Inst. for Med.
Research, 2:7, 1904.
[123] Zeit. f. physiol. Chemie, 10:146; 9 Intern. Hyg. Cong. (London),
1891, p. 118.
[124] Vaughan and Perkins, Arch. f. Hyg., 27:308.
[125] Newton and Wallace (Phila. Med. News, 1887, 50:570) report three
outbreaks at Long Branch, N. J., two of which occurred in summer hotels.
CHAPTER VI.
BACTERIA AND MILK SUPPLIES WITH ESPECIAL REFERENCE TO METHODS OF
PRESERVATION.
To the milk dealer or distributor, bacteria are more or less of a
detriment. None of the organisms that find their way into milk, nor the
by-products formed by their growth, improve the quality of milk
supplies. It is therefore especially desirable from the milk-dealer's
point of view that these changes should be held in abeyance as much as
possible. Then too, the possibility that milk may serve as a medium for
the dissemination of disease-breeding bacteria makes it advisable to
protect this food supply from all possible infection from suspicious
sources.
In considering, therefore, the relation of bacteria to general milk
supplies, the _economic_ and the _hygienic_ standpoints must be taken
into consideration. Ordinarily much more emphasis is laid upon the first
requirement. If the supply presents no abnormal feature as to taste,
odor and appearance, unfortunately but little attention is paid to the
possibility of infection by disease germs. The methods of control which
are applicable to general milk supplies are based on the following
foundations: (1) the exclusion of all bacterial life, as far as
practicable, at the time the milk is drawn, and the subsequent storage
of the same at temperatures unfavorable for the growth of the organisms
that do gain access; (2) the removal of the bacteria, wholly or in part,
after they have once gained access.
Until within comparatively recent years, practically no attention was
given to the character of milk supplies, except possibly as to the
percentage of butter fat, and sometimes the milk solids which it
contained. So long as the product could be placed in the hands of the
consumer in such shape as not to be rejected by him as unfit for food,
no further attention was likely to be given to its character. At
present, however, much more emphasis is being given to the quality of
milk, especially as to its germ content; and the milk dealer is
beginning to recognize the necessity of a greater degree of control.
This control must not merely concern the handling of the product after
it reaches him, but should go back to the milk producer on the farm.
Here especially, it is necessary to inculcate those methods of
cleanliness which will prevent in large measure the wholesale infection
that ordinarily occurs.
The two watch words which are of the utmost importance to the milk
dealer are _cleanliness_ and _cold_. If the milk is properly drawn from
the animal in a clean manner and is immediately and thoroughly chilled,
the dealer has little to fear as to his product. Whenever serious
difficulties do arise, attributable to bacterial changes, it is because
negligence has been permitted in one or both directions. The influence
of cleanliness in diminishing the bacterial life in milk and that of low
temperatures in repressing the growth of those forms which inevitably
gain access has been fully dealt with in preceding chapters. It is of
course not practicable to take all of these precautions to which
reference has been made in the securing of large supplies of market milk
for city use, but great improvement over existing conditions could be
secured if the public would demand a better supervision of this
important food article. Boards of health in our larger cities are
awakening to the importance of this question and are becoming
increasingly active in the matter of better regulations and the
enforcement of the same.
New York City Board of Health has taken an advanced position in
requiring that all milk sold in the city shall be chilled down to 45° F.
immediately after milking and shall be transported to the city in
refrigerator cars.
Reference has already been made to the application of the acid test
(page 52) in the inspection of city milk supplies, and it is the opinion
of the writer that the curd test (see page 76) could also be used with
advantage in determining the sanitary character of milk. This test
reveals the presence of bacteria usually associated with dirt and
permits of the recognition of milks that have been carelessly handled.
From personal knowledge of examinations made of the milk supplies in a
number of Wisconsin cities it appears that this test could be utilized
with evident advantage.
~"Sanitary" or "certified" milk supplies.~ In a number of the larger
cities, the attempt has been made to improve the quality of the milk
supplies by the installation of dairies in which is produced an
especially high grade of milk. Frequently the inspection of the dairy as
well as the examination of the milk at stated intervals is under the
control of milk commissions or medical societies and as it is customary
to distribute the certificate of the examining board with the product,
such milks are frequently known as "certified." In such dairies the
tuberculin test is used at regular intervals, and the herd inspected
frequently by competent veterinarians. The methods of control
inaugurated as to clean milking and subsequent handling are such as to
insure the diminution of the bacteria to the lowest possible point. The
bacterial limit set by the Pediatric Society of Philadelphia is 10,000
organisms per cc. Often it is possible to improve very materially on
this standard and not infrequently is the supply produced where it
contains only a few thousand organisms per cc. Where such a degree of
care is exercised, naturally a considerably higher price must be paid
for the product,[126] and it should be remembered that the development
of such a system is only possible in relatively large centers where the
dealer can cater to a selected high-class trade. Moreover, it should
also be borne in mind that such a method of control is only feasible in
dairies that are under individual control. The impossibility of
exercising adequate control with reference to the milking process and
the care which should be given the milk immediately thereafter, when the
same is produced on different farms under various auspices is evident.
PRESERVATION OF MILK SUPPLIES.
While much can be done to improve the quality of milk supplies by
excluding a large proportion of the bacteria which normally gain access
to the milk, and preventing the rapid growth of those that do find their
way therein, yet for general municipal purposes, any practical method of
preservation[127] that is applicable on a commercial scale must rest
largely upon the destruction of bacteria that are present in the milk.
The two possible methods by which bacteria can be destroyed after they
have once gained access is (1) by the use of chemical preservatives; (2)
by the aid of physical methods.
~Chemical preservatives.~ Numerous attempts have been made to find some
chemical substance that could be added to milk which would preserve it
without interfering with its nutritive properties, but as a general rule
a substance that is toxic enough to destroy or inhibit the growth of
bacterial life exerts a prejudicial effect on the tissues of the body.
The use of chemicals, such as carbolic acid, mercury salts and mineral
acids, that are able to entirely destroy all life, is of course
excluded, except when milk is preserved for analytical purposes; but a
number of milder substances are more or less extensively employed,
although the statutes of practically all states forbid their use.
The substances so used may be grouped in two classes:
1. Those that unite chemically with certain by-products of bacterial
growth to form inert substances. Thus bicarbonate of soda neutralizes
the acid in souring milk, although it does not destroy the lactic acid
bacteria.
2. Those that act directly upon the bacteria in milk, restraining or
inhibiting their development. The substances most frequently utilized
are salicylic acid, formaldehyde and boracic acid. These are nearly
always sold to the milk handler, under some proprietary name, at prices
greatly in excess of what the crude chemicals could be bought for in the
open market. Formaldehyde has been widely advertised of late, but its
use is fraught with the greatest danger, for it practically renders
insoluble all albuminous matter and its toxic effect is greatly
increased in larger doses.
These substances are generally used by milk handlers who know nothing of
their poisonous action, and although it may be possible for adults to
withstand their use in dilute form, without serious results, yet their
addition to general milk supplies that may be used by children is
little short of criminal. The sale of these preparations for use in milk
finds its only outlet with those dairymen who are anxious to escape the
exactions that must be met by all who attempt to handle milk in the best
possible manner. Farrington has suggested a simple means for the
detection of preservalin (boracic acid).[128] When this substance is
added to fresh milk, it increases the acidity of milk without affecting
its taste. As normal milk tastes sour when it contains about 0.3 per
cent lactic acid, a milk that tests as much or more than this without
tasting sour has been probably treated with this antiseptic agent.
~Physical methods of preservation.~ Methods based upon the application of
physical forces are less likely to injure the nutritive value of milk,
and are consequently more effective, if of any value whatever. A number
of methods have been tried more or less thoroughly in an experimental
way that have not yet been reduced to a practical basis, as electricity,
use of a vacuum, and increased pressure.[129] Condensation has long been
used with great success, but in this process the nature of the milk is
materially changed. The keeping quality in condensed milk often depends
upon the action of another principle, viz., the inhibition of bacterial
growth by reason of the concentration of the medium. This condition is
reached either by adding sugar and so increasing the soluble solids, or
by driving off the water by evaporation, preferably in a vacuum pan.
Temperature changes are, however, of the most value in preserving milk,
for by a variation in temperature all bacterial growth can be brought to
a standstill, and under proper conditions thoroughly destroyed.
~Use of low temperatures.~ The effect of chilling or rapid cooling on the
keeping quality of milk is well known. When the temperature of milk is
lowered to the neighborhood of 45° F., the development of bacterial life
is so slow as to materially increase the period that milk remains sweet.
Within recent years, attempts have been made to preserve milk so that it
could be shipped long distances by freezing the product, which in the
form of milk-ice could be held for an indefinite period without
change.[130] A modification of this process known as Casse's system has
been in use more or less extensively in Copenhagen and in several places
in Germany. This consists of adding a small block of milk-ice (frozen
milk) to large cans of milk (one part to about fifty of milk) which may
or may not be pasteurized.[131] This reduces the temperature so that the
milk remains sweet considerably longer. Such a process might permit of
the shipment of milk for long distances with safety but as a matter of
fact, the system has not met with especial favor.
[Illustration: FIG. 22. Microscopic appearance of normal milk showing
the fat-globules aggregated in clusters.]
~Use of high temperatures.~ Heat has long been used as a preserving agent.
Milk has been scalded or cooked to keep it from time immemorial. Heat
may be used at different temperatures, and when so applied exerts a
varying effect, depending upon temperature employed. All methods of
preservation by heat rest, however, upon the application of the heat
under the following conditions:
1. A temperature above the maximum growing-point (105°-115° F.) and
below the thermal death-point (130°-140° F.) will prevent further
growth, and consequently fermentative action.
2. A temperature above the thermal death-point destroys bacteria, and
thereby stops all changes. This temperature varies, however, with the
condition of the bacteria, and for spores is much higher than for
vegetative forms.
Attempts have been made to employ the first principle in shipping milk
by rail, viz., prolonged heating above growing temperature, but when
milk is so heated, its physical appearance is changed.[132] The methods
of heating most satisfactorily used are known as sterilization and
pasteurization, in which a degree of temperature is used approximating
the boiling and scalding points respectively.
[Illustration: FIG. 23. Microscopic appearance of milk heated above 140°
F., showing the homogeneous distribution of fat-globules. The physical
change noted in comparison with Fig. 22 causes the diminished
consistency of pasteurized cream.]
~Effect of heat on milk.~ When milk is subjected to the action of heat, a
number of changes in its physical and chemical properties are to be
noted.
_1. Diminished "body."_ When milk, but more especially cream, is heated
to 140° F. or above, it becomes thinner in consistency or "body," a
condition which is due to a change in the grouping of the fat globules.
In normal milk, the butter fat for the most part is massed in
microscopic clots as (Fig. 22). When exposed to 140° F. or above for ten
minutes these fat-globule clots break down, and the globules become
homogeneously distributed (Fig. 23). A _momentary_ exposure to heat as
high as 158°-160° may be made without serious effect on the cream lime;
but above this the cream rises so poorly and slowly that it gives the
impression of thinner milk.
_2. Cooked Taste._ If milk is heated for some minutes to 160° F., it
acquires a cooked taste that becomes more pronounced as the temperature
is further raised. Milk so heated develops on its surface a pellicle or
"skin." The cause of this change in taste is not well known. Usually it
has been explained as being produced by changes in the nitrogenous
elements in the milk, particularly in the albumen. Thoerner[133] has
pointed out the coincidence that exists between the appearance of a
cooked taste and the loss of certain gases that are expelled by heating.
He finds that the milk heated in closed vessels from which the gas
cannot escape has a much less pronounced cooked flavor than if heated in
an open vessel. The so-called "skin" on the surface of heated milk is
not formed when the milk is heated in a tightly-closed receptacle. By
some[134] it is asserted that this layer is composed of albumen, but
there is evidence to show that it is modified casein due to the rapid
evaporation of the milk serum at the surface of the milk.
_3. Digestibility._ Considerable difference of opinion has existed in
the minds of medical men as to the relative digestibility of raw and
heated milks. A considerable amount of experimental work has been done
by making artificial digestion experiments with enzyms, also digestion
experiments with animals, and in a few cases with children. The results
obtained by different investigators are quite contradictory, although
the preponderance of evidence seems to be in favor of the view that
heating does impair the digestibility of milk, especially if the
temperature attains the sterilizing point.[135] It has been observed
that there is a noteworthy increase in amount of rickets,[136] scurvy
and marasmus in children where highly-heated milks are employed. These
objections do not obtain with reference to milk heated to moderate
temperatures, as in pasteurization, although even this lower temperature
lessens slightly its digestibility. The successful use of pasteurized
milks in children's hospitals is evidence of its usefulness.
_4. Fermentative changes._ The normal souring change in milk is due to
the predominance of the lactic acid bacteria, but as these organisms as
a class do not possess spores, they are readily killed when heated above
the thermal death-point of the developing cell. The destruction of the
lactic forms leaves the spore-bearing types possessors of the field, and
consequently the fermentative changes in heated milk are not those that
usually occur, but are characterized by the curdling of the milk from
the action of rennet enzyms.
_5. Action of rennet._ Heating milk causes the soluble lime salts to be
precipitated, and as the curdling of milk by rennet (in cheese-making)
is dependent upon the presence of these salts, their absence in heated
milks greatly retards the action of rennet. This renders it difficult to
utilize heated milks in cheese-making unless the soluble lime salts are
restored, which can be done by adding solutions of calcium chlorid.
~Sterilization.~ As ordinarily used in dairying, sterilization means the
application of heat at temperatures approximating, if not exceeding,
212° F. It does not necessarily imply that milk so treated is sterile,
i. e., germ-free; for, on account of the resistance of spores, it is
practically impossible to destroy entirely _all_ these hardy forms. If
milk is heated at temperatures above the boiling point, as is done where
steam pressure is utilized, it can be rendered practically germ-free.
Such methods are employed where it is designed to keep milk sweet for a
long period of time. The treatment of milk by sterilization has not met
with any general favor in this country, although it has been more widely
introduced abroad. In most cases the process is carried out after the
milk is bottled; and considerable ingenuity has been exercised in the
construction of devices which will permit of the closure of the bottles
after the sterilizing process has been completed. Milks heated to so
high a temperature have a more or less pronounced boiled or cooked
taste, a condition that does not meet with general favor in this
country. The apparatus suitable for this purpose must, of necessity, be
so constructed as to withstand steam pressure, and consequently is
considerably more expensive than that required for the simpler
pasteurizing process.
~Pasteurization.~ In this method the degree of heat used ranges from 140°
to 185° F. and the application is made for only a limited length of
time. The process was first extensively used by Pasteur (from whom it
derives its name) in combating various maladies of beer and wine. Its
importance as a means of increasing the keeping quality of milk was not
generally recognized until a few years ago; but the method is now
growing rapidly in favor as a means of preserving milk for commercial
purposes. The method does not destroy all germ-life in milk; it affects
only those organisms that are in a growing, vegetative condition; but if
the milk is quickly cooled, it enhances the keeping quality very
materially. It is unfortunate that this same term is used in connection
with the heating of cream as a preparatory step to the use of pure
cultures in cream-ripening in butter-making. The objects to be
accomplished vary materially and the details of the two processes are
also quite different.
While pasteurizing can be performed on a small scale by the individual,
the process can also be adapted to the commercial treatment of large
quantities of milk. The apparatus necessary for this purpose is not
nearly so expensive as that used in sterilizing, a factor of importance
when other advantages are considered. In this country pasteurization has
made considerable headway, not only in supplying a milk that is designed
to serve as children's food, but even for general purposes.
~Requirements essential in pasteurization.~ While considerable latitude
with reference to pasteurizing limits is permitted, yet there are
certain conditions which should be observed, and these, in a sense, fix
the limits that should be employed. These may be designated as (1) the
_physical_, and (2) the _biological_ requirements.
~Physical requirements.~ _1. Avoidance of scalded or cooked taste._ The
English and American people are so averse to a scalded or cooked flavor
in milk that it is practically impossible for a highly heated product to
be sold in competition with ordinary raw milk. In pasteurization then,
care must be taken not to exceed the temperature at which a permanently
cooked flavor is developed. As previously observed, this point varies
with the period of exposure. A momentary exposure to a temperature of
about 170° F. may be made without any material alteration, but if the
heat is maintained for a few minutes (ten minutes or over), a
temperature of 158° to 160° F. is about the maximum that can be employed
with safety.
_2. Normal creaming of the milk._ It is especially desirable that a
sharp and definite cream line be evident on the milk soon after
pasteurization. If this fails to appear, the natural inference of the
consumer is that the milk is skimmed. If the milk be heated to a
temperature sufficiently high to cause the fat-globule clusters to
disintegrate (see Figs. 22 and 23), the globules do not rise to the
surface as readily as before and the cream line remains indistinct.
Where the exposure is made for a considerable period of time (10 minutes
or more), the maximum temperature which can be used without producing
this change is about 140° F.; if the exposure is made for a very brief
time, a minute or less, the milk may be heated to 158°-160 F.° without
injuring the creaming property.
_3. No diminution in cream "body."_ Coincident with this change which
takes place in the creaming of the milk is the change in body or
consistency which is noted where cream is pasteurized at too high a
temperature. For the same reason as given under (2) cream heated above
these temperatures is reduced in apparent thickness and appears to
contain less butter-fat. Of course the pasteurizing process does not
change the fat content, but its "body" is apparently so affected. Thus a
25 per cent. cream may seem to be no thicker or heavier than an 18 per
cent. raw cream. This real reduction in consistency naturally affects
the readiness with which the cream can be whipped.
~Biological requirements.~ _1. Enhanced keeping quality._ In commercial
practice the essential biological requirement is expressed in the
enhanced keeping quality of the pasteurized milk. This expresses in a
practical way the reduction in germ life accomplished by the
pasteurizing process. The improvement in keeping quality depends upon
the temperature and time of exposure, but fully as much also on the way
in which the pasteurized product is handled after heating. The lowest
temperature which can be used with success to kill the active,
vegetative bacteria is about 140° F., at which point it requires about
ten minutes exposure. If this period is curtailed the temperature must
be raised accordingly. An exposure to a temperature of 175° F. for a
minute has approximately the same effect as the lower degree of heat for
the longer time.
The following bacteriological studies as to the effect which a variation
in temperature exerts on bacterial life in milk are of importance as
indicating the foundation for the selection of the proper limits. In the
following table the exposures were made for a uniform period (20
minutes):
_The bacterial content of milk heated at different temperatures._
Number of bacteria per cc. in milk.
45° C. 50° C. 55° C. 60° C. 65° C. 70° C.
Unheated 113° F. 122° F. 131° F. 140° F. 149° F. 158° F.
Series I. 2,895,000 ---- 1,260,000 798,000 32,000 5,770 3,900
Series II. 750,000 665,000 262,400 201,000 950 700 705
Series III. 1,350,000 1,100,000 260,000 215,000 575 610 650
Series IV. 1,750,000 ---- 87,360 ---- 4,000 3,500 3,600
It appears from these results that the most marked decrease in
temperature occurs at 140° F. (60° C.). It should also be observed that
an increase in heat above this temperature did not materially diminish
the number of organisms present, indicating that those forms remaining
were in a spore or resistant condition. It was noted, however, that the
developing colonies grew more slowly in the plates made from the highly
heated milk, showing that their vitality was injured to a greater extent
even though not killed.
_2. Destruction of disease bacteria._ While milk should be pasteurized
so as to destroy all active, multiplying bacteria, it is particularly
important to destroy any organisms of a disease nature that might find
their way into the same. Fortunately most of the bacteria capable of
thriving in milk before or after it is drawn from the animal are not
able to form spores and hence succumb to proper pasteurization. Such is
the case with the diphtheria, cholera and typhoid organisms.
The organism that is invested with most interest in this connection is
the tubercle bacillus. On account of its more or less frequent
occurrence in milk and its reputed high powers of resistance, it may
well be taken as a standard in pasteurizing.
~Thermal death limits of tubercle bacillus.~ Concerning the exact
temperature at which this germ is destroyed there is considerable
difference of opinion. Part of this arises from the inherent difficulty
in determining exactly when the organism is killed (due to its failure
to grow readily on artificial media), and part from the lack of uniform
conditions of exposure. The standards that previously have been most
generally accepted are those of De Man,[137] who found that thirty
minutes exposure at 149° F., fifteen minutes at 155° F., or ten minutes
at 167° F., sufficed to destroy this germ.
More recently it has been demonstrated,[138] and these results
confirmed,[139] that if tuberculous milk is heated in closed receptacles
where the surface pellicle does not form, the vitality of this disease
germ is destroyed at 140° F. in 10-15 minutes, while an exposure at 160°
F. requires only about one minute.[140] If the conditions of heating are
such that the surface of the milk is exposed to the air, the resistance
of bacteria is greatly increased. When heated in open vessels Smith
found that the tubercle organism was not killed in some cases where the
exposure was made for at least an hour. Russell and Hastings[141] have
shown an instance where the thermal death-point of a micrococcus
isolated from pasteurized milk was increased 12.5° F., by heating it
under conditions that permitted of the formation of the scalded layer.
It is therefore apparent that apparatus used for pasteurization should
be constructed so as to avoid this defect.
~Methods of treatment.~ Two different systems of pasteurization have grown
up in the treatment of milk. One of these has been developed from the
hygienic or sanitary aspect of the problem and is used more particularly
in the treatment of cream and relatively small milk supplies. The other
system has been developed primarily from the commercial point of view
where a large amount of milk must be treated in the minimum time. In the
first method the milk is heated for a longer period of time, about
fifteen minutes at a relatively low temperature from 140°-155° F.; in
the other, the milk is exposed to the source of heat only while it is
passing rapidly through the apparatus. Naturally, the exposure under
such conditions must be made at a considerably higher temperature,
usually in the neighborhood of 160° F.
The types of apparatus used in these respective processes naturally
varies. Where the heating is prolonged, the apparatus employed is built
on the principle of a _tank_ or _reservoir_ in which a given volume of
milk may be held at any given temperature for any given period of time.
When the heat is applied for a much shorter period of time, the milk is
passed in a continuous stream through the machine. Naturally the
capacity of a continuous-flow apparatus is much greater than a machine
that operates on the intermittent principle; hence, for large supplies,
as in city distribution, this system has a great advantage. The question
as to relative efficiency is however one which should be given most
careful consideration.
~Pasteurizing apparatus.~ The problems to be solved in the pasteurization
of milk and cream designed for direct consumption are so materially
different from where the process is used in butter-making that the type
of machinery for each purpose is quite different. The equipment
necessary for the first purpose may be divided into two general classes:
1. Apparatus of limited capacity designed for family use.
2. Apparatus of sufficient capacity to pasteurize on a commercial scale.
~Domestic pasteurizers.~ In pasteurizing milk for individual use, it is
not desirable to treat at one time more than will be consumed in one
day; hence an apparatus holding a few bottles will suffice. In this case
the treatment can best be performed in the bottle itself, thereby
lessening the danger of infection. Several different types of
pasteurizers are on the market; but special apparatus is by no means
necessary for the purpose. The process can be efficiently performed by
any one with the addition of an ordinary dairy thermometer to the common
utensils found in the kitchen. Fig. 24 indicates a simple contrivance
that can be readily arranged for this purpose.
The following suggestions indicate the different steps of the process:
1. Use only fresh milk.
2. Place milk in clean bottles or fruit cans, filling to a uniform
level, closing bottles tightly with a cork or cover. If pint and quart
cans are used at the same time, an inverted bowl will equalize the
level. Set these in a flat-bottomed tin pail and fill with warm water to
same level as milk. An inverted pie tin punched with holes will serve as
a stand on which to place the bottles during the heating process.
3. Heat water in pail until the temperature of same reaches 155° to 160°
F.; then remove from source of direct heat, cover with a cloth or tin
cover, and allow the whole to stand for half an hour. In the preparation
of milk for children, it is not advisable to use the low-temperature
treatment (140° F.) that is recommended for commercial city delivery.
[Illustration: FIG. 24. A home-made pasteurizer.]
4. Remove bottles of milk and cool them as rapidly as possible without
danger to bottles and store in a refrigerator.
~Commercial pasteurizers.~ The two methods of pasteurization practiced
commercially for the preservation of milk and cream have been developed
because of the two types of machinery now in use. Apparatus constructed
on the reservoir or tank principle permits of the retention of the milk
for any desired period of time. Therefore, a lower temperature can be
employed in the treatment. In those machines where the milk flows
through the heater in a more or less continuous stream, the period of
exposure is necessarily curtailed, thereby necessitating a higher
temperature.
~Reservoir pasteurizers.~ The simplest type of apparatus suitable for
pasteurizing on this principle is where the milk is placed in shotgun
cans and immersed in water heated by steam. Ordinary tanks surrounded
with water spaces can also be used successfully. The Boyd cream ripening
vat has also been tried. In this the milk is heated by a swinging coil
immersed in the vat through which hot water circulates.
In 1894 the writer[142] constructed a tank pasteurizer which consisted
of a long, narrow vat surrounded by a steam-heated water chamber. Both
the milk and the water chambers were provided with mechanical agitators
having a to-and-fro movement.
[Illustration: FIG. 25. Pott's pasteurizer.]
Another machine which has been quite generally introduced is the Potts'
rotating pasteurizer. This apparatus has a central milk chamber that is
surrounded with an outer shell containing hot water. The whole machine
revolves on a horizontal axis, and the cream or milk is thus thoroughly
agitated during the heating process.
~Continuous-flow pasteurizers.~ The demand for greater capacity than can
be secured in the reservoir machines has led to the perfection of
several kinds of apparatus where the milk is heated momentarily as it
flows through the apparatus. Most of these were primarily introduced for
the treatment of cream for butter-making purposes, but they are
frequently employed for the treatment of milk on a large scale in city
milk trade. Many of them are of European origin although of late years
several have been devised in this country.
The general principle of construction is much the same in most of them.
The milk is spread out in a thin sheet, and is treated by passing it
over a surface, heated either with steam directly or preferably with hot
water.
Where steam is used directly, it is impossible to prevent the "scalding
on" of the milk proteids to the heated surface.
In some of these machines (Thiel, Kuehne, Lawrence, De Laval, and
Hochmuth), a ribbed surface is employed over which the milk flows, while
the opposite surface is heated with hot water or steam. Monrad, Lefeldt
and Lentsch employ a centrifugal apparatus in which a thin layer of milk
is heated in a revolving drum.
In some types of apparatus, as in the Miller machine, an American
pasteurizer, the milk is forced in a thin sheet between two heated
surfaces, thereby facilitating the heating process. In the Farrington
machine heated discs rotate in a reservoir through which the milk flows
in a continuous stream.
One of the most economical types of apparatus is the regenerator type (a
German machine), in which the milk passes over the heating surface in a
thin stream and then is carried back over the incoming cold milk so
that the heated liquid is partially cooled by the inflowing fresh milk.
In machines of this class it requires very much less steam to heat up
the milk than in those in which the cold milk is heated wholly by the
hot water.
A number of machines have been constructed on the principle of a
reservoir which is fed by a constantly flowing stream. In some kinds of
apparatus of this type no attempt is made to prevent the mixing of the
recently introduced milk with that which has been partially heated. The
pattern for this reservoir type is Fjord's heater, in which the milk is
stirred by a stirrer. This apparatus was originally designed as a heater
for milk before separation, but it has since been materially modified so
that it is better adapted to the purposes of pasteurization. Reid was
the first to introduce this type of machine into America.
~Objections to continuous flow pasteurizers.~ In all continuous flow
pasteurizers certain defects are more or less evident. While they
fulfill the important requirement of large capacity, an absolute
essential where large volumes of milk are being handled, it does not of
necessity follow that they conform to all the hygienic and physical
requirements that should be kept in mind. The greatest difficulty is the
shortened period of exposure. The period which the milk is actually
heated is often not more than a minute or so. Another serious defect is
the inability to heat _all_ of the milk for a uniform period of time. At
best, the milk is exposed for an extremely short time, but even then
portions pass through the machine much more quickly than do the
remainder. Those portions in contact with the walls of the apparatus are
retarded by friction and are materially delayed in their passage, while
the particles in the center of the stream, however thin, flow through
in the least possible time.
The following simple method enables the factory operator to test the
period of exposure in the machine: Start the machine full of water, and
after the same has become heated to the proper temperature, change the
inflow to full-cream milk, continuing at the same rate. Note the exact
time of change and also when first evidence of milkiness begins to
appear at outflow. If samples are taken from first appearance of milky
condition and thereafter at different intervals for several minutes, it
is possible, by determining the amount of butter-fat in the same, to
calculate with exactness how long it takes for the milk to entirely
replace the water.
Tests made by the writer[143] on the Miller pasteurizer showed, when fed
at the rate of 1,700 pounds per hour, the minimum period of exposure to
be 15 seconds, and the maximum about 60-70 seconds, while about
two-thirds of the milk passed the machine in 40-50 seconds. This
manifest variation in the rate of flow of the milk through the machine
is undoubtedly the reason why the results of this type of treatment are
subject to so much variation. Naturally, even a fatal temperature to
bacterial life can be reduced to a point where actual destruction of
even vegetating cells does not occur.
~Bacterial efficiency of reservoir pasteurizers.~ The bacterial content of
pasteurized milk and cream will depend somewhat on the number of
organisms originally present in the same. Naturally, if mixed milk
brought to a creamery is pasteurized, the number of organisms remaining
after treatment would be greater than if the raw material was fresh and
produced on a single farm.
An examination of milk and cream pasteurized on a commercial scale in
the Russell vat at the Wisconsin Dairy school showed that over 99.8 per
cent of the bacterial life in raw milk or cream was destroyed by the
heat employed, i. e., 155° F. for twenty minutes duration.[144] In
nearly one-half of the samples of milk, the germ content in the
pasteurized sample fell below 1,000 bacteria per cc., and the average of
twenty-five samples contained 6,140 bacteria per cc. In cream the germ
content was higher, averaging about 25,000 bacteria per cc. This milk
was taken from the general creamery supply, which was high in organisms,
containing on an average 3,675,000 bacteria per cc. De Schweinitz[145]
has reported the germ content of a supply furnished in Washington which
was treated at 158° to 160° F. for fifteen minutes. This supply came
from a single source. Figures reported were from 48-hour-old agar
plates. Undoubtedly these would have been higher if a longer period of
incubation had been maintained. The average of 82 samples, taken for the
period of one year, showed 325 bacteria per cc.
[Illustration: FIG. 26. Effect of pasteurizing on germ content of milk.
Black square represents bacteria of raw milk; small white square, those
remaining after pasteurization.]
~Bacterial efficiency of continuous-flow pasteurizers.~ A quantitative
determination of the bacteria found in milk and cream when treated in
machinery of this class almost always shows a degree of variation in
results that is not to be noted in the discontinuous apparatus.
[Illustration: FIG. 27. Reid's Continuous Pasteurizer.]
Harding and Rogers[146] have tested the efficiency of one of the Danish
type of continuous pasteurizers. These experiments were made at 158°,
176° and 185° F. They found the efficiency of the machine not wholly
satisfactory at the lower temperatures. At 158° F. the average of
fourteen tests gave 15,300 bacteria per cc., with a maximum to minimum
range from 62,790 to 120. Twenty-five examinations at 176° F. showed an
average of only 117, with a range from 300 to 20. The results at 185° F.
showed practically the same results as noted at 176° F. Considerable
trouble was experienced with the "scalding on" of the milk to the walls
of the machine when milk of high acidity was used.
Jensen[147] details the results of 139 tests in 1899, made by the
Copenhagen Health Commission. In 66 samples from one hundred thousand to
one million organisms per cc. were found, and in 22 cases from one to
five millions. Nineteen tests showed less than 10,000 per cc.
In a series of tests conducted by the writer[148] on a Miller
pasteurizer in commercial operation, an average of 21 tests showed
12,350 bacteria remaining in the milk when the milk was pasteurized from
156°-164° F. The raw milk in these tests ran from 115,000 to about one
million organisms per cc.
A recently devised machine of this type (Pasteur) has been tested by
Lehmann, who found that it was necessary to heat the milk as high as
176° to 185° F., in order to secure satisfactory results on the
bacterial content of the cream.
The writer tested Reid's pasteurizer at 155° to 165° F. with the
following results: in some cases as many as 40 per cent. of the bacteria
survived, which number in some cases exceeded 2,000,000 bacteria per
cc.
~Pasteurizing details.~ While the pasteurizing process is exceedingly
simple, yet, in order to secure the best results, certain conditions
must be rigidly observed in the treatment before and after the heating
process.
It is important to select the best possible milk for pasteurizing, for
if the milk has not been milked under clean conditions, it is likely to
be rich in the spore-bearing bacteria. Old milk, or milk that has not
been kept at a low temperature, is much richer in germ-life than
perfectly fresh or thoroughly chilled milk.
The true standard for selecting milk for pasteurization should be to
determine the actual number of bacterial _spores_ that are able to
resist the heating process, but this method is impracticable under
commercial conditions.
The following method, while only approximate in its results, will be
found helpful: Assuming that the age or treatment of the milk bears a
certain relation to the presence of spores, and that the acid increases
in a general way with an increase in age or temperature, the amount of
acid present may be taken as an approximate index of the suitability of
the milk for pasteurizing purposes. Biological tests were carried out in
the author's laboratory[149] on milks having a high and low acid
content, and it was shown that the milk with the least acid was, as a
rule, the freest from spore-bearing bacteria.
This acid determination can be made at the weigh-can by employing the
Farrington alkaline tablet which is used in cream-ripening. Where milk
is pasteurized under general creamery conditions, none should be used
containing more than 0.2 per cent acidity. If only perfectly fresh milk
is used, the amount of acid will generally be about 0.15 per cent with
phenolphthalein as indicator.
[Illustration: FIG. 28. Diagram showing temperature changes in
pasteurizing, and the relation of same to bacterial growth.
Shaded zone represents limits of bacterial growth, 50°-109° F. (10°-43°
C.), the intensity of shading indicating rapidity of development. The
solid black line shows temperature of milk during the process. The
necessity for rapid cooling is evident as the milk falls in temperature
to that of growing zone.]
Emphasis has already been laid on the selection of a proper limit of
pasteurizing (p. 114). It should be kept constantly in mind that the
thermal death-point of any organism depends not alone on the temperature
used, but on the period of exposure. With the lower limits given, 140°
F., it is necessary to expose the milk for not less than fifteen
minutes. If a higher heat is employed (and the cooked flavor
disregarded) the period of exposure may be curtailed.
~Chilling the milk.~ It is very essential in pasteurizing that the heated
milk be immediately chilled in order to prevent the germination of the
resistant spores, for if germination once occurs, growth can go on at
relatively low temperatures.
The following experiments by Marshall[150] are of interest as showing
the influence of refrigeration on germination of spores:
Cultures of organisms that had been isolated from pasteurized milk were
inoculated into bouillon. One set was left to grow at room temperature,
another was pasteurized and allowed to stand at same temperature, while
another heated set was kept in a refrigerator. The unheated cultures at
room temperature showed evidence of growth in thirty trials in an
average of 26 hours; 29 heated cultures at room temperature all
developed in an average of 50 hours, while the heated cultures kept in
refrigerator showed no growth in 45 days with but four exceptions.
Practically all of the rapid-process machines are provided with
especially constructed cooling devices. In some of them, as in the
Miller and Farrington, the cooling is effected by passing the milk
through two separate coolers that are constructed in the same general
way as the heater. With the first cooler, cold running water is
employed, the temperature often being lowered in this way to 58° or 60°
F. Further lessening of the temperature is secured by an additional ice
water or brine cooler which brings the temperature down to 40°-50° F.
In the economical use of ice the ice itself should be applied as closely
as possibly to the milk to be cooled, for the larger part of the
chilling value of ice comes from the melting of the same. To convert a
pound of ice at 32° F. into a pound of water at the same temperature, if
we disregard radiation, would require as much heat as would suffice to
raise 142 pounds of water one degree F., or one pound of water 142° F.
The absorptive capacity of milk for heat (specific heat) is not quite
the same as it is with water, being .847 for milk in comparison with 1.0
for water.[151] Hot milk would therefore require somewhat less ice to
cool it than would be required by any equal volume of water at the same
temperature.
~Bottling the product.~ If the milk has been properly pasteurized, it
should, of course, be dispensed in sterilized bottles. Glass bottles
with plain pulp caps are best, and these should be thoroughly sterilized
in steam before using. The bottling can best be done in a commercial
bottling machine. Care must be taken to thoroughly clean this apparatus
after use each day. Rubber valves in these machines suffer deterioration
rapidly.
[Illustration: FIG. 29. Relative consistency of pasteurized cream before
(A) and after (B) treatment with viscogen as shown by rate of flow down
inclined glass plate.]
~Restoration of "body" of pasteurized cream.~ The action of heat causes
the tiny groupings of fat globules in normal milk (Fig. 22) to break up,
and with this change, which occurs in the neighborhood of 140° F., where
the milk is heated for about 15 minutes and at about 160-165° F. where
rapidly heated in a continuous stream, the consistency of the liquid is
diminished, notwithstanding the fact that the fat-content remains
unchanged. Babcock and the writer[152] devised the following "cure" for
this apparent defect. If a strong solution of cane sugar is added to
freshly slacked lime and the mixture allowed to stand, a clear fluid can
be decanted off. The addition of this alkaline liquid, which is called
"viscogen," to pasteurized cream in proportions of about one part of
sugar-lime solution to 100 to 150 of cream, restores the consistency of
the cream, as it causes the fat globules to cluster together in small
groups.
The relative viscosity of creams can easily be determined by the
following method (Fig. 29):
Take a perfectly clean piece of glass (plate or picture glass is
preferable, as it is less liable to be wavy). Drop on one edge two or
three drops of cream at intervals of an inch or so. Then incline piece
of glass at such an angle as to cause the cream to flow down surface of
glass. The cream, having the heavier body or viscosity, will move more
slowly. If several samples of each cream are taken, then the aggregate
lengths of the different cream paths may be taken, thereby eliminating
slight differences due to condition of glass.
FOOTNOTES:
[126] From 10 to 16 cents per quart is usually paid for such milks.
[127] Much improvement in quality could be made by more careful control
of milk during shipment, especially as to refrigeration; also as to the
care taken on the farms. The use of the ordinary milking machine (see
page 37), would go far to reduce the germ content of milk.
[128] Farrington, Journ. Amer. Chem. Soc., Sept., 1896.
[129] Hite, Bull. 58, West Va. Expt. Stat., 1899.
[130] Milch Zeit., 1895, No. 9.
[131] Ibid., 1897, No. 33.
[132] Bernstein, Milch Zeit., 1894, pp. 184, 200.
[133] Thoerner, Chem. Zeit., 18:845.
[134] Snyder, Chemistry of Dairying, p. 59.
[135] Doane and Price (Bull. 77, Md. Expt. Stat., Aug. 1901) give quite
a full resumé of the work on this subject in connection with rather
extensive experiments made by them on feeding animals with raw,
pasteurized and sterilized milks.
[136] Rickets is a disease in which the bones lack sufficient mineral
matter to give them proper firmness. Marasmus is a condition in which
the ingested food seems to fail to nourish the body and gradual wasting
away occurs.
[137] De Man, Arch. f. Hyg., 1893, 18:133.
[138] Th. Smith, Journ. of Expt. Med., 1899, 4:217.
[139] Russell and Hastings, 17 Rept. Wis. Expt. Stat., 1900, p. 147.
[140] Russell and Hastings, 21 Rept. Ibid., 1904.
[141] Russell and Hastings, 18 Rept. Ibid., 1901.
[142] Russell, Bull. 44, Wis. Expt. Stat.
[143] Russell, 22 Wis. Expt. Stat. Rept., 1905, p. 232.
[144] Russell, 12 Wis. Expt. Stat. Rept., 1895, p. 160.
[145] De Schweinitz, Nat. Med. Rev., 1899, No. 11.
[146] Harding and Rogers. Bull. 182, N. Y. (Geneva) Expt. Stat., Dec.,
1899.
[147] Jensen, Milchkunde und Milch Hygiene, p. 132.
[148] 22 Wis. Expt. Stat. Rept., 1905, p. 236.
[149] Shockley, Thesis, Univ. of Wis., 1896.
[150] Marshall, Mich. Expt. Stat., Bull. 147, p. 47.
[151] Fleischmann, Landw. Versuchts Stat., 17:251.
[152] Babcock and Russell, Bull. 54, Wis. Expt. Stat., Aug. 1896.
CHAPTER VII.
BACTERIA AND BUTTER-MAKING.
In making butter from the butter fat in milk, it is necessary to
concentrate the fat globules into cream, preliminary to the churning
process. The cream may be raised by the gravity process or separated
from the milk by centrifugal action. In either case the bacteria that
are normally present in the milk differentiate themselves in varying
numbers in the cream and the skim-milk. The cream always contains per
cc. a great many more than the skim-milk, the reason for this being that
the bacteria are caught and held in the masses of fat globules, which,
on account of their lighter specific gravity, move toward the surface of
the milk or toward the interior of the separator bowl. This filtering
action of the fat globules is similar to what happens in muddy water
upon standing. As the suspended particles fall to the bottom they carry
with them a large number of the organisms that are in the liquid.
~Various creaming methods.~ The creaming method has an important bearing
on the kind as well as the number of the bacteria that are to be found
in the cream. The difference in species is largely determined by the
difference in ripening temperature, while the varying number is governed
more by the age of the milk.
_1. Primitive gravity methods._ In the old shallow-pan process, the
temperature of the milk is relatively high, as the milk is allowed to
cool naturally. This comparatively high temperature favors especially
the development of those forms whose optimum growing-point is near the
air temperature. By this method the cream layer is exposed to the air
for a longer time than with any other, and consequently the
contamination from this source is greater. Usually cream obtained by the
shallow-pan process will contain a larger number of species and also
have a higher acid content.
_2. Modern gravity methods._ In the Cooley process, or any of the modern
gravity methods where cold water or ice is used to lower the
temperature, the conditions do not favor the growth of a large variety
of species. The number of bacteria in the cream will depend largely upon
the manner in which the milk is handled previous to setting. If care is
used in milking, and the milk is kept so as to exclude outside
contamination, the cream will be freer from bacteria than if
carelessness prevails in handling the milk. Only those forms will
develop in abundance that are able to grow at the low temperature at
which the milk is set. Cream raised by this method is less frequently
infected with undesirable forms than that which is creamed at a higher
temperature.
_3. Centrifugal method._ Separator cream should contain less germ-life
than that which is secured in the old way. It should contain only those
forms that have found their way into the milk during and subsequent to
the milking, for the cream is ordinarily separated so soon that there is
but little opportunity of infection, if care is taken in the handling.
As a consequence, the number of species found therein is smaller.
Where milk is separated, it is always prudent to cool the cream so as to
check growth, as the milk is generally heated before separating in order
to skim efficiently.
Although cream is numerically much richer in bacteria than milk, yet
the changes due to bacterial action are slower; hence milk sours more
rapidly than cream. For this same reason, cream will sour sooner when it
remains on the milk than it will if it is separated as soon as possible.
This fact indicates the necessity of early creaming, so as to increase
the keeping quality of the product, and is another argument in favor of
the separator process.
~Ripening of cream.~ If cream is allowed to remain at ordinary
temperatures, it undergoes a series of fermentation changes that are
exceedingly complex in character, the result of which is to produce in
butter made from the same the characteristic flavor and aroma that are
so well known in this article. We are so accustomed to the development
of these flavors in butter that they are not generally recognized as
being intimately associated with bacterial activity unless compared with
butter made from perfectly fresh cream. Sweet-cream butter lacks the
aromatic principle that is prominent in the ripened product, and while
the flavor is delicate, it is relatively unpronounced.
In the primitive method of butter-making, where the butter was made on
the farm, the ripening of cream became a necessity in order that
sufficient material might be accumulated to make a churning. The
ripening change occurred spontaneously without the exercise of any
especial control. With the development of the creamery system came the
necessity of exercising a control of this process, and therefore the
modern butter-maker must understand the principles which are involved in
this series of complex changes that largely give to his product its
commercial value.
In these ripening changes three different factors are to be taken into
consideration: the development of acid, flavor and aroma. Much confusion
in the past has arisen from a failure to discriminate between these
qualities. While all three are produced simultaneously in ordinary
ripening, it does not necessarily follow that they are produced by the
same cause. If the ripening changes are allowed to go too far,
undesirable rather than beneficial decomposition products are produced.
These greatly impair the value of butter, so that it becomes necessary
to know just to what extent this process should be carried.
In cream ripening there is a very marked bacterial growth, the extent of
which is determined mainly by the temperature of the cream. Conn and
Esten[153] find that the number of organisms may vary widely in
unripened cream, but that the germ content of the ripened product is
more uniform. When cream is ready for the churn, it often contains
500,000,000 organisms per cc., and frequently even a higher number. This
represents a germ content that has no parallel in any natural material.
The larger proportion of bacteria in cream as it is found in the
creamery belong to the acid-producing class, but in the process of
ripening, these forms seem to thrive still better, so that when it is
ready for churning the germ content of the cream is practically made up
of this type.
~Effect on churning.~ In fresh cream the fat globules which are suspended
in the milk serum are surrounded by a film of albuminous material which
prevents them from coalescing readily. During the ripening changes, this
enveloping substance is modified, probably by partial solution, so that
the globules cohere when agitated, as in churning. The result is that
ripened cream churns more easily, and as it is possible to cause a
larger number of the smaller fat-globules to cohere to the butter
granules, the yield is slightly larger--a point of considerable
economic importance where large quantities of butter are made.
~Development of acid.~ The result of this enormous bacterial
multiplication is that acid is produced in cream, lactic being the
principal acid so formed.
Other organic acids are undoubtedly formed as well as certain aromatic
products. While the production of acid as a result of fermentative
activity is usually accompanied with a development of flavor, the flavor
is not directly produced by the formation of acid. If cream is treated
in proper proportions with a commercial acid, as hydrochloric,[154] it
assumes the same churning properties as found in normally ripened cream,
but is devoid of the desired aromatic qualities. Lactic acid[155] has
also been used in a similar way but with no better results.
The amount of acidity that should be developed under natural conditions
so as to secure the optimum quality as to flavor and aroma is the most
important question in cream ripening. Concerning this there have been
two somewhat divergent views as to what is best in practice, some
holding that better results were obtained with cream ripened to a high
degree of acidity than where a less amount was developed.[156] The
present tendency seems to be to develop somewhat more than formerly, as
it is thought that this secures more of the "high, quick" flavor wanted
in the market. On the average, cream is ripened to about 0.5 to 0.65 per
cent. acidity, a higher percentage than this giving a strong-flavored
butter. In the determination of acidity, the most convenient method is
to employ the Farrington alkaline tablet, which permits of an accurate
and rapid estimation of the acidity in the ripening cream. The amount of
acidity to be produced must of necessity be governed by the amount of
butter-fat present, for the formation of acid is confined to the serum
of the cream; consequently, a rich cream would show less acid by
titration than a thinner cream, and still contain really as much acid as
the other. The importance of this factor is evident in gathered-cream
factories.
The rate of ripening is dependent upon the conditions that affect the
rate of growth of bacterial life, such as time and temperature, number
of organisms in cream and also the per cent of butter fat in the cream.
Some years ago it was customary to ripen cream at about 50° to 60° F.,
but more recently better results have been obtained, it is claimed,
where the ripening temperature is increased and the period of ripening
lessened. As high a temperature as 70° to 75° F. has been recommended.
It should be said that this variation in practice may have a valid
scientific foundation, for the temperature of the ripening cream is
undoubtedly the most potent factor in determining what kind of bacteria
will develop most luxuriantly. It is well known that those forms that
are capable of producing bitter flavors are able to thrive better at a
lower temperature than some of the desirable ripening species.
The importance of this factor would be lessened where a pure culture was
used in pasteurized cream, because here practically the selected
organism alone controls the field.
It is frequently asserted that better results are obtained by stirring
the cream and so exposing it to the air as much as possible. Experiments
made at the Ontario Agricultural College, however, show practically no
difference in the quality of the butter made by these two methods. The
great majority of the bacteria in the cream belong to the facultative
class, and are able to grow under conditions where they are not in
direct contact with the air.
~Flavor and aroma.~ The basis for the peculiar flavor or taste which
ripened cream-butter possesses is due, in large part, to the formation
of certain decomposition products formed by various bacteria. Aroma is a
quality often confounded with flavor, but this is produced by volatile
products only, which appeal to the sense of smell rather than taste.
Generally a good flavor is accompanied by a desirable aroma, but the
origin of the two qualities is not necessarily dependent on the same
organisms. The quality of flavor and aroma in butter is, of course, also
affected by other conditions, as, for instance, the presence or absence
of salt, as well as the inherent qualities of the milk, that are
controlled, to some extent at least, by the character of the feed which
is consumed by the animal. The exact source of these desirable but
evanescent qualities in butter is not yet satisfactorily determined.
According to Storch,[157] flavors are produced by the decomposition of
the milk sugar and the absorption of the volatile flavors by the butter
fat. Conn[158] holds that the nitrogenous elements in cream serve as
food for bacteria, and in the decomposition of which the desired
aromatic substance is produced. The change is unquestionably a complex
one, and cannot be explained as a single fermentation.
There is no longer much doubt but that both acid-forming and
casein-digesting species can take part in the production of proper
flavors as well as desirable aromas. The researches of Conn,[159] who
has studied this question most exhaustively, indicate that both of these
types of decomposition participate in the production of flavor and
aroma. He has shown that both flavor and aroma production are
independent of acid; that many good flavor-producing forms belong to
that class which renders milk alkaline, or do not change the reaction at
all. Some of these species liquefied gelatin and would therefore belong
to the casein-dissolving class. Those species that produced bad flavors
are also included in both fermentative types. Conn has found a number of
organisms that are favorable flavor-producers; in fact they were much
more numerous than desirable aroma-yielding species. None of the
favorable aroma forms according to his investigations were lactic-acid
species,--a view which is also shared by Weigmann.[160]
McDonnell[161] has found that the production of aroma in certain cases
varies at different temperatures, the most pronounced being evolved near
the optimum growing temperature, which, as a general rule, is too high
for cream ripening.
The majority of bacteria in ripening cream do not seem to exert any
marked influence in butter. A considerable number of species are
positively beneficial, inasmuch as they produce a good flavor or aroma.
A more limited number are concerned in the production of undesirable
ripening changes. This condition being true, it may seem strange that
butter is as good as it is, because so frequently the requisite care is
not given to the development of proper ripening. In all probability the
chief reason why this is so is that those bacteria that find milk and
cream pre-eminently suited to their development, e. g. the lactic-acid
class, are either neutral or beneficial in their effect on butter.
~Use of starters.~ Experience has amply demonstrated that it is possible
to control the nature of the fermentative changes that occur in ripening
cream to such an extent as to materially improve the quality of the
butter. This is frequently done by the addition of a "starter." While
starters have been employed for many years for the purpose mentioned, it
is only recently that their nature has been understood. A starter may be
selected from widely divergent sources, but in all cases it is sure to
contain a large number of bacteria, and the presumption is that they are
of such a nature as to produce desirable fermentative changes in the
cream.
In the selection of these so-called natural starters, it follows that
they must be chosen under such conditions as experience has shown to
give favorable results. For this purpose, whole milk from a single
animal is often used where the same is observed to sour with the
production of no gas or other undesirable taint. A skim-milk starter
from a mixed supply is recommended by many. Butter milk is frequently
employed, but in the opinion of butter experts is not as suitable as the
others mentioned.
It not infrequently happens that the practical operator may be misled in
selecting a starter that is not desirable, or by continuing its use
after it has become contaminated.
In 1890[162] a new system of cream ripening was introduced in Denmark by
Storch that possesses the merit of being a truly scientific and at the
same time practical method. This consisted in the use of pure cultures
of specific organisms that were selected on account of their ability to
produce a desirable ripening change in cream. The introduction of these
so-called culture starters has become universal in Denmark, and in parts
of Germany. Their use is also rapidly extending in this country,
Australia and New Zealand.
~Principles of pure-culture cream-ripening.~ In the proper use of pure
cultures for ripening cream, it is necessary first to eliminate as far
as possible the bacteria already present in cream before the culture
starter is added. This result is accomplished by heating the cream to a
temperature sufficiently high to destroy the vegetating organisms. The
addition of a properly selected starter will then give the chosen
organism such an impetus as will generally enable it to gain the
ascendency over any other bacteria and so control the character of the
ripening. The principle employed is quite like that practiced in raising
grain. The farmer prepares his soil by plowing, in this way killing the
weeds. Then he sows his selected grain, which is merely a pure culture,
and by the rapid growth of this, other forms are held in check.
The attempt has been made to use these culture starters in raw sweet
cream, but it can scarcely be expected that the most beneficial results
will be attained in this way. This method has been justified on the
basis of the following experiments. Where cream is pasteurized and no
starter is added, the spore-bearing forms frequently produce undesirable
flavors. These can almost always be controlled if a culture starter is
added, the obnoxious form being repressed by the presence of the added
starter. This condition is interpreted as indicating that the addition
of a starter to cream which already contains developing bacteria will
prevent those originally present in the cream from growing.[163] This
repressive action of one species on another is a well-known
bacteriological fact, but it must be remembered that such an explanation
is only applicable in those cases where the culture organism is better
able to develop than those forms that already exist in the cream.
If the culture organism is added to raw milk or cream which already
contains a flora that is well suited to develop in this medium, it is
quite doubtful whether it would gain the supremacy in the ripening
cream. The above method of adding a culture to raw cream renders
cream-ripening details less burdensome, but at the same time Danish
experience, which is entitled to most credence on this question, is
opposed to this method.
~Reputed advantages of culture starters.~ _1. Flavor and aroma._ Naturally
the flavor produced by pure-culture ferments depends upon the character
of the organism used. Those which are most extensively used are able to
produce a perfectly clean but mild flavor, and a delicate but not
pronounced aroma. The "high, quick" flavor and aroma that is so much
desired in the American market is not readily obtained by the use of
cultures. It is quite problematical whether the use of any single
species will give any more marked aroma than normally occurs in natural
ripening.
_2. Uniformity of product._ Culture starters produce a more uniform
product because the type of fermentation is under more complete control,
and herein is the greatest advantage to be derived from their use. Even
the best butter-maker at times will fail to secure uniform results if
his starter is not perfectly satisfactory.
_3. Keeping quality of product._ Butter made from pasteurized cream to
which a pure-culture starter has been added will keep much better than
the ordinary product, because the diversity of the bacterial flora is
less and the milk is therefore not so likely to contain those organisms
that produce an "off" condition.
_4. Elimination of taints._ Many defective conditions in butter are
attributable to the growth of undesirable bacteria in the cream that
result in the formation of "off" flavors and taints. If cream is
pasteurized, thereby destroying these organisms, then ripened with pure
ferments, it is generally possible to eliminate the abnormal
conditions.[164] Taints may also be present in cream due to direct
absorption from the cow or through exposure to foul odors.[165] Troubles
of this sort may thus be carried over to the butter. This is
particularly true in regions where leeks and wild onions abound, as in
some of the Atlantic States. The heating of the cream tends to expel
these volatile taints, so that a fairly good article of butter can be
made from what would otherwise be a relatively worthless product.
~Characteristics desired in culture starters.~ Certain conditions as the
following are desirable in starters made from pure cultures:
1. Vigorous growth in milk at ordinary ripening temperatures.
2. Ability to form acid so as to facilitate churning and increase the
yield of butter.
3. Able to produce a clean flavor and desirable aroma.
4. Impart a good keeping quality to butter.
5. Not easily modified in its flavor-producing qualities by artificial
cultivation.
These different conditions are difficult to attain, for the reason that
some of them seem to be in part incompatible. Weigmann[166] found that a
good aroma was generally an evanescent property, and therefore opposed
to good keeping quality. Conn has shown that the functions of
acid-formation, flavor and aroma production are not necessarily related,
and therefore the chances of finding a single organism that possesses
all the desirable attributes are not very good.
In all probability no one germ possesses all of these desirable
qualities, but natural ripening is the resultant of the action of
several forms.[167] This idea has led to the attempt at mixing selected
organisms that have been chosen on account of certain favorable
characteristics which they might possess. The difficulty of maintaining
such a composite culture in its correct proportions when it is
propagated in the creamery is seemingly well nigh insuperable, as one
organism is very apt to develop more or less rapidly than the other.
A very satisfactory way in which these cultures are marketed is to mix
the bacterial growth with some sterile, inert, dry substance. This is
the method used in most of the Danish cultures. In this country, some of
the more prominent cultures employed are marketed in a liquid form.
~Culture vs. home-made starters.~ One great advantage which has accrued
from the use of culture or commercial starters has been that in
emphasizing the need of closer control of the ripening process, greater
attention has been paid to the carrying out of the details. In the
hands of the better operators, the differences in flavor of butter made
with a culture or a natural starter are not marked,[168] but in the
hands of those who fail to make a good product under ordinary
conditions, an improvement is often secured where a commercial culture
is used.
~Pasteurization as applied to butter-making.~ This process, as applied to
butter making, is often confounded with the treatment of milk and cream
for direct consumption. It is unfortunate that the same term is used in
connection with the two methods, for they have but little in common
except in the use of heat to destroy the germ life of the milk. In
pasteurizing cream for butter-making, it is not necessary to observe the
stringent precautions that are to be noted in the preservation of milk;
for the addition of a rapidly developing starter controls at once the
fermentative changes that subsequently occur. Then again, the physical
requirement as to the production of a cooked taste is not so stringent
in butter-making. While a cooked taste is imparted to milk or even cream
at about 158° F., it is possible to make butter that shows no permanent
cooked taste from cream that has been raised as high as 185° or even
195° F. This is due to the fact that the fat does not readily take up
those substances that give to scalded milk its peculiar flavor.
Unless care is taken in the manipulation of the heated cream, the grain
or body of the butter may be injured. This tendency can be overcome if
the ripened cream is chilled to 48° F. for about two hours before
churning. It is also essential that the heated cream should be quickly
and thoroughly chilled after being pasteurized.
The Danes, who were the first to employ pasteurization in butter-making,
used, in the beginning, a temperature ranging from 158° to 167° F., but
owing to the prevalence of such diseases as tuberculosis and
foot-and-mouth disease, it became necessary to treat all of the skim
milk that was returned from the creameries. For this purpose the skim
milk is heated to a temperature of 176° F., it having been more recently
determined that this degree of heat is sufficient to destroy the seeds
of disease. With the use of this higher temperature the capacity of the
pasteurizing apparatus is considerably reduced, but the higher
temperature is rendered necessary by the prevailing conditions as to
disease.
When the system was first introduced in Denmark, two methods of
procedure were followed: the whole milk was heated to a sufficiently
high temperature to thoroughly pasteurize it before it was separated, or
it was separated first, and the cream pasteurized afterwards. In the
latter case, it is necessary to heat the skim milk after separation to
destroy the disease organisms, but this can be quickly done by the use
of steam directly. Much more care must be used in heating the cream in
order to prevent injury to the grain of the butter. In spite of the
extra trouble of heating the cream and skim milk separately, this method
has practically supplanted the single heating. With the continual spread
of tuberculosis in America the heating of skim milk separately is
beginning to be introduced.[169]
~Use of starters in pasteurized and unpasteurized cream.~ In order to
secure the beneficial results presumably attributable to the use of a
starter, natural as well as a pure culture, it should be employed in
cream in which the bacteria have first been killed out by
pasteurization. This is certainly the most logical and scientific method
and is the way in which the process has been developed in Denmark.
Here in this country, the use of pure cultures has been quite rapidly
extended, but the system of heating the cream has been used in only a
slight measure. The increased labor and expense incurred in pasteurizing
the cream has naturally militated somewhat against the wide-spread use
of the process, but doubtless the main factor has been the inability to
secure as high a flavor where the cream was heated as in the unheated
product. As the demands of the market change from a high, quick flavor
to one that is somewhat milder but of better keeping quality, doubtless
pasteurization of the cream will become more and more popular. That such
a change is gradually occurring is already evident, although as yet only
a small proportion of butter made in this country is now made in this
way. Where the cream is unheated, a considerable number of species will
be found, and even the addition of a pure culture, if that culture is of
the lactic acid-producing species, will to some extent control the type
of fermentation that occurs. Such would not be the case with a culture
composed of the casein-digesting type of bacteria. Only those forms
could thus be used which are especially well suited to development in
raw cream. For this reason the pure culture ferments that are generally
employed in creamery practice are organisms of the lactic acid type,
able to grow rapidly in cream and produce a pure cream flavor in the
butter.
~Purity of commercial starters.~ Naturally the butter maker is forced to
rely on the laboratory for his commercial starter, and the question will
often arise as to the purity and vigor of the various ferments employed.
As there is no way for the factory operator to ascertain the actual
condition of the starter, except by using the same, the greatest care
should be taken by the manufacturer to insure the absolute purity of the
seed used.
A bacteriological examination of the various cultures which have been
placed on the market not infrequently reveals an impure condition. In
several cases the writer has found a not inconsiderable number of
liquefying bacteria mixed with the selected organism. Molds not
infrequently are found in cultures put up in the dry form. Doubtless the
effect of these accidental contaminations is considerably less in the
case of a starter composed of a distinctively lactic acid-producing
organism than with a form which is less capable of thriving vigorously
in milk, and it should be said that these impurities can frequently be
eliminated by continued propagation.
The virility and vigor of the starter is also a fluctuating factor,
dependent in part at least, upon the conditions under which the organism
is grown. In some cases the germ is cultivated in solutions in which
acid cannot be formed in abundance. Where the conditions permit of the
formation of acid, as would be the case if sugar was present with a
lactic acid-producing species, the vitality of the culture is often
impaired by the action of the gradually accumulating acid. Some
manufacturers attempt to minimize this deleterious condition by adding
carbonate of lime which unites with the acid that is formed.
~Propagation of starters for cream-ripening.~ The preparation and
propagation of a starter for cream-ripening is a process involving
considerable bacteriological knowledge, whether the starter is of
domestic origin or prepared from a pure-culture ferment. In any event,
it is necessary that the starter should be handled in a way so as to
prevent the introduction of foreign bacteria as far as possible. It
should be remembered at all times that the starter is a live thing and
must be handled throughout its entire history in a way so as to retain
its vitality and vigor unimpaired. The following points should be taken
into consideration in growing the starter and transferring it from day
to day:
1. If a commercial starter is used, see that it is fresh and that the
seal has not been broken. If the culture is too old, the larger part of
the organisms may have died out before it is transferred, in which case
the effect of its addition to the sterilized milk would be of little
value.
When the commercial ferment is received, it should be stored in the
refrigerator pending its use so as to retard as much as possible the
changes that naturally go on in the culture liquid. Be careful that the
bottle is not exposed to the influence of direct sunlight for in a
transparent medium the organisms may be readily killed by the
disinfecting action of the sun's rays.
2. If a home-made starter is employed, use the greatest possible care in
selecting the milk that is to be used as a basis for the starter.
3. For the propagation and perpetuation of the starter from day to day,
it is necessary that the same should be grown in milk that is as
germ-free as it is possible to secure it. For this purpose sterilize
some fresh skim-milk in a covered can that has previously been well
steamed. This can be done easily by setting cans containing skim-milk in
a vat filled with water and heating the same to 180° F. or above for
one-half hour or more. Steam should not be introduced directly. This
process destroys all but a few of the most resistant spore-bearing
organisms. This will give a cooked flavor to the milk, but will not
affect the cream to which the starter is added. Dairy supply houses are
now introducing the use of starter cans that are specially made for this
purpose.
4. After the heated milk is cooled down to about 70° or 80° F., it can
be inoculated with the desired culture. Sometimes it is desirable to
"build up" the starter by propagating it first in a smaller volume of
milk, and then after this has developed, adding it to a larger amount.
This method is of particular value where a large amount of starter is
needed for the cream-ripening.
5. After the milk has been inoculated, it should be kept at a
temperature that is suitable for the rapid development of the contained
bacteria, 65°-75° F., which temperature should be kept as uniform as
possible.
This can best be done by setting the covered can in a vat filled with
warm water. The starter cans are often arranged so that temperature can
be controlled by circulating water.
6. The starter should not be too thoroughly curdled when it is needed
for use, but should be well soured and only partially curdled for it is
difficult to break up thoroughly the curd particles if the starter is
completely curdled. If these curd masses are added to ripening cream,
white specks may appear in the butter.
7. The vigor of the starter is in all probability stronger when the milk
is on the point of curdling than it is after the curd has been formed
some time. The continued formation of lactic acid kills many of the
bacteria and thus weakens the fermentative action. It is therefore
highly important that the acidity of the starter should be closely
watched.
8. Do not refrigerate the starter when it has reached the proper stage
of development, as this retards the bacterial growth in the same manner
as cold weather checks the growth of grain. It is preferable to dilute
the starter, if it cannot be used when ready, with sufficient freshly
sterilized sweet milk to hold the acidity at the proper point and thus
keep the bacteria in the starter in a condition which will favor
vigorous growth.
9. The starter should be propagated from day to day by adding a small
quantity to a new lot of freshly prepared milk. For this purpose two
propagating cans should be provided so that one starter may be in use
while the other is being prepared.
~How long should a starter be propagated?~ No hard-and-fast rule can be
given for this, for it depends largely upon how carefully the starter is
handled during its propagation. If the starter is grown in sterilized
milk kept in steamed vessels and is handled with sterile dippers, it is
possible to maintain it in a state of relative purity for a considerable
period of time; if, however, no especial care is given, it will soon
become infected by the air, and the retention of its purity will depend
more upon the ability of the contained organism to choke out foreign
growths than upon any other factor. Experience seems to indicate that
pure-culture starters "run out" sooner than domestic starters. While it
is possible, by bacteriological methods, to determine with accuracy the
actual condition of a starter as to its germ content, still such methods
are inapplicable in creamery practice. Here the maker must rely largely
upon the general appearance of the starter as determined by taste and
smell. The supply houses that deal in cultures of this class generally
expect to supply a new culture at least every month.
~Bacteria in butter.~ As ripened cream is necessarily rich in bacteria, it
follows that butter will also contain germ life in varying amounts, but
as butter-fat is not well adapted for bacterial food, the number of
germs in butter is usually less than in ripened cream.
Sweet-cream butter is naturally poorer in germ life than that made from
ripened cream. Grotenfelt reports in sweet-cream butter, the so-called
"Paris butter," only a few bacteria while in acid cream butter the germ
content runs from scores to hundreds of thousands.
~Effect of bacteria in wash water.~ An important factor in contamination
may be the wash water that is used. Much carelessness often prevails
regarding the location and drainage of the creamery well, and if same
becomes polluted with organic matter, bacterial growth goes on apace.
Melick[170] has made some interesting studies on using pasteurized and
sterilized well waters for washing. He found a direct relation to exist
between the bacterial content of the wash water and the keeping quality
of the butter. Some creameries have tried filtered water but under
ordinary conditions a filter, unless it is tended to with great
regularity, becomes a source of infection rather than otherwise.
~Changes in germ content.~ The bacteria that are incorporated with the
butter as it first "comes" undergo a slight increase for the first few
days. The duration of this period of increase is dependent largely upon
the condition of the butter. If the buttermilk is well worked out of the
butter, the increase is slight and lasts for a few days only, while the
presence of so nutritious a medium as buttermilk affords conditions much
more favorable for the continued growth of the organisms.
While there may be many varieties in butter when it is fresh, they are
very soon reduced in kind as well as number. The lactic acid group of
organisms disappear quite rapidly; the spore-bearing species remaining
for a somewhat longer time. Butter examined after it is several months
old is often found to be almost free from germs.
In the manufacture of butter there is much that is dependent upon the
mechanical processes of churning, washing, salting and working the
product. These processes do not involve any bacteriological principles
other than those that are incident to cleanliness. The cream, if ripened
properly, will contain such enormous numbers of favorable forms that the
access of the few organisms that are derived from the churn, the air, or
the water in washing will have little effect, unless the conditions are
abnormal.
BACTERIAL DEFECTS IN BUTTER.
~Rancid change in butter.~ Fresh butter has a peculiar aroma that is very
desirable and one that enhances the market price, if it can be retained;
but this delicate flavor is more or less evanescent, soon disappearing,
even in the best makes. While a good butter loses with age some of the
peculiar aroma that it possesses when first made, yet a gilt-edged
product should retain its good keeping qualities for some length of
time. All butters, however, sooner or later undergo a change that
renders them worthless for table use. This change is usually a rancidity
that is observed in all stale products of this class. The cause of this
rancid condition in butter was at first attributed to the formation of
butyric acid, but it is now recognized that other changes also enter
in.[171] Light and especially air also exert a marked effect on the
flavor of butter. Where butter is kept in small packages it is much more
prone to develop off flavors than when packed in large tubs. From the
carefully executed experiments of Jensen it appears that some of the
molds as well as certain species of bacteria are able to incite these
changes. These organisms are common in the air and water and it
therefore readily follows that inoculation occurs.
Practically, rancidity is held in check by storing butter at low
temperatures where germ growth is quite suspended.
~Lack of flavor.~ Often this may be due to improper handling of the cream
in not allowing it to ripen far enough, but sometimes it is impossible
to produce a high flavor. The lack of flavor in this case is due to the
absence of the proper flavor-producing organisms. This condition can
usually be overcome by the addition of a proper starter.
~Putrid butter.~ This specific butter trouble has been observed in
Denmark, where it has been studied by Jensen.[172] Butter affected by it
rapidly acquires a peculiar putrid odor that ruins it for table use.
Sometimes, this flavor may be developed in the cream previous to
churning.
Jensen found the trouble to be due to several different putrefactive
bacteria. One form which he called _Bacillus foetidus lactis_, a close
ally of the common feces bacillus, produced this rotten odor and taste
in milk in a very short time. Fortunately, this organism was easily
killed by a comparatively low heat, so that pasteurization of the cream
and use of a culture starter quickly eliminated the trouble, where it
was tried.
~Turnip-flavored butter.~ Butter sometimes acquires a peculiar flavor
recalling the order of turnips, rutabagas, and other root crops. Often
this trouble is due to feeding, there being in several of these crops,
aromatic substances that pass directly into the milk, but in some
instances the trouble arises from bacteria that are able to produce
decomposition products,[173] the odor and taste of which strongly
recalls these vegetables.
~"Cowy" butter.~ Frequently there is to be noted in milk a peculiar odor
that resembles that of the cow stable. Usually this defect in milk has
been ascribed to the absorption of impure gases by the milk as it cools,
although the gases and odors naturally present in fresh milk have this
peculiar property that is demonstrable by certain methods of aeration.
Occasionally it is transmitted to butter, and recently Pammel[174] has
isolated from butter a bacillus that produced in milk the same peculiar
odor so commonly present in stables.
~Lardy and tallowy butter.~ The presence of this unpleasant taste in
butter may be due to a variety of causes. In some instances, improper
food seems to be the source of the trouble; then again, butter exposed
to direct sunlight bleaches in color and develops a lardy flavor.[175]
In addition to these, cases have been found in which the defect has been
traced to the action of bacteria. Storch[176] has described a
lactic-acid form in a sample of tallowy butter that was able to produce
this disagreeable odor.
~Oily butter.~ Jensen has isolated one of the causes of the dreaded oily
butter that is reported quite frequently in Denmark. The specific
organism that he found belongs to the sour-milk bacteria. In twenty-four
hours it curdles milk, the curd being solid like that of ordinary sour
milk. There is produced, however, in addition to this, an unpleasant
odor and taste resembling that of machine oil, a peculiarity that is
transmitted directly to butter made from affected cream.
~Bitter butter.~ Now and then butter develops a bitter taste that may be
due to a variety of different bacterial forms. In most cases, the bitter
flavor in the butter is derived primarily from the bacteria present in
the cream or milk. Several of the fermentations of this character in
milk are also to be found in butter. In addition to these defects
produced by a biological cause, bitter flavors in butter are sometimes
produced by the milk being impregnated with volatile, bitter substances
derived from weeds.
~Moldy butter.~ This defect is perhaps the most serious because most
common. It is produced by the development of a number of different
varieties of molds. The trouble appears most frequently in packed butter
on the outside of the mass of butter in contact with the tub. Mold
spores are so widely disseminated that if proper conditions are given
for their germination, they are almost sure to develop. In some cases
the mold is due to the growth of the ordinary bread mold, _Penicillium
glaucum_; in other cases a black mold develops, due often to
_Cladosporium butyri_. Not infrequently trouble of this character is
associated with the use of parchment wrappers. The difficulty can easily
be held in check by soaking the parchment linings and the tubs in a
strong brine, or paraffining the inside of the tub.
~Fishy butter.~ Considerable trouble has been experienced in Australian
butter exported to Europe in which a fishy flavor developed. It was
noted that the production of this defect seemed to be dependent upon the
storage temperature at which the butter was kept. When the butter was
refrigerated at 15° F. no further difficulty was experienced. It is
claimed that the cause of this condition is due to the formation of
trimethylamine (herring brine odor) due to the growth of the mold fungus
_Oidium lactis_, developing in combination with the lactic-acid
bacteria.
A fishy taste is sometimes noted in canned butter. Rogers[177] has
determined that this flavor is caused by yeasts (_Torula_) which produce
fat-splitting enzyms capable of producing this undesirable change.
FOOTNOTES:
[153] Conn and Esten, Cent. f. Bakt., II Abt., 1901, 7:746.
[154] Tiemann, Milch Zeit., 23:701.
[155] Milch Zeit., 1889, p. 7; 1894, p. 624; 1895, p. 383.
[156] Dean, Ont. Agr. Coll., 1897, p. 66.
[157] Storch, Nogle, Unders. over Floed. Syrning, 1890.
[158] Conn, 6 Storrs Expt. Stat., 1893, p. 66.
[159] Conn, 9 Storrs Expt. Stat., 1896, p. 17.
[160] Weigmann, Milch Zeit., 1891, p. 793
[161] McDonnell, ü. Milchsäure Bakterien (Diss. Kiel, 1899), p. 43.
[162] Storch, Milch Zeit., 1890, p. 304.
[163] Conn, 9 Storrs Expt. Stat., 1896, p. 25.
[164] Milch Zeit., 1891, p. 122; 1894, p. 284; 1895, p. 56; 1896, p.
163.
[165] McKay, Bull. 32, Iowa Expt. Stat., p. 47
[166] Weigmann, Landw. Woch. f. Schl. Hol., No. 2, 1890.
[167] Weigmann, Cent. f. Bakt., II Abt., 3:497, 1897.
[168] At the National Creamery Buttermakers' Association for 1901, 193
out of 240 exhibitors used starters. Of those that employed starters,
nearly one-half used commercial cultures. There was practically no
difference in the average score of the two classes of starters, but
those using starters ranked nearly two points higher in flavor than
those that did not.
[169] Russell, Bull. 143, Wis. Expt. Stat., Feb. 1907.
[170] Melick, Bull. 138, Kansas Expt. Stat., June 1906.
[171] Reinmann, Cent. f. Bakt., 1900, 6:131; Jensen, Landw. Jahr. d.
Schweiz, 1901.
[172] Jensen, Cent. f. Bakt., 1891, 11:409.
[173] Jensen, Milch Zeit., 1892, 6, Nos. 5 and 6.
[174] Pammel, Bull. 21, Iowa Expt. Stat., p. 803.
[175] Fischer, Hyg. Rund., 5:573.
[176] Storch, 18 Rept. Danish Agric. Expt. Stat., 1890.
[177] Rogers Bull. 57, B. A. I. U. S. Dept Agric., 1904.
CHAPTER VIII.
BACTERIA IN CHEESE.
The art of cheese-making, like all other phases of dairying, has been
developed mainly as a result of empirical methods. Within the last
decade or so, the subject has received more attention from the
scientific point of view and the underlying causes determined to some
extent. Since the subject has been investigated from the bacteriological
point of view, much light has been thrown on the cause of many changes
that were heretofore inexplicable. Our knowledge, as yet, is quite
meager, but enough has already been determined to indicate that the
whole industry is largely based on the phenomena of ferment action, and
that the application of bacteriological principles and ideas is sure to
yield more than ordinary results, in explaining, in a rational way, the
reasons underlying many of the processes to be observed in this
industry.
The problem of good milk is a vital one in any phase of dairy activity,
but it is pre-eminently so in cheese-making, for the ability to make a
first-class product depends to a large extent on the quality of the raw
material. Cheese contains so large a proportion of nitrogenous
constituents that it is admirably suited, as a food medium, to the
development of bacteria; much better, in fact, than butter.
INFLUENCE OF BACTERIA IN NORMAL CHEESE PROCESSES.
In the manufacture of cheddar cheese bacteria exert a marked influence
in the initial stages of the process. To produce the proper texture that
characterizes cheddar cheese, it is necessary to develop a certain
amount of acid which acts upon the casein. This acidity is measured by
the development of the lactic-acid bacteria that normally abound in the
milk; or, as the cheese-maker expresses it, the milk is "ripened" to the
proper point. The action of the rennet, which is added to precipitate
the casein of the milk, is markedly affected by the amount of acid
present, as well as the temperature. Hence it is desirable to have a
standard amount of acidity as well as a standard temperature for
coagulation, so as to unify conditions. It frequently happens that the
milk is abnormal with reference to its bacterial content, on account of
the absence of the proper lactic bacteria, or the presence of forms
capable of producing fermentative changes of an undesirable character.
In such cases the maker attempts to overcome the effect of the unwelcome
bacteria by adding a "starter;" or he must vary his method of
manufacture to some extent to meet these new conditions.
~Use of starters.~ A starter may be employed to hasten the ripening of
milk that is extremely sweet, so as to curtail the time necessary to get
the cheese to press; or it may be used to overcome the effect of
abnormal conditions.
The starter that is employed is generally one of domestic origin, and is
usually taken from skim milk that has been allowed to ferment and sour
under carefully controlled conditions. Of course much depends upon the
quality of the starter, and in a natural starter there is always the
possibility that it may not be perfectly pure.
Within recent years the attempt has been made to control the effect of
the starter more thoroughly by using pure cultures of some desirable
lactic-acid form.[178] This has rendered the making of cheese not only
more uniform, but has aided in repressing abnormal fermentations
particularly those that are characterized by the production of gas.
Recently, pure cultures of Adametz's _B. nobilis_, a digesting organism
that is claimed to be the cause of the breaking down of the casein and
also of the peculiar aroma of Emmenthaler cheese, has been placed on the
market under the name _Tyrogen_. It is claimed that the use of this
starter, which is added directly to the milk and also rubbed on the
surface of the cheese, results in the improvement of the curds, assists
in the development of the proper holes, imparts a favorable aroma and
hastens ripening.[179]
Campbell[180] states that the discoloration of cheese in England, which
is due to the formation of white spots that are produced by the
bleaching of the coloring matter in the cheese, may be overcome by the
use of lactic-acid starters.
The use of stringy or slimy whey has been advocated in Holland for some
years as a means of overcoming the tendency toward gas formation in Edam
cheese which is made from practically sweet milk. This fermentation, the
essential feature of which is produced by a culture of _Streptococcus
Hollandicus_,[181] develops acid in a marked degree, thereby inhibiting
the production of gas.
The use of masses of moldy bread in directing the fermentation of
Roquefort cheese is another illustration of the empirical development of
starters, although in this instance it is added after the curds have
been prepared for the press.
~Pasteurizing milk for cheese-making.~ If it were possible to use properly
pasteurized milk in cheese-making, then practically all abnormal
conditions could be controlled by the use of properly selected starters.
Numerous attempts have been made to perfect this system with reference
to cheddar cheese, but so far they have been attended with imperfect
success. The reason for this is that in pasteurizing milk, the soluble
lime salts are precipitated by the action of heat, and under these
conditions rennet extract does not curdle the casein in a normal manner.
This condition can be restored, in part at least, by the addition of
soluble lime salts, such as calcium chlorid; but in our experience,
desirable results were not obtained where heated milks to which this
calcium solution had been added were made into cheddar cheese.
Considerable experience has been gained in the use of heated milks in
the manufacture of certain types of foreign cheese. Klein[182] finds
that Brick cheese can be successfully made even where the milk is heated
as high as 185° F. An increased weight is secured by the addition of the
coagulated albumin and also increased moisture.
~Bacteria in rennet.~ In the use of natural rennets, such as are
frequently employed in the making of Swiss cheese, considerable numbers
of bacteria are added to the milk. Although these rennets are preserved
in salt, alcohol or boric acid, they are never free from bacteria.
Adametz[183] found ten different species and from 640,000 to 900,000
bacteria per cc. in natural rennets. Freudenreich has shown that rennet
extract solutions can be used in Swiss cheese-making quite as well as
natural rennets; but to secure the best results, a small quantity of
pure lactic ferment must be added to simulate the conditions that
prevail when natural rennets are soaked in whey, which, it must be
remembered, is a fluid rich in bacterial life.
Where rennet extract or tablets are used, as is generally the case in
cheddar making, the number of bacteria added is so infinitesimal as to
be negligible.
~Development of acid.~ In the manufacture of cheddar cheese, the
development of acid exerts an important influence on the character of
the product. This is brought about by holding the curds at temperatures
favorable to the growth of the bacteria in the same. Under these
conditions the lactic-acid organisms, which usually predominate, develop
very rapidly, producing thereby considerable quantities of acid which
change materially the texture of the curds. The lactic acid acts upon
the casein in solutions containing salt, causing it to dissolve to some
extent, thus forming the initial compounds of digestion.[184] This
solution of the casein is expressed physically by the "stringing" of the
curds on a hot iron. This causes the curds to mat, producing a close,
solid body, free from mechanical holes. Still further, the development
of this acid is necessary for the digestive activity of the pepsin in
the rennet extract.
In some varieties of cheese, as the Swiss, acid is not developed and the
character of the cheese is much different from that of cheddar. In all
such varieties, a great deal more trouble is experienced from the
production of "gassy" curds, because the development of the
gas-producing bacteria is held in check by the rapid growth of the
lactic acid-producing species.
~Bacteria in green cheese.~ The conditions under which cheese is made
permit of the development of bacteria throughout the entire process. The
cooking or heating of curds to expel the excessive moisture is never so
high as to be fatal to germ life; on the contrary, the acidity of the
curd and whey is continually increased by the development of bacteria in
the same.
The body of green cheese fresh from the press is, to a considerable
extent, dependent upon the acid produced in the curds. If the curds are
put to press in a relatively sweet condition the texture is open and
porous. The curd particles do not mat closely together and "mechanical
holes," rough and irregular in outline, occur. Very often, at relatively
high temperatures, such cheese begin to "huff," soon after being taken
from the press, a condition due to the development of gas, produced by
gas-generating bacteria acting on the sugar in the curd. This gas finds
its way readily into these ragged holes, greatly distending them, as in
Fig. 30.
[Illustration: FIG. 30. _L_, a sweet curd cheese direct from the press.
"Mechanical" holes due to lack of acid development; _P_, same cheese
four days later, mechanical holes distended by development of gas.]
~Physical changes in ripening cheese.~ When a green cheese is taken from
the press, the curd is tough, firm, but elastic. It has no value as a
food product for immediate use, because it lacks a desirable flavor and
is not readily digestible. It is nothing but precipitated casein and
fat. In a short time, a deep-seated change occurs. Physically this
change is demonstrated in the modification that the curd undergoes.
Gradually it breaks down and becomes plastic, the elastic, tough curd
being changed into a softened mass. This change in texture of the cheese
is also accompanied by a marked change in flavor. The green cheese has
no distinctively cheese flavor, but in course of time, with the gradual
change of texture, the peculiar flavor incident to ripe cheese is
developed.
The characteristic texture and flavor are susceptible of considerable
modification that is induced not only by variation in methods of
manufacture, but by the conditions under which the cheese are cured. The
amount of moisture incorporated with the curd materially affects the
physical appearance of the cheese, and the rate of change in the same.
The ripening temperature, likewise the moisture content of the
surrounding air, also exerts a marked influence on the physical
properties of the cheese. To some extent the action of these forces is
purely physical, as in the gradual loss by drying, but in other respects
they are associated with chemical transformations.
~Chemical changes in ripening cheese.~ Coincident with the physical
breaking down of the curd comes a change in the chemical nature of the
casein. The hitherto insoluble casein is gradually transformed into
soluble nitrogenous substances (_caseone_ of Duclaux, or _caseogluten_
of Weigmann). This chemical phenomenon is a breaking-down process that
is analogous to the peptonization of proteids, although in addition to
the peptones and albumoses characteristic of peptic digestion,
amido-acids and ammonia are to be found. The quantity of these lower
products increases with the age of the cheese.
The chemical reaction of cheese is normally acid to phenolphthalein,
although there is generally no free acid, as shown by Congo red, the
lactic acid being converted into salts as fast as formed. In very old
cheese, undergoing putrefactive changes, especially on the outside, an
alkaline reaction may be present, due to the formation of free ammonia.
The changes that occur in a ripening cheese are for the most part
confined to the proteids. According to most investigators the fat
remains practically unchanged, although the researches of Weigmann and
Backe[185] show that fatty acids are formed from the fat. In the green
cheese considerable milk-sugar is present, but, as a result of the
fermentation that occurs, this is rapidly converted into acid products.
~Bacterial flora of cheese.~ It might naturally be expected that the green
cheese, fresh from the press, would contain practically the same kind of
bacteria that are in the milk, but a study of cheese shows a peculiar
change in the character of the flora. In the first place, fresh cottage
cheese, made by the coagulation of the casein through the action of
acid, has a more diversified flora than cheese made with rennet, for the
reason, as given by Lafar,[186] that the fermentative process is farther
advanced.
When different varieties of cheese are made from milk in the same
locality, the germ content of even the ripened product has a marked
similarity, as is illustrated by Adametz's work[187] on Emmenthaler or
Swiss hard cheese, and Schweitzer Hauskäse, a soft variety. Of the nine
species of bacilli and cocci found in mature Emmenthaler, eight of them
were also present in ripened Hauskäse.
Different investigators have studied the bacterial flora of various
kinds of cheese, but as yet little comparative systematic work has been
done. Freudenreich[188] has determined the character and number of
bacteria in Emmenthaler cheese, and Russell[189] the same for cheddar
cheese. The same general law has also been noted in Canadian[190] and
English[191] cheese. At first a marked decrease in numbers is usually
noted, lasting for a day or two. This is followed by an enormous
increase, caused by the rapid growth of the lactic-acid type. The
development may reach scores of millions and often over a hundred
million organisms per gram. Synchronous with this increase, the
peptonizing and gas-producing bacteria gradually disappear. This rapid
development, which lasts only for a few weeks, is followed by a general
decline.
In the ripening of cheese a question arises as to whether the process
goes on throughout the entire mass of cheese, or whether it is more
active at or near the surface. In the case of many of the soft cheese,
such as Brie and limburger, bacterial and mold development is
exceedingly active on the exterior, and the enzyms secreted by these
organisms diffuse toward the interior. That such a condition occurs in
the hard type of cheese made with rennet is extremely improbable. Most
observers agree that in this type of cheese the ripening progresses
throughout the entire mass, although Adametz opposes this view and
considers that in Emmenthaler cheese the development of the specific
aroma-producing organism occurs in the superficial layers. Jensen has
shown, however, that the greatest amount of soluble nitrogenous products
are to be found in the innermost part of the cheese, a condition that is
not reconcilable with the view that the most active ripening is on the
exterior.[192]
The course of development of bacteria in cheddar cheese is materially
influenced by the ripening temperature. In cheese ripened at relatively
low temperatures (50°-55° F.),[193] a high germ content is maintained
for a much longer period of time than at higher temperatures. Under
these conditions the lactic-acid type continues in the ascendancy as
usual. In cheese cured at high temperatures (80°-86° F.) the number of
organisms is greatly diminished, and they fail to persist in appreciable
numbers for as long a time as in cheese cured at temperatures more
frequently employed.
~Influence of temperature on curing.~ Temperature exerts a most potent
influence on the quality of the cheese, as determined not only by the
rate of ripening but the nature of the process itself. Much of the poor
quality of cheese is attributable to the effect of improper curing
conditions. Probably in the initial stage of this industry cheese were
allowed to ripen without any sort of control, with the inevitable result
that during the summer months the temperature generally fluctuated so
much as to impair seriously the quality. The effect of high temperatures
(70° F. and above) is to produce a rapid curing, and, therefore, a short
lived cheese; also a sharp, strong flavor, and generally a more or less
open texture. Unless the cheese is made from the best quality of milk,
it is very apt to undergo abnormal fermentations, more especially those
of a gassy character.
[Illustration: FIG. 31. Influence of curing temperature on texture of
cheese. Upper row ripened eight months at 60° F.; lower row at 40° F.]
Where cheese is ripened at low temperatures, ranging from 50° F. down to
nearly the freezing temperatures, it is found that the quality is
greatly improved.[194] Such cheese are thoroughly broken down from a
physical point of view even though they may not show such a high per
cent of soluble nitrogenous products. They have an excellent texture,
generally solid and firm, free from all tendency to openness; and,
moreover, their flavor is clean and entirely devoid of the sharp,
undesirable tang that so frequently appears in old cheese. The keeping
quality of such cheese is much superior to the ordinary product. The
introduction of this new system of cheese-curing promises much from a
practical point of view, and undoubtedly a more complete study of the
subject from a scientific point of view will aid materially in
unraveling some of the problems as to flavor production.
~Theories of cheese curing.~ Within the last few years considerable study
has been given the subject of cheese curing or ripening, in order to
explain how this physical and chemical transformation is brought about.
Much of the misconception that has arisen relative to the cause of
cheese ripening comes from a confusion of terms. In the ordinary use of
the word, ripening or curing of cheese is intended to signify the sum
total of all the changes that result in converting the green product as
it comes from the press into the edible substance that is known as cured
cheese. As previously shown, the most marked chemical transformation
that occurs is that which has to do with the peptonization or breaking
down of the casein. It is true that under ordinary conditions this
decomposition process is also accompanied with the formation of certain
flavor-producing substances, more or less aromatic in character; but it
by no means follows that these two processes are necessarily due to the
same cause. The majority of investigators have failed to consider these
two questions of casein decomposition and flavor as independent, or at
least as not necessarily related. They are undoubtedly closely bound
together, but it will be shown later that the problems are quite
different and possibly susceptible of more thorough understanding when
considered separately.
In the earlier theories of cheese ripening it was thought to be purely a
chemical change, but, with the growth of bacteriological science,
evidence was forthcoming that seemed to indicate that the activity of
organisms entered into the problem. Schaffer[195] showed that if milk
was boiled and made into cheese, the casein failed to break down.
Adametz[196] added to green cheese various disinfectants, as creolin and
thymol, and found that this practically stopped the curing process. From
these experiments he drew the conclusion that bacteria must be the cause
of the change, because these organisms were killed; but when it is
considered that such treatment would also destroy the activity of enzyms
as well as vital ferments, it is evident that these experiments were
quite indecisive.
A determination of the nature of the by-products found in maturing
cheese indicates that the general character of the ripening change is a
peptonization or digestion of the casein.
Until recently the most widely accepted views relating to the cause of
this change have been those which ascribed the transformation to the
activity of micro-organisms, although concerning the nature of these
organisms there has been no unanimity of opinion. The overwhelming
development of bacteria in all cheeses naturally gave support to this
view; and such experiments as detailed above strengthened the idea that
the casein transformation could not occur where these ferment organisms
were destroyed.
The very nature of the changes produced in the casein signified that to
take part in this process any organism must possess the property of
dissolving the proteid molecule, casein, and forming therefrom
by-products that are most generally found in other digestive or
peptonizing changes of this class.
~Digestive bacterial theory.~ The first theory propounded was that of
Duclaux,[197] who in 1887 advanced the idea that this change was due to
that type of bacteria which is able to liquefy gelatin, peptonize milk,
and cause a hydrolytic change in proteids. To this widely-spread group
that he found in cheese, he gave the generic name _Tyrothrix_ (cheese
hairs). According to him, these organisms do not function directly as
ripening agents, but they secrete an enzym or unorganized ferment to
which he applies the name _casease_. This ferment acts upon the casein
of milk, converting it into a soluble product known as _caseone_. These
organisms are found in normal milk, and if they function as casein
transformers, one would naturally expect them to be present, at least
frequently, if not predominating in the ripening cheese; but such is not
the case. In typical cheddar or Swiss cheese, they rapidly disappear (p.
168), although in the moister, softer varieties, they persist for
considerable periods of time. According to Freudenreich, even where
these organisms are added in large numbers to the curd, they soon
perish, an observation that is not regarded as correct by the later
adherents to the digestive bacterial theory, as Adametz and Winkler.
Duclaux's experiments were made with liquid media for isolation
purposes, and his work, therefore, cannot be regarded as satisfactory as
that carried out with more modern technical methods. Recently this
theory has been revived by Adametz,[198] who claims to have found in
Emmenthaler cheese a digesting species, one of the Tyrothrix type, which
is capable of peptonizing the casein and at the same time producing the
characteristic flavor of this class of cheese. This organism, called by
him _Bacillus nobilis_, the Edelpilz of Emmenthaler cheese, has been
subjected to comparative experiments, and in the cheese made with pure
cultures of this germ better results are claimed to have been secured.
Sufficient experiments have not as yet been reported by other
investigators to warrant the acceptance of the claims made relative to
the effect of this organism.
~Lactic-acid bacterial theory.~ It has already been shown that the
lactic-acid bacteria seems to find in the green cheese the optimum
conditions of development; that they increase enormously in numbers for
a short period, and then finally decline. This marked development,
coincident with the breaking down of the casein, has led to the view
which has been so ably expounded by Freudenreich[199] that this type of
bacterial action is concerned in the ripening of cheese. This group of
bacteria is, under ordinary conditions, unable to liquefy gelatin, or
digest milk, or, in fact, to exert, under ordinary conditions, any
proteolytic or peptonizing properties. This has been the stumbling-block
to the acceptance of this hypothesis, as an explanation of the breaking
down of the casein. Freudenreich has recently carried on experiments
which he believes solve the problem. By growing cultures of these
organisms in milk, to which sterile, freshly precipitated chalk had been
added, he was able to prolong the development of bacteria for a
considerable period of time, and as a result finds that an appreciable
part of the casein is digested; but this action is so slow compared with
what normally occurs in a cheese, that exception may well be taken to
this type of experiment alone. Weigmann[200] inclines to the view that
the lactic-acid bacteria are not the true cause of the peptonizing
process, but that their development prepares the soil, as it were, for
those forms that are more directly concerned in the peptonizing process.
This they do by developing an acid substratum that renders possible the
more luxuriant growth of the aroma-producing species. According to
Gorini,[201] certain of the Tyrothrix forms function at high
temperatures as lactic acid producing bacteria, while at lower
temperatures they act as peptonizers. On this basis he seeks to
reconcile the discrepancies that appear in the experiments of other
investigators.
~Digestive milk enzym theory.~ In 1897 Babcock and the writer[202] showed
that milk underwent digestive changes spontaneously when bacterial
activity was suspended by the addition of such anaesthetics as ether,
chloroform and benzol. The chemical nature of the by-products produced
by this auto-digestion of milk resembles quite closely those found in
ripened cheese, except that ammonia is not produced as is the case in
old cheese. The cause of the decomposition of the casein, they found to
be due to the action of a milk enzym which is inherent to the milk
itself. This digestive ferment may be separated from fresh milk by
concentrating centrifuge slime extracts by the usual physiological
reagents. This ferment, called by them _galactase_, on account of its
origin in milk, is a proteolytic enzym of the tryptic type. Its activity
is destroyed by strong chemicals such as formaldehyde, corrosive
sublimate, also when heated to 175° F. or above. When such extracts are
added to boiled milk, the digestive process is started anew, and the
by-products produced are very similar to those noted in a normal cheese.
Jensen[203] has also shown that the addition of pancreatic extracts to
cheese accelerated the formation of soluble nitrogenous products.
The action of galactase in milk and cheese has been confirmed by
Freudenreich[204] and Jensen,[205] as well as by American investigators,
and this enzym is now generally accepted as one of the factors concerned
in the decomposition of the casein. Freudenreich believes it is able to
change casein into albumose and peptones, but that the lactic-acid
bacteria are chiefly responsible for the further decomposition of the
nitrogen to amid form.
Failure before to recognize the presence of galactase in milk is
attributable to the fact that all attempts to secure sterile milk had
been made by heating the same, in which case galactase was necessarily
destroyed. A brief exposure at 176° F. is sufficient to destroy its
activity, and even an exposure at lower temperatures weakens its action
considerably, especially if the reaction of the medium is acid. This
undoubtedly explains the contradictory results obtained in the ripening
of cheese from pasteurized milk, such cheese occasionally breaking down
in an abnormal manner.
The results mentioned on page 172, in which cheese failed to ripen when
treated with disinfectants,--experiments which were supposed at that
time to be the foundation of the bacterial theory of casein
digestion--are now explicable on an entirely different basis. In these
cases the casein was not peptonized, because these strong disinfectants
destroyed the activity of the enzyms as well as the bacteria.
Another important factor in the breaking down of the casein is the
_pepsin_ in the rennet extract. The digestive influence of this agent
was first demonstrated for cheddar cheese by Babcock, Russell and
Vivian,[206] and simultaneously, although independently, by Jensen[207]
in Emmenthaler cheese. In this digestive action, only albumoses and
higher peptones are produced. The activity of pepsin does not become
manifest until there is about 0.3 per cent. acid which is approximately
the amount developed in the cheddar process. These two factors
undoubtedly account for by far the larger proportion of the changes in
the casein; and yet, the formation of ammonia in well ripened cheese is
not accounted for by these factors. This by-product is the main end
product of proteid digestion by the liquefying bacteria but their
apparent infrequency in cheese makes it difficult to understand how they
can function prominently in the change, unless the small quantity of
digestive enzyms excreted by them in their growth in milk is capable of
continuing its action until a cumulative effect is obtained. Although
much light has been thrown on this question by the researches of the
last few years, the matter is far from being satisfactorily settled at
the present time and the subject needs much more critical work. If
liquefying bacteria abound in the milk, doubtless they exert some
action, but the rôle of bacteria is doubtless much greater in the
production of flavor than in the decomposition of the curd.
~Conditions determining quality.~ In determining the quality of cheese,
several factors are to be taken into consideration. First and foremost
is the flavor, which determines more than anything else the value of the
product. This should be mild and pleasant, although with age the
intensity of the same generally increases but at no time should it have
any bitter, sour, or otherwise undesirable taste or aroma. Texture
registers more accurately the physical nature of the ripening. The
cheese should not be curdy and harsh, but should yield quite readily to
pressure under the thumb, becoming on manipulation waxy and plastic
instead of crumbly or mealy. Body refers to the openness or closeness of
the curd particles, a close, compact mass being most desirable. The
color of cheese should be even, not wavy, streaked or bleached.
For a cheese to possess all of these characteristics in an optimum
degree is to be perfect in every respect--a condition that is rarely
reached.
So many factors influence this condition that the problem of making a
perfect cheese becomes exceedingly difficult. Not only must the quality
of the milk--the raw material to be used in the manufacture--be
perfectly satisfactory, but the factory management while the curds are
in the vat demands great skill and careful attention; and finally, the
long period of curing in which variation in temperature or moisture
conditions may seriously affect the quality,--all of these stages, more
or less critical, must be successfully gone through, before the product
reaches its highest state of development.
It is of course true that many phases of this complex series of
processes have no direct relation to bacteria, yet it frequently happens
that the result attained is influenced at some preceding stage by the
action of bacteria in one way or another. Thus the influence of the
acidity developed in the curds is felt throughout the whole life of the
cheese, an over-development of lactic-acid bacteria producing a sour
condition that leaves its impress not only on flavor but texture. An
insufficient development of acid fails to soften the curd-particles so
as to permit of close matting, the consequence being that the body of
the cheese remains loose and open, a condition favorable to the
development of gas-generating organisms.
~Production of flavor.~ The importance of flavor as determining the
quality of cheese makes it imperative that the nature of the substances
that confer on cheese its peculiar aromatic qualities and taste be
thoroughly understood. It is to be regretted that the results obtained
so far are not more satisfactory, for improvement in technique is hardly
to be expected until the reason for the process is thoroughly
understood.
The view that is most generally accepted is that this most important
phase of cheese curing is dependent upon bacterial activity, but the
organisms that are concerned in this process have not as yet been
satisfactorily determined. In a number of cases, different species of
bacteria have been separated from milk and cheese that have the power of
producing aromatic compounds that resemble, in some cases, the peculiar
flavors and odors that characterize some of the foreign kinds of cheese;
but an introduction of these into curd has not resulted in the
production of the peculiar variety, even though the methods of
manufacture and curing were closely followed. The similarity in germ
content in different varieties of cheese made in the same locality has
perhaps a bearing on this question of flavor as related to bacteria. Of
the nine different species of bacteria found in Emmenthaler cheese by
Adametz, eight of them were also present in ripened Hauskäse. If
specific flavors are solely the result of specific bacterial action, it
might naturally be expected that the character of the flora would
differ.
Some suggestive experiments were made by Babcock and Russell on the
question of flavor as related to bacterial growth, by changing the
nature of the environment in cheese by washing the curds on the racks
with warm water. In this way the sugar and most of the ash were removed.
Under such conditions the character of the bacterial flora was
materially modified. While the liquefying type of bacteria was very
sparse in normal cheddar, they developed luxuriantly in the washed
cheese. The flavor at the same time was markedly affected. The control
cheddar was of good quality, while that made from the washed curds was
decidedly off, and in the course of ripening became vile. It may be
these two results are simply coincidences, but other data[208] bear out
the view that the flavor was to some extent related to the nature of the
bacteria developing in the cheese. This was strengthened materially by
adding different sugars to washed curds, in which case it was found that
the flavor was much improved, while the more normal lactic-acid type of
bacteria again became predominant.
~Ripening of moldy cheese.~ In a number of foreign cheeses, the peculiar
flavor obtained is in part due to the action of various fungi which grow
in the cheese, and there produce certain by-products that flavor the
cheese. Among the most important of these are the Roquefort cheese of
France, Stilton of England, and Gorgonzola of Italy.
Roquefort cheese is made from goat's or cow's milk, and in order to
introduce the desired mold, which is the ordinary bread-mold,
_Penicillium glaucum_, carefully-prepared moldy bread-crumbs are added
to the curd.
At ordinary temperatures this organism develops too rapidly, so that the
cheese to ripen properly must be kept at a low temperature. The town of
Roquefort is situated in a limestone country, in a region full of
caves, and it is in these natural caves that most of the ripening is
done. These caverns are always very moist and have a temperature ranging
from 35° to 44° F., so that the growth of the fungus is retarded
considerably. The spread of the mold throughout the ripening mass is
also assisted in a mechanical way. The partially-matured cheese are run
through a machine that pricks them full of small holes. These slender
canals allow the mold organism to penetrate the whole mass more
thoroughly, the moldy straw matting upon which the ripening cheese are
placed helping to furnish an abundant seeding of the desired germ.
When new factories are constructed it is of advantage to introduce this
necessary germ in quantities, and the practice is sometimes followed of
rubbing the walls and cellars of the new location with material taken
from the old established factory. In this custom, developed in purely an
empirical manner, is to be seen a striking illustration of a
bacteriological process crudely carried out.
In the Stilton cheese, one of the highly prized moldy cheeses of
England, the desired mold fungus is introduced into the green cheese by
exchanging plugs taken with a cheese trier from a ripe Stilton.
~Ripening of soft cheese.~ The type of ripening which takes place in the
soft cheeses is materially different from that which occurs in the hard
type. The peptonizing action does not go on uniformly throughout the
cheese, but is hastened by the development of molds and bacteria on the
outside that exert a solvent action on the casein. For this reason, soft
cheeses are usually made up in small sizes, so that this action may be
hastened. The organisms that take part in this process are those that
are able to form enzyms (similar in their action to trypsin, galactase,
etc.), and these soluble ferments gradually diffuse from the outside
through the cheese.
Most of these peptonizing bacteria are hindered in their growth by the
presence of lactic acid, so that in many cases the appearance of the
digesting organisms on the surface is delayed until the acidity of the
mass is reduced to the proper point by the development of other
organisms, principally molds, which prefer an acid substratum for their
growth.
In Brie cheese a blue coating of mold develops on the surface. In the
course of a few weeks, a white felting appears which later changes to
red. This slimy coat below the mold layer is made up of diverse species
of bacteria and fungi that are able to grow after the acid is reduced by
the blue mold. The organisms in the red slimy coat act upon the casein,
producing an alkaline reaction that is unfavorable to the growth of the
blue mold. Two sets of organisms are, therefore essential in the
ripening process, one preparing the soil for the ferment that later
produces the requisite ripening changes. As ordinarily carried on, the
process is an empirical one, and if the red coat does not develop as
expected, the maker resorts to all kinds of devices to bring out the
desired ferment. The appearance of the right form is dependent, however,
upon the proper reaction of the cheese, and if this is not suitable, the
wished-for growth will not appear.
INFLUENCE OF BACTERIA IN ABNORMAL CHEESE PROCESSES.
The reason why cheese is more subject to abnormal fermentation than
butter is because its high nitrogen content favors the continued
development of bacteria for some time after it is made. It must be
borne in mind, in considering the more important of these changes, that
not all defective conditions in cheese are attributable to the influence
of living organisms. Troubles frequently arise from errors in
manufacturing details, as too prolonged cooking of curds, too high
heating, or the development of insufficient or too much acid. Then
again, the production of undesirable flavors or impairment in texture
may arise from imperfect curing conditions.
Our knowledge regarding the exact nature of these indefinite faults is
as yet too inadequate to enable many of these undesirable conditions to
be traced to their proper source; but in many cases the taints observed
in a factory are due to the abnormal development of certain bacteria,
capable of evolving unpleasant or even putrid odors. Most of them are
seeded in the milk before it comes to the factory and are due to
careless manipulation of the milk while it is still on the farm. Others
gain access to the milk in the factory, owing to unclean conditions of
one sort or another. Sometimes the cheese-maker is able to overcome
these taints by vigorous treatment, but often they pass on into the
cheese, only to detract from the market value of the product. Most
frequently these "off" flavors appear in cheese that are cured at too
high temperatures, say above 65° F.
~"Gassy" fermentations in cheese.~ One of the worst and at the same time
most common troubles in cheese-making is where the cheese undergoes a
fermentation marked by the evolution of gas. The presence of gas is
recognized by the appearance either of spherical or lens-shaped holes of
various sizes in the green cheese; often they appear in the curd before
it is put to press. Usually in this condition the curds look as if they
had been punctured with a pin, and are known as "pin holey" curds. Where
the gas holes are larger, they are known as "Swiss holes" from their
resemblance to the normal holes in the Swiss product. If the development
of gas is abundant, these holes are restricted in size. Often the
formation of gas may be so intense as to cause the curds to float on the
surface of the whey before they are removed. Such curds are known as
"floaters" or "bloaters."
If "gassy" curds are put to press, the abnormal fermentation may
continue. The further production of gas causes the green cheese to
"huff" or swell, until it may be considerably distorted as in Fig. 33.
In such cases the texture of the cheese is greatly injured, and the
flavor is generally impaired.
[Illustration: FIG. 33. Cheese made from gassy milk.]
Such abnormal changes may occur at any season of the year, but the
trouble is most common in summer, especially in the latter part.
This defect is less likely to occur in cheese that is well cheddared
than in sweet curd cheese. When acidity is produced, these gassy
fermentations are checked, and in good cheddar the body is so close and
firm as not readily to permit of gaseous changes.
In Swiss cheese, which is essentially a sweet curd cheese, these
fermentations are very troublesome. Where large holes are formed in
abundance (blähen), the trouble reaches its maximum. If the gas holes
are very numerous and therefore small it is called a "nissler."
Sometimes the normal "eyes" are even wanting when it is said to be
"blind" or a "gläsler."
[Illustration: FIG. 34. Block Swiss cheese showing "gassy"
fermentation.]
One method of procedure which is likely to cause trouble in Swiss
factories is often produced by the use of sour, fermented whey in which
to soak the natural rennets. Freudenreich and Steinegger[209] have shown
that a much more uniform quality of cheese can be made with rennet
extract if it is prepared with a starter made from a pure lactic
ferment.
The cause of the difficulty has long been charged to various sources,
such as a lack of aeration, improper feeding, retention of animal gases,
etc., but in all these cases it was nothing more than a surmise. Very
often the milk does not betray any visible symptom of fermentation when
received, and the trouble is not to be recognized until the process of
cheese-making is well advanced.
Studies from a biological standpoint have, however, thrown much light on
this troublesome problem; and it is now known that the formation of gas,
either in the curd or after it has been put to press, is due entirely to
the breaking down of certain elements, such as the sugar of milk, due to
the influence of various living germs. This trouble is, then, a type
fermentation, and is, therefore, much more widely distributed than it
would be if it was caused by a single specific organism. These
gas-producing organisms are to be found, sparingly at least, in almost
all milks, but are normally held in check by the ordinary lactic
species. Among them are a large number of the bacteria, although yeasts
and allied germs are often present and are likewise able to set up
fermentative changes of this sort. In these cases the milk-sugar is
decomposed in such a way as to give off CO_{2} and H, and in some cases,
alcohol. Russell and Hastings[210] found a lactose-splitting yeast in a
severe outbreak of gassy cheese in a Swiss factory. In this case the gas
did not develop until the cheese were a few weeks old. In severe cases
the cheese actually cracked to pieces.
According to Guillebeau, a close relation exists between those germs
that are able to produce an infectious inflammation (mastitis) in the
udder of the cow and some forms capable of gas evolution.
If pure cultures of these gas-producing bacteria are added to perfectly
sweet milk, it is possible to artificially produce the conditions in
cheese that so frequently appear in practice.
~Treatment of "pin-holey" curds.~ When this type of fermentation appears
during the manufacture of the cheese, the maker can control it in part
within certain limits. These methods of treatment are, as a rule, purely
mechanical, as when the curds are piled and turned, and subsequently
ground in a curd mill. After the gas has been forced out, the curds are
then put to press and the whole mats into a compact mass.
Another method of treatment based upon bacteriological principles is the
addition of a starter to induce the formation of acid. Where acid is
developed as a result of the growth of the lactic-acid bacteria, the
gas-producing species do not readily thrive. Another reason why acid
aids in repressing the development of gas is that the curd particles are
partially softened or digested by the action of the acid. This causes
them to mat together more closely, and there is not left in the cheese
the irregular mechanical openings in which the developing gas may find
lodgment.
Another method that is also useful with these curds is to employ salt.
This represses gaseous fermentations, and the use of more salt than
usual in making the cheese will very often restrain the production of
gas. Tendency to form gas in Edam cheese is controlled by the addition
of a starter prepared from slimy whey (lange wei) which is caused by the
development of an acid-forming organism.
Some have recommended the custom of washing the curds to remove the whey
and the gas-producing bacteria contained therein. Care must be taken not
to carry this too far, for the removal of the sugar permits
taint-producing organisms to thrive.[211]
The temperature at which the cheese is cured also materially affects the
development of gas. At high curing temperatures, gas-producing organisms
develop rapidly; therefore more trouble is experienced in summer than at
other seasons.
If milks which are prone to undergo "gassy" development are excluded
from the general supply, it would be possible to eliminate the source of
the entire trouble. To aid in the early recognition of such milks that
are not apparently affected when brought to the factory, fermentation or
curd tests (p. 76) are of great value. The use of this test in the hands
of the factory operator often enables him to detect the exact source of
the trouble, which may frequently be confined to the milk delivered by a
single patron.
~"Fruity" or "sweet" flavor.~ Not infrequently the product of a factory
may acquire during the process of ripening what is known as a "sweet" or
"fruity" flavor. This flavor resembles the odor of fermented fruit or
the bouquet of certain kinds of wine. It has been noted in widely
different sections of the country and its presence bears no relation to
the other qualities of the cheese. The cause of this trouble has
recently been traced[212] to the presence of various kinds of yeasts.
Ordinarily yeasts are rarely present in good cheese, but in cheese
affected with this trouble they abound. The addition of starters made
from yeast cultures resulted in the production of the undesirable
condition.
~Mottled cheese.~ The color of cheese is sometimes cut to that extent that
the cheese presents a wavy or mottled appearance. This condition is apt
to appear if the ripening temperature is somewhat high, or larger
quantities of rennet used than usual. The cause of the defect is
obscure, but it has been demonstrated that the same is communicable if a
starter is made by grating some of this mottled cheese into milk. The
bacteriology of the trouble has not yet been worked out, but the defect
is undoubtedly due to an organism that is able to grow in the ripening
cheese. It has been claimed that the use of a pure lactic ferment as a
starter enables one to overcome this defect.
~Bitter cheese.~ Bitter flavors are sometimes developed in cheese
especially where the ripening process is carried on at a low temperature
in the presence of an excess of moisture for a considerable length of
time.
Guillebeau[213] isolated several forms from Emmenthaler cheese which he
connected with udder inflammation that were able to produce a bitter
substance in cheese.
Von Freudenreich[214] has described a new form _Micrococcus casei amari_
(micrococcus of bitter cheese) that was found in a sample of bitter
cheese. This germ is closely related to Conn's micrococcus of bitter
milk. It develops lactic acid rapidly, coagulating the milk and
producing an intensely bitter taste in the course of one to three days.
When milk infected with this organism is made into cheese, there is
formed in a few days a decomposition product that imparts a marked
bitter flavor to the cheese.
Harrison[215] has recently found a yeast that grows in the milk and also
in the cheese which produces an undesirable bitter change.
It is peculiar that some of the organisms that are able to produce
bitter products in milk do not retain this property when the milk is
worked up into cheese.
~Putrid or rotten cheese.~ Sometimes cheese undergoes a putrefactive
decomposition in which the texture is profoundly modified and various
foul smelling gases are evolved. These often begin on the exterior as
small circumscribed spots that slowly extend into the cheese, changing
the casein into a soft slimy mass. Then, again, the interior of the
cheese undergoes this slimy decomposition. The soft varieties are more
prone toward this fermentation than the hard, although the firm cheeses
are by no means exempt from the trouble. The "Verlaufen" or "running" of
limburger cheese is a fermentation allied to this. It is where the
inside of the cheese breaks down into a soft semi-fluid mass. In severe
cases, the rind may even be ruptured, in which case the whole interior
of the cheese flows out as a thick slimy mass, having sometimes a putrid
odor. The conditions favoring this putrid decomposition are usually
associated with an excess of moisture, and an abnormally low ripening
temperature.
~Rusty spot.~ This name is applied to the development of small
yellowish-red or orange spots that are formed sometimes throughout the
whole mass of cheddar cheese. A close inspection shows the colored
points to be located along the edges of the curd particles. According to
Harding,[216] this trouble is most common in spring and fall. The cause
of the difficulty has been traced by Connell[217] to the development of
a chromogenic bacterium, _Bacillus rudensis_. The organism can be most
readily isolated on a potato surface rather than with the usual
isolating media, agar or gelatin.
~Other pigment changes.~ Occasionally, with the hard type of cheese, but
more frequently with the softer foreign varieties, various abnormal
conditions arise that are marked by the production of different pigments
in or on the cheese. More frequently these are merely superficial and
affect only the outer layers of the cheese. Generally they are
attributable to the development of certain chromogenic organisms
(bacteria, molds and yeasts), although occasionally due to other causes,
as in the case of a blue discoloration sometimes noted in foreign cheese
made in copper kettles.[218]
De Vries[219] has described a blue condition that is found in Edam
cheese. It appears first as a small blue spot on the inside, increasing
rapidly in size until the whole mass is affected. This defect he was
able to show was produced by a pigment-forming organism, _B.
cyaneo-fuscus_. By the use of slimy whey (lange wei) this abnormal
change was controlled.
~Moldy cheese.~ With many varieties of cheese, especially some of the
foreign types, the presence of mold on the exterior is not regarded as
detrimental; in fact a limited development is much desired. In hard
rennet cheese as cheddar or Swiss, the market demands a product free
from mold, although it should be said that this condition is imposed by
the desire to secure a good-looking cheese rather than any injury in
flavor that the mold causes. Mold spores are so widely distributed that,
if proper temperature and moisture conditions prevail, these spores will
always develop. At temperatures in the neighborhood of 40° F. and
below, mold growth is exceedingly slow, and often fructification does
not occur, the only evidence of the mold being the white, felt-like
covering that is made up of the vegetating filaments. The use of
paraffin has been suggested as a means of overcoming this growth, the
cheese being dipped at an early stage into melted paraffin. Recent
experiments have shown that "off" flavors sometimes develop where cheese
are paraffined directly from the press. If paraffin is too hard, it has
a tendency to crack and separate from the rind, thus allowing molds to
develop beneath the paraffin coat, where the conditions are ideal as to
moisture, for evaporation is excluded and the air consequently
saturated. The use of formalin (2% solution) has been suggested as a
wash for the outside of the cheese. This substance or sulfur is also
applied in a gaseous form. Double bandaging is also resorted to as a
means of making the cheese more presentable through the removal of the
outer bandage.
The nature of these molds has not been thoroughly studied as yet. The
ordinary blue-green bread mold, _Penicillium glaucum_, is most
frequently found, but there are numerous other forms that appear,
especially at low temperatures.
~Poisonous cheese.~ Cases of acute poisoning arising from the ingestion of
cheese are reported from time to time. Vaughan has succeeded in showing
that this condition is due to the formation of a highly poisonous
alkaloid which he has isolated, and which he calls _tyrotoxicon_.[220]
This poisonous ptomaine has also been demonstrated in milk and other
milk products, and is undoubtedly due to the development of various
putrefactive bacteria that find their way into the milk. It seems quite
probable that the development of these toxic organisms can also go on
in the cheese after it is taken from the press.
~Prevention or cheese defects.~ The defective conditions previously
referred to can rarely be overcome in cheese so as to improve the
affected product, for they only become manifest in most cases during the
later stages of the curing process. The only remedy against future loss
is to recognize the conditions that are apt to prevail during the
occurrence of an outbreak and see that the cheese are handled in such a
way as to prevent a recurrence of the difficulty.
Many abnormal and undesirable results are incident to the manufacture of
the product, such as "sour" or "mealy" cheese, conditions due to the
development of too much acid in the milk or too high a "cook." These are
under the direct control of the maker and for them he alone is
responsible. The development of taints due to the growth of unwelcome
bacteria that have gained access to the milk while it is yet on the farm
are generally beyond the control of the cheese maker, unless they are so
pronounced as to appear during the handling of the curds. If this does
occur he is sometimes able, through the intervention of a starter or by
varying some detail in making, to handle the milk in such a way as to
minimize the trouble, but rarely is he able to eliminate it entirely.
One of the most strenuous duties which the maker must perform at all
times is to point out to his patrons the absolute necessity of their
handling the milk in such a way as to prevent the introduction of
organisms of a baleful type.
FOOTNOTES:
[178] Russell, 13 Rept. Wis. Expt. Stat., 1896, p. 112; Campbell, Trans.
High. & Agr. Soc. Scotland, 5 ser., 1898, 10:181.
[179] Winkler, Milch Zeit. (Hildesheim), Nov. 24, 1900.
[180] Campbell, No. Brit., Agric., May 12, 1897.
[181] Weigmann, Milch Zeit., No. 50, 1889.
[182] Klein, Milch Zeit. (Hildesheim), No. 17, 1900.
[183] Adametz, Landw. Jahr., 18:256.
[184] Van Slyke and Hart, Bull. 214, N. Y. Expt. Stat., July 1902.
[185] Milch Zeit., 1898, No. 49.
[186] Lafar, Technical Mycology, p. 216.
[187] Adametz, Landw. Jahr., 18:228.
[188] Freudenreich, Landw. Jahr. d. Schweiz, 4:17; 5:16.
[189] Russell, 13 Rept. Wis. Expt. Stat., 1896, p. 95.
[190] Harrison and Connell, Rev. gen. du Lait, Nos. 4, 5, 6, 7 and 8,
1903-04.
[191] Lloyd, Bath and West of Eng. Soc. Rept., 1892, 2:180.
[192] Freudenreich, Landw. Jahr. d. Schweiz, 1900; Adametz, Oest. Molk.
Zeit., 1899, No. 7.
[193] Russell, 14 Wis. Expt. Stat., 1897, p. 203. Harrison and Connell,
Rev. gen. du Lait Nos. 4, etc., 1903-04.
[194] Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901. Dean,
Harrison and Harcourt, Bull. 121, Ont. Agr'l. Coll., June 1902.
[195] Schaffer, Milch Zeit., 1889, p. 146.
[196] Adametz, Landw. Jahr., 18:261.
[197] Duclaux, Le Lait, p. 213.
[198] Adametz, Oest. Molk. Zeit., 1900, Nos. 16-18.
[199] Freudenreich, Landw. Jahr. d. Schweiz, 1897, p. 85.
[200] Weigmann, Cent. f. Bakt., II Abt., 1898, 4:593; also 1899, 5:630.
[201] Gorini, Abs. in Expt. Stat. Rec., 11:388.
[202] Babcock and Russell, 14 Rept. Wis. Expt. Stat., 1897, p. 161.
[203] Jensen, Cent. f. Bakt., II Abt., 3:752.
[204] Freudenreich, Cent. f. Bakt., II Abt., 1900, 6:332.
[205] Jensen, Ibid., 1900, 6:734.
[206] 17 Rept. Wis. Expt. Stat., 1900, p. 102.
[207] Jensen, Landw. Jahr. d. Schweiz, 1900.
[208] Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901.
[209] Cent. f. Bakt. 1899, p. 14.
[210] Bull. 128, Wis. Expt. Stat., Sept. 1905.
[211] Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901.
[212] Harding, Rogers and Smith, Bull. 183, N. Y. (Geneva) Expt. Stat.,
Dec., 1900.
[213] Guillebeau, Landw. Jahr., 1890, p. 27.
[214] Freudenreich, Füehl. Landw. Ztg., 43:361.
[215] Harrison, Bull. 123 Ont. Agr'l. Coll., May, 1902.
[216] Bull. 183, N. Y. (Geneva) Expt. Stat., Dec. 1900.
[217] Connell, Bull. Canadian Dept. of Agr., 1897.
[218] Schmöger, Milch Zeit., 1883, p. 483.
[219] De Vries, Milch Zeit., 1888, pp. 861, 885.
[220] Zeit. f. physiol. Chemie, 10:146.
INDEX.
Acid, effect of, on churning, 137;
in butter-making, 138.
Acid test, 52.
Aeration of milk, 59.
Aerobic bacteria, 7.
Alcoholic fermentation in milk, 72.
Anaerobic bacteria, 7.
Animal, influence of, on milk infection, 34.
Animal odor, 56.
Anthrax, 94.
Antiseptics, 9, 88.
Aroma, of butter, 140.
Bacillus: definition of, 2.
_acidi lactici_, 64;
_cyaneo-fuscus_, 188;
_cyanogenus_, 74;
_foetidus lactis_, 157;
_lactis aerogenes_, 65;
_lactis erythrogenes_, 74;
_lactis saponacei_, 67;
_lactis viscosus_, 71;
_nobilis_, 162, 174;
_prodigiosus_, 74;
_rudensis_, 188;
_synxanthus_, 75;
_tuberculosis_, 84.
Bacteria:
on hairs, 35;
kinds in milk, 63;
in barn air, 42;
in milk pails, 27;
in butter, 154;
classification of, 4;
in cheese, 160;
culture of, 17;
in cream, 128;
discovery of, 1;
external conditions affecting, 8;
form of, 2;
in butter, 142;
in butter-making, 127;
in centrifuge slime, 39;
In fore milk, 28;
in rennet, 163;
In separator slime, 39;
manure, 37;
number of, in milk, 50.
Distribution of:
milk of American cities, 50;
European cities, 50;
in relation to cheese, 168.
Of disease:
anthrax, 94;
cholera, 98;
diphtheria, 99;
lockjaw, 94;
toxic, 100;
tuberculosis, 84;
typhoid fever, 98.
Methods of study of:
culture, 15;
culture media, 13;
isolation, 14.
Bitter butter, 158;
cheese, 189;
milk, 72.
Bloody milk, 74.
Blue cheese, 191;
milk, 74.
Bovine tuberculosis, 84.
Brie cheese, 182.
Butter:
bacteria in, 154;
bitter, 158;
"cowy," 157;
fishy, 159;
lardy, 157;
moldy, 158;
mottled, 156;
oily, 158;
putrid, 156;
rancid, 155;
tallowy, 157;
turnip flavor in, 157.
Making:
aroma, 140;
flavor in, 140;
pure culture, 143;
in ripening of cream, 136.
Butyric acid fermentation, 69.
By-products of factory, methods of preserving, 25.
Casease, 68.
Caseone, 68.
Centrifugal force, cleaning milk by, 38.
Cheese:
bacterial flora of, 168;
bitter, 189;
blue, 187;
Brie, 182;
Edam, 72, 162;
Emmenthaler, 185;
flavor of, 179;
gassy fermentations in, 183;
Gorgonzola, 180;
molds on, 191;
mottled, 189;
"nissler," 185;
poisonous, 192;
putrid, 190;
ripening of moldy, 180;
ripening of soft, 181;
Roquefort, 180;
rusty spot in, 188;
Stilton, 180;
Swiss, 185.
Making and curing:
chemical changes in curing, 166;
influence of temperature on curing, 169;
influence of rennet, 177;
physical changes in curing, 165;
prevention of defects, 193;
starters in, 161;
temperature in relation to bacterial influence, 169.
Theories of curing:
digestive, 173;
galactase, 175, 177;
lactic acid, 174.
Chemical changes in cheese-ripening, 166.
Chemical disinfectants in milk:
bleaching powder, 81;
corrosive sublimate, 81;
formalin, 80;
sulfur, 80;
whitewash, 81;
vitriol, 81.
Chemical preservatives, 80.
Children, milk for, 45.
Cholera in milk, 98.
Classification by separator, 38.
Coccus, definition of, 2.
Cold, influence on bacteria, 8, 48.
Contamination of milk through disease germs, 95, 191.
Covered milk pails, 41.
Cream, bacterial changes in, 135;
mechanical causes for bacteria in, 135;
pasteurized, 113;
restoration of consistency of pasteurized, 132.
Ripening of, 136;
advantage of pure cultures in, 144;
by natural starters, 142;
characteristics of pure cultures in, 145;
objections to pure cultures in, 146;
principles of pure cultures in, 143;
propagation of pure cultures, 151;
purity of commercial starters, 150;
home-made starters in, 146.
Creaming methods, 134.
Curd test, 76.
Dairy utensils a source of contamination, 21.
Diarrhoeal diseases, 100.
Digesting bacteria, 67.
Digestibility of heated milk, 111.
Diphtheria, 99.
Dirt in milk, 34.
Dirt, exclusion of, 36.
Disease germs in milk, 95;
effect of heat on, 91;
origin of, 83.
Disinfectants, 9:
carbolic acid, 81;
chloride of lime, 81;
corrosive sublimate, 81;
formalin, 80;
sulfur, 80;
vitriol salts, 81;
whitewash, 79.
Disinfectants in milk:
alkaline salts, 106;
boracic acid, 106;
formalin, 106;
preservaline, 107;
salicylic acid, 106.
Domestic pasteurizing apparatus, 119.
Drugs, taints in milk due to, 56.
Drying, effect of, 8.
Edam cheese, 72, 162.
Emmenthaler cheese, 185.
Endospores, 3.
Enzyms, 10.
Factory by-products, 22;
treatment of, 25.
Farrington alkaline tablet, 52.
Fecal bacteria, effect of, on butter, 35.
Fermentation:
In cheese: gassy, 183.
In milk:
alcoholic, 72;
bitter, 72;
blue, 74;
butyric, 69;
digesting, 67;
gassy, 66;
kephir, 72;
koumiss, 72;
lactic acid, 63;
lange-wei, 72;
red, 74;
ropy, 69;
slimy, 69;
soapy, 73;
souring, 63;
sweet curdling, 67;
treatment of, 75.
Tests, 76;
Gerber's, 76;
Walther's, 76;
Wisconsin curd, 76.
Filtration of milk, 38.
Fishy butter, 159.
Flavor:
of butter, 140;
of cheese, 179.
Foot and mouth disease, 93.
Fore milk, 28.
Formaldehyde, 80.
Formalin, 80.
Fruity flavor in cheese, 188.
Galactase in cheese, 175.
Gassy fermentations:
in cheese, 183;
in milk, 67;
in Swiss cheese, 167.
Gläsler, 185.
Gorgonzola cheese, 180.
Growth of bacteria, essential conditions for, 4;
in milk, 46.
Hair, bacteria on, 35.
Heat, influence on bacterial growth, 8.
Heated milk:
characteristics of, 109;
action toward rennet, 112;
body, 110;
digestibility, 111;
fermentative changes, 111;
flavor, 110;
hydrogen peroxid test in, 23;
Storch's test, 23.
Hygienic milk, bacteria in, 45.
Infection of milk:
animal, 34;
dairy utensils, 21;
fore milk, 28;
milker, 36.
Isolation of bacteria, methods of, 14.
Kephir, 72.
Koumiss, 72.
Lactic acid:
fermentation in milk, 63;
theory in cheese-curing, 174.
Lange-wei, 72.
Lardy butter, 157.
Light, action on bacteria, 9.
Manure, bacteria in, 33.
Methods:
of isolation, 14;
culture, 15.
_Micrococcus casei amari_, 189.
Microscope, use of, 17.
Milk:
a bacterial food medium, 19;
bacteria in, 48.
Disease organisms in:
anthrax, 94;
cholera, 98;
diphtheria, 99;
foot and mouth disease, 93;
poisonous, 101;
ptomaines, 101;
scarlet fever, 99;
tuberculosis, 84;
typhoid fever, 98.
Contamination, 20:
from air, 42;
from animal odors, 55;
dirt, 34;
distinction between bacterial and non-bacterial, 57;
fore milk, 28;
infection in factory, 59;
milker, 36;
relative importance of various kinds, 43;
utensils, 21.
Milk fermentations:
alcoholic, 72;
bitter, 72;
bloody, 74;
blue, 74;
butyric acid, 69;
gassy, 66, 167;
kephir, 72;
koumiss, 72;
lactic acid, 63;
red, 72;
ropy, 69;
slimy, 69;
soapy, 74;
souring, 63;
sweet curdling, 67;
tests for, 76;
treatment of, 75;
yellow, 75.
Milk, heated:
action towards rennet, 112;
digestibility, 111;
flavor of, 110;
fermentative changes in, 111;
hydrogen peroxid test, 110.
Milking machines, influence of, on germ content, 37.
Milk preservation:
chemical agents in, 106;
condensation, 107;
freezing, 108;
heat, 108;
pasteurization, 113;
sterilization, 112.
Milk-sugar as bacterial food, 19.
Mold, in butter, 158;
in cheese, 191.
Mottled cheese, 189.
"Nissler" cheese, 185.
Odors, direct absorption of, in milk, 55.
_Oidium lactis_, 159.
Oily butter, 158.
Pasteurization of milk;
acid test in, 128;
bacteriological study of, 124, 126, 149;
for butter, 147;
for cheese, 162;
for direct use, 113;
of skim milk, 25;
details of, 128;
temperature and time limit in, 118.
Pasteurizing apparatus:
continuous flow, 122;
coolers, 131;
Danish, 123;
domestic, 119;
Farrington, 122;
intermittant flow, 121;
Miller, 122;
Potts, 121;
regenerator, 122;
Reid, 126;
Russell, 121;
testing rate of flow, 124.
_Penicillium glaucum_, 159, 180, 190.
Pepsin, 10.
Physical changes in cheese-ripening, 165.
Poisonous bacteria:
in cheese, 192;
in milk, 100, 101.
Preservaline, 167.
Preservation of milk:
by exclusion, 103;
chemical agents, 106;
condensing, 107;
filtration, 38;
freezing, 108;
pasteurization, 112;
physical agents, 107;
sterilization, 112.
Ptomaine poisoning, 101.
Pure cultures, 15.
Pure culture starters:
advantages of, 144;
characteristics of, 145;
home-made cultures compared with, 146;
propagation of, 151.
Putrid cheese, 190;
butter, 156.
Rancidity in butter, 155.
Red milk, 74.
Rennet:
action in heated milk, 112;
bacteria in, 163;
influence of, on cheese-ripening, 177.
Restoration of consistency in pasteurized cream, 132.
Ripening of cheese:
moldy cheese, 180;
soft cheese, 181.
Of cream, 136;
artificial starters, 143;
natural starters, 142;
principles of pure culture starters in, 143.
Ropy milk, 69.
Roquefort cheese, 180.
Rusty spot in cheese, 190.
Rusty cans: effect of, on acidity, 53.
Sanitary milk, 45, 104.
Sanitary pails, 41.
Scarlet fever in milk, 99.
Separator slime:
bacteria in, 39;
tubercle bacillus in, 93.
Scalded layer, resistance of bacteria in, 91.
Skim-milk, a distributor of disease, 96.
Slimy milk, 69.
Soapy milk, 74.
Soft cheese, ripening of, 186.
Sources of contamination in milk:
barn air, 42;
dairy utensils, 21;
dirt from animals, 34;
factory cans, 25;
fore-milk, 28;
milker, 36.
Souring of milk, 63.
Spirillum, definition of, 2.
Spores, 3.
Starters:
in cheese-making, 161;
in butter-making, 142;
propagation of, 151;
pure cultures in cream-ripening, 143.
Sterilization of milk, 112.
_Streptococcus Hollandicus_, 72, 162.
Stilton cheese, 181.
Storch's test, 23.
Sulfur as a disinfectant, 81.
Sweet curdling milk, 68.
Sweet flavor in cheese, 188.
Swiss cheese, 177;
gassy fermentations in, 24, 185.
Taints, absorption of, 55.
Taints, bacterial vs. physical, 58.
Taints in milk, absorption of, 55.
Taints, use of starters in overcoming, 79.
Taints in butter:
putrid, 156;
rancidity, 155;
turnip flavor, 157.
Tallowy butter, 157.
Temperature:
effect on bacterial development, 6, 48;
effect of low, 108;
effect of high, 108;
and time limit in milk pasteurization, 113.
Tests for milk:
fermentation, 76;
Storch's, 23;
acid, 52.
Theories in cheese-curing:
digestive, 171;
galactase, 175, 177;
lactic acid, 174.
Trypsin, 10.
Tubercle bacillus:
in milk, 88;
in separator slime, 93;
thermal death limits, 117.
Tuberculin test, 86.
Tuberculosis, bovine, 84.
Turnip flavor in butter, 157.
Typhoid fever, 98.
Tyrogen, 162.
Tyrotoxicon, 101, 190.
Udder:
artificial introduction of bacteria into, 32;
milk germ-free in, 19;
infection of, 28;
washing, 89;
tuberculosis in, 87.
Viscogen, 132.
Water: as a source of infection, 61.
Whey, pollution of vats, 23;
method of preserving, 25;
treatment of, in vats, 25.
Whitewash, 81.
Wisconsin curd test, 76.
Yeasts:
alcoholic ferments in milk, 73;
fruity flavor in cheese, 186;
gassy due to yeasts, 186;
in bitter cheese, 189;
in canned butter, 159;
kephir, 72.
End of the Project Gutenberg EBook of Outlines of Dairy Bacteriology, 8th
edition, by H. L. Russell
*** END OF THIS PROJECT GUTENBERG EBOOK DAIRY BACTERIOLOGY, 8TH EDITION ***
***** This file should be named 27778-8.txt or 27778-8.zip *****
This and all associated files of various formats will be found in:
https://www.gutenberg.org/2/7/7/7/27778/
Produced by Mark C. Orton, Linda McKeown, Josephine Paolucci
and the Online Distributed Proofreading Team at
https://www.pgdp.net.
Updated editions will replace the previous one--the old editions
will be renamed.
Creating the works from public domain print editions means that no
one owns a United States copyright in these works, so the Foundation
(and you!) can copy and distribute it in the United States without
permission and without paying copyright royalties. Special rules,
set forth in the General Terms of Use part of this license, apply to
copying and distributing Project Gutenberg-tm electronic works to
protect the PROJECT GUTENBERG-tm concept and trademark. Project
Gutenberg is a registered trademark, and may not be used if you
charge for the eBooks, unless you receive specific permission. If you
do not charge anything for copies of this eBook, complying with the
rules is very easy. You may use this eBook for nearly any purpose
such as creation of derivative works, reports, performances and
research. They may be modified and printed and given away--you may do
practically ANYTHING with public domain eBooks. Redistribution is
subject to the trademark license, especially commercial
redistribution.
*** START: FULL LICENSE ***
THE FULL PROJECT GUTENBERG LICENSE
PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK
To protect the Project Gutenberg-tm mission of promoting the free
distribution of electronic works, by using or distributing this work
(or any other work associated in any way with the phrase "Project
Gutenberg"), you agree to comply with all the terms of the Full Project
Gutenberg-tm License (available with this file or online at
https://gutenberg.org/license).
Section 1. General Terms of Use and Redistributing Project Gutenberg-tm
electronic works
1.A. By reading or using any part of this Project Gutenberg-tm
electronic work, you indicate that you have read, understand, agree to
and accept all the terms of this license and intellectual property
(trademark/copyright) agreement. If you do not agree to abide by all
the terms of this agreement, you must cease using and return or destroy
all copies of Project Gutenberg-tm electronic works in your possession.
If you paid a fee for obtaining a copy of or access to a Project
Gutenberg-tm electronic work and you do not agree to be bound by the
terms of this agreement, you may obtain a refund from the person or
entity to whom you paid the fee as set forth in paragraph 1.E.8.
1.B. "Project Gutenberg" is a registered trademark. It may only be
used on or associated in any way with an electronic work by people who
agree to be bound by the terms of this agreement. There are a few
things that you can do with most Project Gutenberg-tm electronic works
even without complying with the full terms of this agreement. See
paragraph 1.C below. There are a lot of things you can do with Project
Gutenberg-tm electronic works if you follow the terms of this agreement
and help preserve free future access to Project Gutenberg-tm electronic
works. See paragraph 1.E below.
1.C. The Project Gutenberg Literary Archive Foundation ("the Foundation"
or PGLAF), owns a compilation copyright in the collection of Project
Gutenberg-tm electronic works. Nearly all the individual works in the
collection are in the public domain in the United States. If an
individual work is in the public domain in the United States and you are
located in the United States, we do not claim a right to prevent you from
copying, distributing, performing, displaying or creating derivative
works based on the work as long as all references to Project Gutenberg
are removed. Of course, we hope that you will support the Project
Gutenberg-tm mission of promoting free access to electronic works by
freely sharing Project Gutenberg-tm works in compliance with the terms of
this agreement for keeping the Project Gutenberg-tm name associated with
the work. You can easily comply with the terms of this agreement by
keeping this work in the same format with its attached full Project
Gutenberg-tm License when you share it without charge with others.
1.D. The copyright laws of the place where you are located also govern
what you can do with this work. Copyright laws in most countries are in
a constant state of change. If you are outside the United States, check
the laws of your country in addition to the terms of this agreement
before downloading, copying, displaying, performing, distributing or
creating derivative works based on this work or any other Project
Gutenberg-tm work. The Foundation makes no representations concerning
the copyright status of any work in any country outside the United
States.
1.E. Unless you have removed all references to Project Gutenberg:
1.E.1. The following sentence, with active links to, or other immediate
access to, the full Project Gutenberg-tm License must appear prominently
whenever any copy of a Project Gutenberg-tm work (any work on which the
phrase "Project Gutenberg" appears, or with which the phrase "Project
Gutenberg" is associated) is accessed, displayed, performed, viewed,
copied or distributed:
This eBook is for the use of anyone anywhere at no cost and with
almost no restrictions whatsoever. You may copy it, give it away or
re-use it under the terms of the Project Gutenberg License included
with this eBook or online at www.gutenberg.org
1.E.2. If an individual Project Gutenberg-tm electronic work is derived
from the public domain (does not contain a notice indicating that it is
posted with permission of the copyright holder), the work can be copied
and distributed to anyone in the United States without paying any fees
or charges. If you are redistributing or providing access to a work
with the phrase "Project Gutenberg" associated with or appearing on the
work, you must comply either with the requirements of paragraphs 1.E.1
through 1.E.7 or obtain permission for the use of the work and the
Project Gutenberg-tm trademark as set forth in paragraphs 1.E.8 or
1.E.9.
1.E.3. If an individual Project Gutenberg-tm electronic work is posted
with the permission of the copyright holder, your use and distribution
must comply with both paragraphs 1.E.1 through 1.E.7 and any additional
terms imposed by the copyright holder. Additional terms will be linked
to the Project Gutenberg-tm License for all works posted with the
permission of the copyright holder found at the beginning of this work.
1.E.4. Do not unlink or detach or remove the full Project Gutenberg-tm
License terms from this work, or any files containing a part of this
work or any other work associated with Project Gutenberg-tm.
1.E.5. Do not copy, display, perform, distribute or redistribute this
electronic work, or any part of this electronic work, without
prominently displaying the sentence set forth in paragraph 1.E.1 with
active links or immediate access to the full terms of the Project
Gutenberg-tm License.
1.E.6. You may convert to and distribute this work in any binary,
compressed, marked up, nonproprietary or proprietary form, including any
word processing or hypertext form. However, if you provide access to or
distribute copies of a Project Gutenberg-tm work in a format other than
"Plain Vanilla ASCII" or other format used in the official version
posted on the official Project Gutenberg-tm web site (www.gutenberg.org),
you must, at no additional cost, fee or expense to the user, provide a
copy, a means of exporting a copy, or a means of obtaining a copy upon
request, of the work in its original "Plain Vanilla ASCII" or other
form. Any alternate format must include the full Project Gutenberg-tm
License as specified in paragraph 1.E.1.
1.E.7. Do not charge a fee for access to, viewing, displaying,
performing, copying or distributing any Project Gutenberg-tm works
unless you comply with paragraph 1.E.8 or 1.E.9.
1.E.8. You may charge a reasonable fee for copies of or providing
access to or distributing Project Gutenberg-tm electronic works provided
that
- You pay a royalty fee of 20% of the gross profits you derive from
the use of Project Gutenberg-tm works calculated using the method
you already use to calculate your applicable taxes. The fee is
owed to the owner of the Project Gutenberg-tm trademark, but he
has agreed to donate royalties under this paragraph to the
Project Gutenberg Literary Archive Foundation. Royalty payments
must be paid within 60 days following each date on which you
prepare (or are legally required to prepare) your periodic tax
returns. Royalty payments should be clearly marked as such and
sent to the Project Gutenberg Literary Archive Foundation at the
address specified in Section 4, "Information about donations to
the Project Gutenberg Literary Archive Foundation."
- You provide a full refund of any money paid by a user who notifies
you in writing (or by e-mail) within 30 days of receipt that s/he
does not agree to the terms of the full Project Gutenberg-tm
License. You must require such a user to return or
destroy all copies of the works possessed in a physical medium
and discontinue all use of and all access to other copies of
Project Gutenberg-tm works.
- You provide, in accordance with paragraph 1.F.3, a full refund of any
money paid for a work or a replacement copy, if a defect in the
electronic work is discovered and reported to you within 90 days
of receipt of the work.
- You comply with all other terms of this agreement for free
distribution of Project Gutenberg-tm works.
1.E.9. If you wish to charge a fee or distribute a Project Gutenberg-tm
electronic work or group of works on different terms than are set
forth in this agreement, you must obtain permission in writing from
both the Project Gutenberg Literary Archive Foundation and Michael
Hart, the owner of the Project Gutenberg-tm trademark. Contact the
Foundation as set forth in Section 3 below.
1.F.
1.F.1. Project Gutenberg volunteers and employees expend considerable
effort to identify, do copyright research on, transcribe and proofread
public domain works in creating the Project Gutenberg-tm
collection. Despite these efforts, Project Gutenberg-tm electronic
works, and the medium on which they may be stored, may contain
"Defects," such as, but not limited to, incomplete, inaccurate or
corrupt data, transcription errors, a copyright or other intellectual
property infringement, a defective or damaged disk or other medium, a
computer virus, or computer codes that damage or cannot be read by
your equipment.
1.F.2. LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right
of Replacement or Refund" described in paragraph 1.F.3, the Project
Gutenberg Literary Archive Foundation, the owner of the Project
Gutenberg-tm trademark, and any other party distributing a Project
Gutenberg-tm electronic work under this agreement, disclaim all
liability to you for damages, costs and expenses, including legal
fees. YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
PROVIDED IN PARAGRAPH F3. YOU AGREE THAT THE FOUNDATION, THE
TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
DAMAGE.
1.F.3. LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
defect in this electronic work within 90 days of receiving it, you can
receive a refund of the money (if any) you paid for it by sending a
written explanation to the person you received the work from. If you
received the work on a physical medium, you must return the medium with
your written explanation. The person or entity that provided you with
the defective work may elect to provide a replacement copy in lieu of a
refund. If you received the work electronically, the person or entity
providing it to you may choose to give you a second opportunity to
receive the work electronically in lieu of a refund. If the second copy
is also defective, you may demand a refund in writing without further
opportunities to fix the problem.
1.F.4. Except for the limited right of replacement or refund set forth
in paragraph 1.F.3, this work is provided to you 'AS-IS' WITH NO OTHER
WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
WARRANTIES OF MERCHANTIBILITY OR FITNESS FOR ANY PURPOSE.
1.F.5. Some states do not allow disclaimers of certain implied
warranties or the exclusion or limitation of certain types of damages.
If any disclaimer or limitation set forth in this agreement violates the
law of the state applicable to this agreement, the agreement shall be
interpreted to make the maximum disclaimer or limitation permitted by
the applicable state law. The invalidity or unenforceability of any
provision of this agreement shall not void the remaining provisions.
1.F.6. INDEMNITY - You agree to indemnify and hold the Foundation, the
trademark owner, any agent or employee of the Foundation, anyone
providing copies of Project Gutenberg-tm electronic works in accordance
with this agreement, and any volunteers associated with the production,
promotion and distribution of Project Gutenberg-tm electronic works,
harmless from all liability, costs and expenses, including legal fees,
that arise directly or indirectly from any of the following which you do
or cause to occur: (a) distribution of this or any Project Gutenberg-tm
work, (b) alteration, modification, or additions or deletions to any
Project Gutenberg-tm work, and (c) any Defect you cause.
Section 2. Information about the Mission of Project Gutenberg-tm
Project Gutenberg-tm is synonymous with the free distribution of
electronic works in formats readable by the widest variety of computers
including obsolete, old, middle-aged and new computers. It exists
because of the efforts of hundreds of volunteers and donations from
people in all walks of life.
Volunteers and financial support to provide volunteers with the
assistance they need are critical to reaching Project Gutenberg-tm's
goals and ensuring that the Project Gutenberg-tm collection will
remain freely available for generations to come. In 2001, the Project
Gutenberg Literary Archive Foundation was created to provide a secure
and permanent future for Project Gutenberg-tm and future generations.
To learn more about the Project Gutenberg Literary Archive Foundation
and how your efforts and donations can help, see Sections 3 and 4
and the Foundation web page at https://www.pglaf.org.
Section 3. Information about the Project Gutenberg Literary Archive
Foundation
The Project Gutenberg Literary Archive Foundation is a non profit
501(c)(3) educational corporation organized under the laws of the
state of Mississippi and granted tax exempt status by the Internal
Revenue Service. The Foundation's EIN or federal tax identification
number is 64-6221541. Its 501(c)(3) letter is posted at
https://pglaf.org/fundraising. Contributions to the Project Gutenberg
Literary Archive Foundation are tax deductible to the full extent
permitted by U.S. federal laws and your state's laws.
The Foundation's principal office is located at 4557 Melan Dr. S.
Fairbanks, AK, 99712., but its volunteers and employees are scattered
throughout numerous locations. Its business office is located at
809 North 1500 West, Salt Lake City, UT 84116, (801) 596-1887, email
[email protected]. Email contact links and up to date contact
information can be found at the Foundation's web site and official
page at https://pglaf.org
For additional contact information:
Dr. Gregory B. Newby
Chief Executive and Director
[email protected]
Section 4. Information about Donations to the Project Gutenberg
Literary Archive Foundation
Project Gutenberg-tm depends upon and cannot survive without wide
spread public support and donations to carry out its mission of
increasing the number of public domain and licensed works that can be
freely distributed in machine readable form accessible by the widest
array of equipment including outdated equipment. Many small donations
($1 to $5,000) are particularly important to maintaining tax exempt
status with the IRS.
The Foundation is committed to complying with the laws regulating
charities and charitable donations in all 50 states of the United
States. Compliance requirements are not uniform and it takes a
considerable effort, much paperwork and many fees to meet and keep up
with these requirements. We do not solicit donations in locations
where we have not received written confirmation of compliance. To
SEND DONATIONS or determine the status of compliance for any
particular state visit https://pglaf.org
While we cannot and do not solicit contributions from states where we
have not met the solicitation requirements, we know of no prohibition
against accepting unsolicited donations from donors in such states who
approach us with offers to donate.
International donations are gratefully accepted, but we cannot make
any statements concerning tax treatment of donations received from
outside the United States. U.S. laws alone swamp our small staff.
Please check the Project Gutenberg Web pages for current donation
methods and addresses. Donations are accepted in a number of other
ways including including checks, online payments and credit card
donations. To donate, please visit: https://pglaf.org/donate
Section 5. General Information About Project Gutenberg-tm electronic
works.
Professor Michael S. Hart was the originator of the Project Gutenberg-tm
concept of a library of electronic works that could be freely shared
with anyone. For thirty years, he produced and distributed Project
Gutenberg-tm eBooks with only a loose network of volunteer support.
Project Gutenberg-tm eBooks are often created from several printed
editions, all of which are confirmed as Public Domain in the U.S.
unless a copyright notice is included. Thus, we do not necessarily
keep eBooks in compliance with any particular paper edition.
Most people start at our Web site which has the main PG search facility:
https://www.gutenberg.org
This Web site includes information about Project Gutenberg-tm,
including how to make donations to the Project Gutenberg Literary
Archive Foundation, how to help produce our new eBooks, and how to
subscribe to our email newsletter to hear about new eBooks.
