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Title: Outlines of dairy bacteriology
A concise manual for the use of students in dairying
Author: H. L. Russell
E. G. Hastings
Release Date: May 14, 2010 [EBook #32367]
Language: English
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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
AND
E. G. HASTINGS
Professor of Agricultural Bacteriology
University of Wisconsin
_TENTH EDITION_
MADISON, WISCONSIN
H. L. RUSSELL
1914
Copyright 1914
BY
H. L. RUSSELL AND E. G. HASTINGS
PREFACE TO THE TENTH EDITION.
This text was originally the outgrowth of a series of lectures on
the subject of dairy bacteriology to practical students in the
winter Dairy Course in the University of Wisconsin. The importance
of bacteriology in dairy processes has now come to be so widely
recognized that no student of dairying regards his training as
complete until he has had the fundamental principles of this
subject.
The aim of this volume is not to furnish an exhaustive treatise of
the subject, but an outline and sufficient detail to enable the
general student of dairying to obtain as comprehensive an idea of
the bacteria and their effects on milk and other dairy products as
may be possible without the aid of laboratory practice. When
possible the dairy student is urged to secure a laboratory knowledge
of these organisms, but lacking this, the student and general reader
should secure a general survey of the field of bacteriology in
relation to dairying.
In this, the tenth edition, the effort has been made to include all
of the recent developments of the subject. Especially is this true
in regard to the subject of market milk, a phase of dairying that
has gained greatly in importance in the last few years. The changes
in the methods of handling market milk have been marked. The results
of these changes in influencing the quality of milk offered to the
consumer are fully discussed.
H. L. R.
E. G. H.
CONTENTS
Structure, Growth and Distribution of Bacteria 7
Methods of Studying Bacteria 20
Contamination of Milk 28
Infection of Milk with Pathogenic Bacteria 62
Fermentations of Milk 82
Preservation of Milk 113
Bacteria and Butter Making 136
Bacteria and Cheese Making 161
Bacteria in Market Milk 189
CHAPTER I.
STRUCTURE, GROWTH AND DISTRIBUTION.
=Relation of bacteriology to dairying.= The arts which have been
developed by mankind have been the outgrowth of experience. Man
first learned by doing, _how_ to perform these various activities,
and a scientific knowledge of the underlying principles which govern
these processes was later developed.
The art of dairying has been practiced from time immemorial, but a
correct understanding of the fundamental principles on which the
practice of dairying rests is of recent origin. In working out these
principles, chemistry has been of great service, but in later years,
bacteriology has also been most successfully applied to the problems
of modern dairying. Indeed, it may be said that the science of
dairying, as related to the problems of dairy manufacture is, in
large degree, dependent upon an understanding of bacteriological
principles. It is therefore essential that the student of dairying,
even though he is concerned in large measure with the practical
aspects of the subject, should acquire as complete an understanding
of these principles as possible.
While bacteriology is concerned primarily with the activities of
those microscopic forms of plant life known as the bacteria, yet the
general principles governing the life of this particular class of
organisms are sufficiently similar to those governing the molds and
other types of microscopic life that affect milk and its products to
make it possible to include all of these types in a general
consideration of the subject.
=Nature of bacteria.= The vegetable kingdom to which the bacteria
belong consists of plants of the most varying size and nature. Those
of most common acquaintance are the green plants varying in size
from those not visible to the naked eye to the largest trees.
Another class of plants known as fungi or fungous plants do not
contain chlorophyll, the green coloring matter, but are usually
colorless and, as a rule, of small size; among them are included
such forms as the mushrooms, smuts, rusts and mildews, as well as
the molds and yeasts. The bacteria are closely allied to this latter
class. When first discovered they were thought to be animals because
of the ability of some forms to move about in liquids.
The bacteria, like other kinds of living organisms, possess a
definite form and shape. They are the simplest in structure of all
the plants, the individual organism consisting of a single cell. The
larger and more highly organized forms of life are made up of many
microscopic cells, and the life of the individual consists of the
work of all the cells. The bacteria are very comparable to the
single cells of the higher plants and animals, but in the case of
the bacteria the single cell is able to exist apart from all other
cells and to carry out all of its life processes including
reproduction.
=Forms of bacteria.= With the multicellular organisms much variation
in form is possible, but with these single-celled organisms the
possible variation in form is greatly limited. Three well marked
types occur among the bacteria: the round or coccus form (plural
cocci); the rod-shaped or bacillus (plural bacilli); and the
twisted or spirillum type (plural spirilla). Most organisms of
special significance in dairying belong to the coccus or bacillus
group.
=Size of bacteria.= The bacteria, as a class, are among the smallest
of living objects. None of them are individually visible to the
naked eye, and they can be so seen only when clumps or masses are
formed in the process of growth.
[Illustration: Fig. 1.--Forms of Bacteria. A, coccus; B, bacillus;
C, spirillum.]
While there is considerable relative variation in size, yet in
actual dimensions, this difference is so small as to make careful
microscopic determinations necessary. An average diameter may be
taken as about one thirty-thousandth of an inch, while the length
varies naturally several fold, depending upon whether the type under
observation is a coccus or a bacillus.
It is very difficult to conceive of the minuteness of the bacteria;
the following may give some idea of their size. In a drop of cream
ready for churning may be found as many as 10,000,000 and in a piece
of fresh cheese as large as a cherry there may be as many living
bacteria as there are people on our earth. While the bacteria are
very minute, the effect which they exert in milk and other dairy
products is great on account of their enormous numbers.
=Manner of growth.= The cells of which all plants and animals consist
increase in numbers by the division of each cell into two cells
through the formation of a division wall across the cell. The new
cells divide and the plant or animal continues to grow. The same
cell division occurs in the bacteria but since the bacteria are
single celled, division of the cells means an increase in numbers
rather than growth as in the higher forms of life.
[Illustration: Fig. 2.--Division of Bacteria.
The bacteria increase in numbers by the division of each cell into
two cells. (After Novy.)]
In the case of those bacteria that have a greater length than
diameter, the new wall is formed at right angles to the long axis of
the cell. As soon as the division is complete each cell is a
complete individual, capable of carrying on all of its life
processes. The cells may, however, cohere and thus form distinctive
groupings that may serve to identify certain types. Some of the
cocci form long chains and the term _streptococcus_ is applied to
such. Other groupings may be similar to a bale of twine or they may
be massed in clusters with no regularity distinguishable.
=Spores.= Just as ordinary plants form resistant structures, known as
seeds, capable of retaining vitality under conditions unfavorable
for growth thereby perpetuating the species, so with certain of the
bacteria, definite structures, known as _spores_, that are analogous
in some respects to the seeds of the higher plants, are produced
within the mother cell. The spores are exceedingly resistant to the
influence of an unfavorable environment, such as heat, cold, drying,
and even chemical agents. It is this property of the spores which
makes it so difficult to destroy the bacterial life in the process
of sterilizing milk. The property of spore-formation is fortunately
confined to a comparatively small number of different species of
bacilli.
=Movement.= Many of the bacteria are provided with vibratory organs of
locomotion, known as _cilia_ (singular cilium) which are variously
distributed on the surface of the cell. By the movement of these
relatively long, thread-like appendages the individual cell is able
to move in liquids. It must be remembered, when these moving cells
are observed under the microscope, that their apparent rate of
movement is magnified relatively as much as their size.
=Conditions for growth.= All kinds of living things need certain
conditions for growth such as food, moisture, air and a favorable
temperature. The bacteria prefer as food such organic matter as
milk, meat, and vegetable infusions. Those living on dead organic
matter are known as _saprophytes_, while those which are capable of
thriving in the tissues of the living plant or animal are known as
_parasites_. Certain of the parasitic forms are capable of causing
disease in plants and animals. In the first group are embraced most
of the bacteria that are able to develop in milk or its products,
such as those forms concerned in the spoiling of milk or its
fermentation. It is true that milk may contain disease-producing
bacteria coming either from a diseased animal or from a diseased
human being. It is also true that some of such harmful forms are
able to grow in milk, such as the organisms causing typhoid fever
and diphtheria.
=Food.= The bacteria like all other plants must have their food in
solution. Where they apparently live on solids, such as meats,
fruits, etc., they dissolve the food substances before utilizing the
same. If the solutions are highly concentrated, as in the case of
syrups, preserves and condensed milk, the bacteria cannot readily
grow, although all of the necessary food ingredients are present.
When such concentrated solutions are diluted, bacterial growth will
take place and the solutions will spoil.
[Illustration: Fig. 3.--Photomicrograph of Lactic Acid Bacteria.
Each cell is an individual organism, magnified 1250 diameters.]
Generally speaking the bacteria grow best in a neutral or slightly
alkaline solution rather than in acid liquids.
=Temperature.= One of the most important conditions influencing the
rate of growth of bacteria is the temperature. Each form has a
_minimum_ temperature below which growth can not take place; also a
_maximum_ above which growth is again impossible. For the majority
of species the minimum temperature ranges from 40 to 45° F. the
maximum from 105 to 110° F. Growth takes place most rapidly at the
optimum temperature, which, for each species, lies close to the
maximum temperature at which growth can occur. Most of the bacteria
of importance in the dairy grow well at from 70 to 100° F.
There are forms that can grow below the freezing point of water when
they are in solutions that do not freeze at this temperature. There
are still other bacteria that can grow at 140° F. a temperature that
is quickly fatal to most forms. These are of importance in the dairy
since they limit the temperatures at which milk can be stored for
long periods of time.
=Air supply.= Living organisms, both plant and animal, require air or
oxygen for the combustion of their food and for the production of
energy. Most bacteria use, as do the green plants and animals, the
free oxygen of the air for their respiration. Such organisms are
called _aerobic_ or air-living. A much smaller group possess the
power of taking oxygen from organic compounds such as sugar and the
like and therefore are able to live under conditions where air is
excluded. These are called _anaerobic_ bacteria. A large number of
bacteria are able to live either in the presence or in the absence
of free oxygen. Most of the bacteria of importance in the dairy are
of this nature.
=Rate of growth.= When there is an abundant supply of food and when
the temperature conditions are favorable, the bacteria increase in
numbers with astounding rapidity. It has been determined by actual
experiment that the process of cell division under favorable
conditions takes place in a few moments. Barber has shown that one
of the forms of bacteria constantly found in milk will divide in 17
minutes at 98° F. and that a single organism kept at this
temperature for ten hours would increase to 1,240,000,000. If the
temperature is reduced to 50° F., the time required for division is
increased to several hours. The explanation for the rapid spoiling
of milk that is not well cooled is thus apparent. The initial rapid
rate of increase cannot be maintained for any length of time as the
conditions become more and more unfavorable as growth continues, due
to the accumulation of the by-products of the cell activity. Thus,
the growth of acid-forming organisms in milk becomes checked by the
formation of acid from the fermentation of the sugar.
=Detrimental effect of external conditions.= Environmental conditions
of a detrimental character are constantly at work tending to repress
the activity of bacteria or to destroy them. These act more readily
on the vegetating cells than on the more resistant spores. It is of
the utmost importance that those engaged in dairy work be familiar
with these antagonistic forces since it is constantly necessary to
repress or to kill outright the bacteria in milk and other dairy
products. In many lines of dairy work it is likewise important to be
familiar with the conditions favorable for bacterial growth.
=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 growing
condition. Numerous non-spore-bearing species remain alive in ice
for a prolonged period, and experiments with liquid air show that
even a temperature of-310° F. maintained for hours does not 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 spore stage.
The temperature at which the vitality of the cell is lost is known
as the _thermal death point_. This limit is dependent not only upon
the nature of the organism, but upon 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 growing organisms, the thermal death point of most species
ranges from 130° to 140° F. for ten minutes. When spores are
present, resistance is greatly increased, some forms being able to
withstand steam at 212° F. from one to three hours. In the
sterilization of milk, it is often necessary to heat for several
hours, where a single exposure is made, to destroy the resistant
spores, that seem to be more abundant under summer than winter
conditions. Steam under pressure is a much more effective agent, as
the temperature is thus raised considerably beyond 212° F. An
exposure of twenty minutes, at a temperature of 230° to 240° F. will
kill all spores. Where heat is used in a dry state, it is much less
effective, a baking temperature of 260° to 300° F. for an hour being
necessary to kill spores. This condition is of the utmost importance
in the destruction of bacteria in the dairy and creamery.
=Effect of drying.= The spore-bearing bacteria withstand
effects of desiccation without serious injury, and many of the
non-spore-producing types retain their vitality for some months. The
bacteria found in the air are practically all derived from the soil,
and exist in the air in a dried condition, in which they are able to
remain alive for considerable periods of time. In a dried condition,
active cell growth is not possible, but when other conditions, such
as moisture and food supply are present, resumption of growth
quickly begins. This property is also of importance in the dairy as
in the preparation of dry starters for creameries and cheese
factories.
=Effect of light.= Bright sunlight exerts a markedly injurious effect
on bacterial life, both in a spore and in a growing condition. Where
the direct sunlight strikes, more or less complete disinfection
results in the course of a few hours, the effect being produced by
the chemical or violet rays, and not by the heat or red rays of the
spectrum. This action, however, does not penetrate opaque objects,
and is therefore confined to the surface. In diffused light, the
effect is much lessened, although it is exerted to some extent.
Sunlight exerts a beneficial effect on the general health and
well-being of animal life, and is a matter of importance to be taken
into consideration in the erection of buildings for animals as well
as for people.
=Effect of chemicals.= A great many chemical substances exert a more
or less powerful toxic action on various kinds of life. Many of
these are of great service in destroying bacteria or holding them 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 large amounts. Disinfectants have
no place in dairy work, except to destroy disease-producing
bacteria, or to preserve milk for analytical purposes. The so-called
chemical preservatives used to "keep" milk depend for their effect
on the inhibition of bacterial growth. In this country, most states
prohibit the use of these substances in milk. Their only function in
the dairy should be to check fermentative and putrefactive processes
outside of milk and so keep the air free from taints.
=Products of growth.= All bacteria, as a result of their growth in
food substances, form more or less characteristic compounds that are
known as _by-products_. The changes brought about are those of
decomposition and are collectively known as _fermentations_; they
are characterized by the production of a large amount of by-products
as the result of the development of a relatively small amount of
cell life. The souring of milk, the rotting of eggs, the spoiling of
meats, the making of vinegar from cider are examples of
fermentations caused by different bacteria.
If the substances decomposed contain but little sugar, as do animal
tissues, the conditions are favorable for the growth of the
putrefactive bacteria, and foul-smelling gases are formed. When
sugars are present, as in milk, the environmental conditions are
most favorable for the acid-forming bacteria that do not as a rule
produce offensive odors.
Many of the bacteria form substances known as enzymes which are able
to produce certain decomposition changes in the absence of the
living cells, and it is by virtue of these enzymes that the
organisms are able to break down such enormous quantities of
organic matter. Most of these enzymes 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 conditions where the
activity of the cell is wholly suspended as in a saturated ether or
chloroform atmosphere. The production of enzymes is not confined to
bacteria, but they are found throughout the animal and plant world,
especially in those processes that are concerned in digestion.
Rennet, used in cheese making, is an example of an animal enzyme.
=Distribution of bacteria.= As bacteria possess greater powers of
resistance than almost any other form of life, they are found very
widely distributed over the surface of the earth. In soil they are
abundant, because of the fact that all of the conditions necessary
for growth are here best satisfied. They are, however, distributed
with reference to the layers of the soil; the soil proper, i.e.,
that turned over by the plow, is extremely rich in them on account
of the abundance of organic matter. But at the depth of a few feet
they decrease rapidly in numbers, and in the deeper layers, from six
to ten feet, or more, they are normally not present, because of the
lack of proper food supply and oxygen. The fertility of the soil is
closely associated with their presence.
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 from the soil 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 generally contains 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 bacteria. 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 contaminated with soil organisms, as the
water issues from the ground. Wells vary in their germ content,
depending upon manner of construction, ease of contamination at
surface, etc. Wells are too frequently insufficiently protected from
surface leachings, and consequently may contain all kinds of
organisms found in the surface soil. Typhoid fever is very
frequently disseminated in this way, as is cholera 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 readily infected.
This is particularly true with reference to the intestinal tract,
and in the undigested residue of the food, bacterial activity is at
a maximum. The result is that fecal matter of all kinds contains
enormous numbers of organisms so that the pollution of any food
medium, such as milk, with such material is sure to introduce
elements that seriously affect its quality.
CHAPTER II.
METHODS OF STUDYING BACTERIA.
=Necessity of artificial cultivation.= The bacteria are so extremely
small, that it is impossible to study individual germs separately
without the aid of powerful microscopes. 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 the resulting mass of cells would be
visible to the unaided eye. This is done by growing the bacteria on
various kinds of nutrient media that are prepared for the purpose,
but inasmuch as bacteria are so universally distributed, it becomes
an impossibility to cultivate any special form alone, unless the
medium in which they are grown is first freed from all pre-existing
forms of germ life.
=Food materials.= Many kinds of food substances are used for the
cultivation of bacteria in the laboratory. In fact, bacteria will
grow on almost any organic substance, whether it is solid or liquid,
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 culture media may be either permanently solid, like potatoes
and coagulated egg, or they may retain their solid properties only
at certain temperatures, like gelatin or agar. The latter two, which
were devised by Robert Koch, are of utmost importance in
bacteriological research, for their use permits the separation of
the different forms of bacteria that may happen to be in any
mixture. Gelatin is advantageously used, 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 80° F. Agar, a gelatinous product derived
from a Japanese seaweed, has a much higher melting point, and is
used especially with those organisms whose optimum temperature for
growth is above the melting point of gelatin.
Besides these solid culture 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.
=Sterilization.= The various ingredients that are used in the
preparation of culture media are not free from micro-organisms,
hence the media would soon spoil if they were not destroyed, and the
media subsequently protected from contamination from the air, etc.
The process of rendering the media free from living micro-organisms
is known as _sterilization_. It may be accomplished in a number of
ways, but most often is done by the use of heat. For culture
material, which is always organic in character, moist heat is
employed. The various culture media, in appropriate containers, are
subjected to a thorough steaming in a steam cooker. This destroys
all of the vegetating cells but not the resistant spores that may be
present. The media are then stored, for twenty-four hours, at
temperatures favorable for the germination of the spores and are
then again heated. Three such applications on successive days are
usually sufficient to free the media from all living germs, since
between the heating periods the spores germinate and the resulting
vegetative cells are more easily destroyed. The sterile media will
keep for an indefinite period in a moist place.
The media are usually placed in glass containers which may be
sterilized before use by heating them in an oven, it being possible
to thus secure a much higher temperature than with streaming steam.
All glass or metal articles may be sterilized by the use of dry heat
but for organic media, to avoid burning, moist heat must be used.
All kinds of materials may be sterilized by treatment with steam
under pressure. An exposure for a few moments at 250° F., a
temperature attained with 15 pounds steam pressure, will destroy all
kinds of bacteria and their spores. This method of sterilization is
used in the canning of meats and vegetables and in the preparation
of evaporated milk. To avoid contamination of the media after
sterilization, the flasks and tubes are, after being filled,
stoppered with plugs of cotton-wool, which effectually filter out
all bacteria and mold spores from the air, and yet allow the air to
pass freely in and out of the containers.
=Methods of determining the number of bacteria.= The method of
determining the number and kinds of bacteria in any substance can be
illustrated by the process as applied to milk. For this purpose the
method of procedure is as follows: Sterile gelatin in glass tubes is
melted and then cooled until it is barely warm. To this melted
gelatin a definite quantity of milk is added. The medium is gently
shaken, so as to thoroughly mix the milk and gelatine, and the
mixture then poured into a sterile, flat, glass dish, and quickly
covered, where it is allowed to cool 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 spots are
called _colonies_, and are composed of an almost infinite number of
individual cells, the result of the continued growth of a single
organism that was in the drop of milk and which was firmly held in
place when the gelatin solidified. The number of these colonies
represents approximately the number of living bacteria that were
present in the amount of milk added to the tube of gelatin. If the
plate is not too thickly sown with the bacteria, 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.
[Illustration: Fig. 4.--Plate Culture.
Each of the dots is a colony that has been formed by the growth of
an organism embedded in the solid culture-medium. By counting the
colonies, the number of living bacteria in the amount of milk added
to the culture is determined.]
In order to make sure that the number of colonies is not so numerous
as to prevent counting and further study of their characteristics, a
series of plate cultures is usually made in which varying amounts of
milk are added to the tubes of gelatine. This is attained by adding
a definite amount of the milk or other substance to be examined to a
measured amount of sterile water, e.g., one cubic centimeter of milk
to ninety-nine cubic centimeters of water. One cubic centimeter of
this mixture may be used for the inoculation of the plate culture.
This dilution may be carried on to any desired extent; in the
examination of many dairy products, it is necessary to use very
minute quantities of material, often only one one-millionth of a
cubic centimeter.
To study further the peculiarities of the different bacteria, small
portions of the individual colonies are transferred to tubes of
sterile culture-media. In order to do this the colony is touched
with a piece of platinum wire; the minute amount of growth that
adheres to the wire is sufficient to seed the tube of fresh
culture-medium. The inoculating needle must always be sterilized
before use by passing it through a gas flame.
A culture thus obtained is called a _pure culture_ since it contains
but a single kind of an organism, as the colony is the result of the
growth of a single cell. These cultures then serve as a basis for
continued study, and must be planted and grown upon the different
kinds of media that are obtainable. In this way the slightest
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.
[Illustration: Fig. 5.--Different Kinds of Bacteria Growing in
Gelatin.
A, meager growth, no liquefaction or surface growth; B, profuse
surface growth, radiating filaments from the growth below the
surface; C, a rapid liquefying form; D, a gas producer that grows
equally well in the presence or absence of air; E, form that grows
only in the absence of air, an anaerob.]
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 in a pure state.
=The microscope in bacterial investigations.= 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 high magnifying powers (600 to 800 diameters), combined with
sharp definition.
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 colored 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 made in
different ways, 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 experimental animals, like rabbits, guinea pigs and
white mice.
The methods of the bacteriologist in his laboratory are in their
effect not dissimilar to those which the farmer employs in securing
his crop of pure-bred grain. The laboratory farmer kills the weed
seeds in his culture field by the application of heat. His field,
which is embraced in his culture dish, has been fertilized and
prepared by the addition of certain favorable ingredients. When he
has garnered his crop, he maintains its purity by keeping his
selected seed, the pure culture, free from all contamination. The
dairyman, even though he may not expect to carry on the detailed
operations of the laboratory, will understand the reason for the
directions which he is often required to follow much better if he
knows how the simple operations of the laboratory are carried out.
For a fuller knowledge of these matters, the reader is referred to
the special texts on bacteriology.
CHAPTER III.
CONTAMINATION OF MILK.
=Spoiling of milk.= Materials of animal origin are peculiarly prone to
undergo changes, rendering them unfit for use, and of these, milk is
exceedingly susceptible to such changes. This is due to the fact
that the composition of milk is especially adapted to bacterial
growth, and that the opportunity for entrance of such organisms is
likewise such as to permit of abundant contamination. The
consequence is that milk readily undergoes fermentative changes, due
to the development of one or another type of micro-organism.
=Milk, a suitable bacterial food.= While milk is designed by nature
for the nourishment of mammalian life, it is, curiously enough,
equally well adapted to the growth of these lowest forms of
vegetable life. The nutritive substances required by bacteria are
here sufficiently dilute to make possible rapid growth.
Milk also contains all the necessary chemical substances to make a
suitable bacterial food supply. Of the nitrogenous compounds,
albumen is in a readily assimilable form. Casein, the principal
nitrogenous constituent of milk, exists in an insoluble condition,
and cannot be directly utilized, until it is acted upon by digesting
enzymes. The fat in milk does not readily decompose, and while there
are a few bacteria capable of splitting this substance, the majority
of organisms are unable to utilize it. Milk sugar, on the other
hand, is an excellent food for most species.
[Illustration: Fig. 6.--Fat Globules and Bacteria.
Note the relative size of the fat globules of milk and the lactic
acid bacteria.]
=Sources of contamination.= Inasmuch as milk is especially exposed to
the inroads of bacterial growth, and because of the fact that much
of the contamination can easily be prevented, it is highly important
that the milk producer and dealer should be thoroughly cognizant of
the various sources of contamination. The different factors
concerned in contamination may be grouped as follows: the interior
of the udder; utensils, including all apparatus with which the milk
is brought in contact subsequent to withdrawal from the animal;
infection coming from the animal herself, from the milker, and the
surrounding air.
=Condition of milk when secreted.= Immediately after withdrawal from
the udder, milk always contains bacteria, yet in the secreting
cells of the udder of a healthy cow, germ life does not seem to be
present. 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.
A study of the structure of the udder shows the manner in which such
infection occurs.
=Structure of the udder.= The udder is composed of secreting tissue
(_gland cells_) that is supported by fibrous connective tissue. The
milk is elaborated in these cells and is discharged into microscopic
cavities, from whence it flows through the numerous channels (_milk
sinuses_) that ramify through the substance of the udder, until
finally it is conveyed into the _milk cistern_, a common receptacle
holding about one half pint that is located just above the teat.
This cavity is connected with the outside by a direct opening (_milk
duct_) through the teat. During the process of milking, the milk is
elaborated rapidly in the gland cells, and their contents upon
rupture of the milk cells, flow down into the cistern. The normal
contraction of the muscles at the lower opening of the outer duct
prevents the milk from passing out except when pressure is applied,
as in milking. The inner walls of the milk duct and cistern are
always more or less moist, and therefore afford a suitable place for
bacteria to develop, if infection once occurs, and conditions are
favorable for growth.
=Manner of invasion.= Two possible sources of invasion of the udder by
bacteria may exist. If bacteria are present in the circulating
blood, there is the possibility of organisms passing directly
through the tissues into the milk-secreting cells. The other
alternative is the possible direct contamination from the outside by
organisms passing up through the milk duct, and so spreading through
the open channels in the udder.
[Illustration: Fig. 7.--Sectional View of Udder.
Teat with milk duct connecting the exterior with the milk cistern.
Milk sinuses which conduct the milk from the secreting tissue to the
milk cistern. (After Moore & Ward.)]
=Number of bacteria in fore-milk.= If a bacteriological examination is
made of the milk drawn from each teat at different periods during
the milking process, it will be found that the fore-milk, _i.e._,
the first few streams, contains, as a rule, many more organisms per
cubic centimeter than that removed later. Not infrequently thousands
of organisms per cubic centimeter may be found in the first streams
while the middle milk, or strippings, will contain much smaller
numbers.
=Distribution and nature of bacteria in udder.= If the udder itself is
carefully examined as to its bacterial content, it appears that the
majority of organisms found is confined to the lower portion of this
organ, in the teat, milk-cistern and large milk-ducts; while
bacteria occur in contact with the secreting tissue, they are
relatively less abundant. This would seem to indicate that the more
probable mode of infection is through the open teat.
While there is no constant type of bacteria found in the fore-milk,
yet it is noteworthy that nearly all observers agree that the
organisms most commonly found are not usually the acid-producing, or
gas-generating type, so abundant on the skin or hairy coat of the
udder and which predominate in ordinary milks. Coccus forms,
belonging to both liquefying and non-liquefying types are most
generally present. Many of these produce acid slowly and in small
quantities.
The bacteria coming from the interior of the udder are of small
practical significance since they do not grow rapidly at the
temperatures at which milk is stored. If the milk is protected from
contamination from other sources, the bacteria from the udder will
ultimately cause it to spoil, but under ordinary conditions other
forms are present in such greater numbers, and grow so much more
rapidly in milk, that the udder forms have small opportunity to
exert any effect.
It is interesting to note that the bacteria found in the udder are
similar to those that seem to be most abundant in such glandular
tissues as the liver and spleen. This fact increases the probability
that these comparatively inert coccus forms of the udder may
originate directly from the blood stream. The organisms that
normally are found in the udder exert no harmful effects on the
gland. It might be thought that due to the presence of abundant food
and a favorable temperature that growth would be abundant, but such
is not the case. At times the udder may be invaded by forms that are
not held in check by the natural factors and an inflammation of the
udder is likely to result.
=Germicidal property of milk.= It has been claimed that freshly drawn
milk, like other body fluids, possesses germicidal properties,
_i.e._, the power of destroying bacteria with which it may be
brought in contact. If milk is carefully examined bacteriologically,
hour by hour, after it is withdrawn from the udder, it will
generally be found that there is at first not only no increase in
number of organisms during a longer or shorter period when it is
kept at temperatures varying from 40° to 70° F., but that an actual
reduction not infrequently takes place. When cultures of bacteria,
such as _B. prodigiosus_, a red organism, lactic acid organisms, and
even the yellow, liquefying coccus, so commonly found in the
fore-milk, are artificially introduced into the udder, it has been
found that no growth occurs and that in the course of a few days the
introduced organisms actually disappear. Whether this failure to
colonize can be regarded as evidence of a germicidal property or not
is questionable. In fact, this question is a matter of but little
practical importance in the handling of milk since, under the best
of conditions, the keeping quality of the milk is not materially
enhanced. It may be of importance in inhibiting growth in the udder.
=Rejection of fore-milk.= The fact that the fore-milk contains per
cubic centimeter so much more germ life than the remainder of the
milk has led some to advocate its rejection when a sanitary milk
supply is under consideration. While from a purely quantitative
point of view, this custom may be considered advantageous, in
practice, however, it is hardly worth while since it is not at all
certain that the rejection will have any effect on the keeping
quality or healthfulness of milk. This is especially true if the
ends of the teats are thoroughly cleaned before milking. It is true
that the fore-milk is relatively deficient in fat so that the loss
of butter fat occasioned by the rejection of the first few streams
is comparatively slight.
=Contamination from utensils.= One of the most important phases of
contamination is that which comes from the utensils used to hold the
milk from the time it is drawn until it is utilized. Not only is
this important because it is a leading factor in the infection of
milk, but because much improvement can be secured with but little
trouble, and it is especially necessary that the dairy student
should be made familiar with the various conditions that obtain.
Pails and cans used to hold milk may be apparently clean to the eye,
and yet contribute materially to the germ content of the milk placed
in them. Not only does much depend upon their condition, but it is
equally important to take into consideration their manner of
construction. Dairy utensils should be simple in construction,
rather than complex. They should be made so that they can be readily
and easily cleaned, or otherwise the cleaning process is apt to be
neglected.
Of first importance are those utensils that are used to collect the
milk and in which it is handled while on the farm. The warm milk is
first received in pails, and unless these are scrupulously cleaned,
an important initial contamination then occurs. As ordinarily
washed, the process falls far short of ridding the utensils of the
bacterial life that is adherent to the inner surface of the pail.
Then, too, all angles or crevices afford an excellent hiding place
for bacteria, and it is very important to see that all seams are
well soldered. Round corners and angles flushed with solder greatly
facilitate thorough cleaning of utensils. Tin utensils are
recognized as most satisfactory.
Shipping cans are likely to serve as greater infecting agents than
pails for they are subject to more wear and tear and are harder to
clean. As long as the surface is bright and smooth, it may be easily
cleaned, but large utensils, such as cans, are likely to become
dented and rusty in spots on the inner side. The storage of milk in
such utensils results in its rapid deterioration. The action of
rennet has been found to be greatly retarded where milk comes in
contact with a rusty iron surface. It is also probable that some of
the abnormal flavors in butter are due to the action of acid cream
on iron or copper surfaces from which the tin has been worn. It is
equally important that attention be paid to the care of strainers,
coolers, and the small utensils. Cloth strainers are more or less of
a hotbed for bacterial growth, for unless they are boiled, and then
dried quickly and thoroughly, germ growth will continue apace in
them, as long as they contain any moisture.
=Milking machines and farm separators.= The introduction of these
special types of dairy machinery in the handling of milk on the farm
has materially complicated the question of the care of milk. Both of
these types of apparatus are much more complicated than the usual
milk utensil; consequently, the danger of imperfect cleaning is
thereby increased. This is still further accentuated by the fact
that cleansing of utensils on the farm can never be done so well as
at the factory or milk depot where steam is available. The milking
machine may be easily kept in a comparatively germ-free condition,
but unless this is done, it contributes its quota of germ life to
the milk.
The farm separator is more widely used than the milking machine and
in actual practice the grossest carelessness prevails in the matter
of its care. Frequently it is not taken apart and thoroughly
cleansed, but is rinsed out by passing water through the machine. It
is impossible by such a treatment to remove the slime that collects
on the wall of the bowl; the machine remains moist and bacterial
growth can go on. Such a machine represents a most important source
of contamination of milk and cream and it is probable that the
widespread introduction of the hand separator has contributed more
to lower the quality of cream delivered at the factory than any
other single factor.
=Contamination from factory by-products.= The custom of returning
factory by-products in the same set of cans that is used to bring
fresh milk is a prominent cause of bad milk. Whey and skim milk are
rich in bacterial life, and not infrequently are so handled as to
become a foul, fermenting mass. If the cans used to transport this
material are not scrupulously cleaned on the farm, transfer of
harmful bacteria to the milk is made possible. In this way the
carelessness of a single patron may be the means of seeding the
whole factory supply. This custom is not only liable to produce a
poor quality of milk, but it is more or less of a menace to all the
patrons of a factory, inasmuch as the opportunity always obtains
that disease-producing organisms may thus be introduced into the
supply. Not infrequently is tuberculosis thus spread through the
medium of factory by-products.
[Illustration: Fig. 8.--Whey Disposal.
Whey barrels at a Wisconsin Swiss cheese factory. Each patron's
share is placed in a barrel which is so situated that it is
impossible to empty it completely; thus it is not cleaned during the
season.]
The manufacture of Swiss cheese presents a striking example of the
disregard which factory operators show toward the employment of
bacteriological principles. In these factories, the custom is widely
practiced of apportioning the patrons' allotment of whey into
individual barrels which are supposed to be emptied each day. As
these barrels are, however, rarely ever cleaned from the beginning
to the end of the season, they become very foul, and the whey placed
in them from day to day highly polluted. It is this material which
is taken back to the farms in the same set of cans that is used for
the fresh milk. When one recalls that the very best type of milk is
essential for the making of a prime quality of Swiss cheese, and
that to secure such, the maker insists that the patron bring the
product to the factory twice daily, the before mentioned practice
appears somewhat inconsistent.
=Treatment of factory by-products.= To overcome the danger of
infecting milk from factory by-products with either undesirable
fermentative organisms, or disease-producing bacteria, the most
feasible process is to destroy these organisms by the application of
heat. In Denmark, some portions of Germany, and in some of the
states in this country, laws exist which require the heating of all
skim milk before it is returned to the farm. This is done by the
direct use of exhaust steam, or running the product through heaters.
The treatment of whey in cheese factory practice is especially
important since the warm whey must be stored for a number of hours
before it is returned to the farms. Even under the best of
conditions the whey is certain to be in an advanced state of
fermentation when placed in the milk cans, and it only needs the
infection of the whey tank with harmful bacteria to cause great loss
on account of the injury of the product by these bacteria. Among
Canadian factories the custom of heating the whey as it passes from
the cheese vat to whey tank has been introduced, and where ever
adopted has been retained, because, it has resulted in such an
improvement of the cheese that the gain was much greater than the
cost, which is estimated at not over fifty cents per ton of cheese.
The whey is heated not to exceed 155° F.; the hot whey serves to
scald the whey tank and as the mass of whey is usually quite large,
it does not cool to a point where bacterial growth can take place
for a number of hours. The whey is thus quite sweet when returned to
the farm and has greater feeding value. The heating also prevents
the creaming of the whey in the tank and thus avoids the soiling of
the cans with grease which is most difficult to remove.
Where compulsory legislation is in force it is generally required
that these by-products be heated to a temperature of at least 176°
F. This is done so as to destroy effectually the organisms of
tuberculosis, and especially to permit of the utilization of the
so-called Storch test,[1] which enables a person to determine
quickly whether milk or whey has been heated or not.
[1] Storch (40 Rept. 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 enzyme known as
peroxidase which has the property of decomposing hydrogen
peroxid. If milk is heated to 176° F., (80° C.) or above, this
enzyme is destroyed, so that the above reaction no longer takes
place. If potassium iodide 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.
=Cleaning utensils.= Various processes are applied to dairy utensils
to cleanse them. In removing visible dirt and foreign matter, much
of the bacterial life is mechanically eliminated, but most of the
cleaning processes fail to destroy the germ life in these utensils.
In rinsing, washing, or even scalding, the water is not applied at a
sufficiently high temperature to destroy effectively the bacteria.
These processes are primarily used for the removal of dirt and other
matter. To facilitate such removal, washing powders of various kinds
are frequently employed; some of these possess considerable
disinfecting action. All utensils after cleansing should be
thoroughly rinsed in clean, hot water. Even where no further
treatment is given, a careful cleaning may so reduce the germ
content on the inner surface of utensil as to render contamination
therefrom relatively unimportant. Most of the contamination in a
well cleaned utensil comes from the cracks and angles, which permit
of the collection of the dirt. If these are properly attended to,
thorough cleaning and rinsing alone will accomplish much.
To exert an actual germ-destroying effect on the bacterial content
of the utensil, resort must be had to boiling or steaming. To treat
utensils so as to render them wholly germ-free would be impractical
under ordinary commercial conditions, as it would consume too much
time, although with proper apparatus, this process is not
impossible, but it is well within the limits of practicability in
factory treatment to apply steam for a short period of time. Where
cans, pails and such utensils, are steamed for a minute or so after
being thoroughly cleaned, the germ content is greatly reduced. In a
series of tests by Harrison, the germ content of a set of cans
cleaned in an ordinary way was 442,000 bacteria per cubic centimeter
in 100 cubic centimeters of wash water; in a set washed in tepid
water and then scalded--the best farm practice--it was 54,000 per
cubic centimeter, while in cans carefully washed and then steamed
for 5 minutes, it was reduced to 880 per cubic centimeter. It would
not be worth while to institute measures that would accomplish the
destruction of this small residual content.
The use of steam, therefore, is of great service in eliminating
bacterial life in all utensils. In apparatus of at all complicated
design, it is absolutely necessary. Of course, ordinarily, steam can
be applied only at the factory, as the farm does not usually afford
facilities for its easy generation. This fact has led in some cases
to the adoption of the method of cleaning and sterilizing the cans
at the factory rather than to await their arrival at the farm. This
custom is most frequently followed in milk supply plants.
It is also very important in cleaning dairy utensils to see that
they are rapidly and thoroughly dried after being washed and
steamed. As pointed out above, the short period of steaming that can
be followed in practice does not kill all the bacteria. If moisture
is retained, conditions permit of the growth of the undestroyed
organisms. Tests made on glass milk bottles showed that considerable
growth occurred in the condensation water even after quite thorough
sterilization. Some of the devices used for the sterilization of
such utensils as milk cans are so arranged that, after steam has
been introduced, hot air is passed into the can until it is
thoroughly dried. Other utensils such as cloth strainers become
sources of contamination unless the articles are thoroughly and
quickly dried after cleaning.
In a general way, it may be said that whenever a utensil is so
constructed and in such a condition that every portion of its
surface can be reached by a cloth or a brush, it can be kept in a
sanitary condition. But whenever any portion cannot be thus reached,
whether it is an angle or a seam in a pail or can, the interior of
the separator bowl, or in the pipes used for conducting milk,
contamination is certain to result from such places, unless extreme
care is taken to destroy the bacteria therein by steaming.
=Contamination from the animal.= In the process of milking, the
bacterial content of the milk is materially increased. In part this
comes from the utensils into which the milk is drawn, but the animal
herself, the milker, as well as the surrounding air, also contribute
to a varying extent. Of these factors, the one fraught by far with
the most consequence, is the influence of the animal herself. It is
a popular belief that the organisms found in milk are derived from
the feed and water which the animal consumes, but under normal
conditions, the bacteria consumed in food pass through the
intestinal canal and do not appear in the circulation. It must not
be assumed, however, that the character of feed and water supply is
of no moment. Stock should be given pure and wholesome water 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 the
ration fed, the softer are the fecal discharges, producing a
condition 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.--Bacteria on Hairs.
Each colony on the hair represents one or more bacteria that were
adherent to the hair when it was placed on the surface of the solid
culture-medium.]
The animal 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 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. All of this
material contains organisms that are more or less undesirable in
milk. Bacteria concerned in gassy fermentations and those capable of
producing obnoxious taints are particularly common, so that this
type of pollution is especially undesirable in milk.
=Amount of dirt in milk.= When one remembers that the larger part of
fresh manure is of such a nature that it does not appear as
sediment, the presence of evident filth in milk must bespeak
careless methods of handling.
The sediment or dirt test is used quite extensively to ascertain the
amount of dirt milk may contain. By means of a cotton filter, the
insoluble residue is removed and is made evident upon a layer of
absorbent cotton. Milk that would show with difficulty any evidence
of dirt upon ordinary examination reveals such defects very readily
in this test.
=Exclusion of dirt.= It is better to keep bacteria out of milk, so far
as practicable, rather than to attempt to remove them after they
have once gained entrance. As is usual, prevention of trouble is
much more easily accomplished than removing the difficulty after it
once occurs.
[Illustration: Fig. 10.--Dirt from Milk.
The dirt adherent to each of the filters was obtained from one pint
of milk. The milks tested were produced on different farms.]
Much reduction as to the amount of dirt that finds its way into milk
may be accomplished by improved stable environment. The fouling of
the udder and flanks comes from wading in dirty water, muddy yards,
and from improper type of stalls. Barnyards are often a disgrace
through the accumulation of manure and seepage. Cows wading in such
mire cannot but accumulate mud and filth to a material degree on the
teats and udder. Greater care as to drainage of the barnyard and the
paving of same with gravel, cinders, etc., will permit of its being
kept clean, and so prevent the fouling of animals. But more
important than the yard is the stall which the animal occupies in
the stable. The essential feature is to have a stall of such
construction as to keep the animal out of her own manure when she
lies down. To accomplish this, it is necessary to have a manure drop
behind the stall proper so that the feces and urine are kept out of
the bed of the stall as much as possible.
[Illustration: Fig. 11.--The Model Stall.
A stall of this type keeps the animals clean, and thus aids greatly
in producing good milk.]
Most of the stalls widely advertised in the farm press seek to
accomplish this in one way or another, usually by some arrangement
by which the cow is forced back when standing and drawn forward on
lying down. In Fig. 11 a type of stall is illustrated that
accomplishes this most successfully; the essential feature being a
2×3-inch wood strip nailed to the stall floor immediately in front
of the hind feet of the animal when in a standing position. When the
animal lies down, she crowds forward to avoid lying on this strip,
and thus is out of contact with the manure, except such as is
carried onto the bedding by the hind feet. By the use of this stall
it is possible to keep the animals free from all accumulations of
manure.
Effort should be made to prevent fouling of the animals rather than
in cleaning them after once soiled. It is very evident that where
the cattle come to the milker with muddy udders, they will not be so
cleaned before milking as to prevent a large amount of such dirt
from entering the milk. However, when all that can be done towards
keeping the cows clean has been accomplished, a small amount of
grooming will greatly reduce the contamination coming from them.
The kind of bedding used in the stalls may have a marked influence
on the contamination coming from the animal. If the straw is dusty,
partially rotten and moldy, the bacteria and molds adhere to the
coat of the animal and are thus introduced into the milk. In the
case of cattle on pasture, no visible evidences of dirt are usually
present but the hair is covered with the dust coming from the soil.
There is very good reason to believe that the quality of milk is
influenced by the type of pasture on which the cows graze, due to
the difference in the types of bacteria in the surface soil. The
milk from animals on low land is more likely to show undesirable
fermentations than that from those grazing on higher lands. This is
not due to the influence of the feed as is often supposed but rather
to the dirt from the coat of the animal.
=Washing the udder.= If a surface is moist, dust and the adherent
bacteria cannot be easily dislodged. The air over snow-covered
mountains or over oceans is relatively free from bacteria. The udder
and flanks of the animals can be carded to remove the loose hairs
and the evident dirt; the fine dust can now be removed by wiping
with a clean damp cloth just before the milking process. The actual
washing and wiping of the udder and flanks still further reduces the
contamination coming from the animal; experiments show a reduction
of fully three-fourths of total contamination. Clipping the udder
and flanks also aids in keeping the animal clean.
It is often asserted that the treatment of the animals in these ways
reduces the yield of milk. It is certain that such an effect will
persist for only a short time and there is reason to believe that
grooming increases the yield.
[Illustration: Fig. 12.--Sanitary Milk Pails.
The small opening is very efficient in keeping the dirt out of
milk.]
=Sanitary milk pails.= The entrance of organisms into the milk can be
greatly reduced by lessening the area of the milk pail exposed to
the dust shower. To accomplish this purpose a number of so-called
sanitary or hygienic milk pails have been devised. In some cases,
these are the regular type of pail provided with a cover having a
small opening through which the milk is received. In other cases, a
strainer is interposed so as to remove more effectually the coarse
particles. While pails of this type are successful in the removal of
a large part of the dirt, and consequently reduce materially the
bacterial content of the milk, yet they must be of simple
construction, so that they can be kept in a clean condition in order
to adapt them for general practical use. The use of such a utensil
increases materially the keeping quality of the milk.
[Illustration: Fig. 13.--Sanitary Milk Pails.
The Stadtmueller pail and the Truman pail, two of the most practical
of the small-topped pails.]
Stocking has shown that under ordinary barn conditions, the use of
small-topped pails reduced the number of bacteria 95 per cent; with
dirty cows the reduction in bacteria amounted to 97 per cent. A
six-inch opening presents only one-fourth as large an exposure as a
twelve inch, so that the reduction in bacterial content is greater
than the lessening in the size of the openings of the pails. The
ordinary pail receives dust not only from the udder, but also from
the flank which is usually a more important source of contamination
than the udder itself, while the small-topped pail receives only
that from the udder.
[Illustration: Fig. 14.--use of Sanitary Milk Pails.
The open pail is fully exposed to the falling dust while the hooded
pail excludes much of the dust and dirt coming from the animal.]
=Milking machines.= Where the milk is removed from the udder by
machine methods, instead of by hand, it is possible to eliminate
nearly all external contamination from the animal and her
surroundings. The only opportunity for infection is then through the
leakage of air around the teat cups. Care should be taken to see
that the teats are in a clean condition before applying the suction
cups. The main problem in the use of a milking machine is to keep
the apparatus in an aseptic condition. Immersion of the teat cups
and the rubber connections in lime water, brine solution, or other
mild antiseptics, prevents bacterial development. Hastings has found
that milk having a germ content of less than 10,000 bacteria per
cubic centimeter may be produced by the use of a properly handled
milking machine.
=Contamination from the milker.= While the milker is a small factor in
comparison with the animal in the matter of contamination, yet he
can not be neglected, as it is within his power to affect profoundly
the quality of the milk. His personal habits as to cleanliness and
his appreciation of the precautions necessary in the production of
clean milk have much to do with the contamination of the milk. The
milking should be done with dry hands, although a little vaseline
may be used with effect. The hands should be washed before milking
as milk is certain to come in contact with them to some extent. The
milking should be done with the whole hand rather than stripping
between the thumb and finger; the clothing should be covered with
clean overalls and jumper, or at least a clean apron should be worn
during the milking. If these are of white material, more frequent
laundering is likely to result.
=Contamination from air.= It is difficult to disassociate the
contamination arising from the condition of the air from that
derived directly from the animal. Barn operations of various kinds
result in the production of dust, particularly where dry forage,
such as hay or straw, is handled. Where manure is given an
opportunity to dry, dust is readily produced, and such material is
particularly replete with bacterial life. Some kinds of dust, such
as that originating from ground grains, or shavings that may be used
for bedding, contain a small amount of bacterial life in comparison
with the dust from hay, or other dry fodder. In a dried condition,
the slightest movement is apt to dislodge these fine particles, and
they float in the air for considerable periods of time. If milk is
drawn and exposed to the air of the barn during the feeding
operations, it is subject to the dust shower that is present. Where
the storage can is allowed to stand in the stable during the
milking, even though it is covered with a strainer, this
accumulation of microscopic particles is added to the milk, as they
readily pass the meshes of the finest strainer.
[Illustration: Fig. 15.--contamination From the Air.
This culture plate, three inches in diameter, was exposed for 30
seconds in the barn during feeding of dry fodder. A 12-inch pail
exposes over 18 times the surface of this plate.]
=Removal of dirt after introduction.= The more primitive method of
improving the quality of milk, so far as its dirt content is
concerned, is to attempt to remove the grosser particles of
contamination after entrance. In the case of straining, the method
is usually applied at the time of milking, but in the case of
filtering and clarifying, it is carried out at the milk station, in
an effort to improve the appearance of milk and overcome the
influence of careless methods of the producer. By the use of
strainers, either metallic or cloth, it is possible to remove
particles of hair, undissolved dirt and manure, but it must be
remembered that these grosser _visible_ particles of pollution are
not really the cause of the troubles which may ensue in improperly
handled milk. The bacteria which are adherent to these foreign
particles are in large measure washed off in the process of
straining, and pass through the meshes of the finest strainer. The
main service, therefore, of straining is to improve the appearance
of the milk, and it has no effect on the quality in any way.
=Production of clean milk.= The problem of clean milk is important,
whatever may be the use to which milk may be put. It is important in
the manufacture of butter, but owing to the fact that the fat is not
readily acted upon by bacteria, it is not so sensitive to bacterial
conditions, as when the milk is made into cheese. In this product,
the bacterial condition of the milk is a matter of prime importance.
In milk destined for direct consumption, the exclusion of the
bacteria becomes yet more important. While it is impossible to
exclude bacteria so completely that milk will not undergo
fermentative changes, yet for domestic consumption it is preferable
to have milk with as low bacterial content as can readily be
secured. The highest type of market milk, that known as sanitary, or
certified, is produced under such extreme conditions of care as to
contain the minimum germ content. To accomplish these results
requires such stringent control as to increase greatly the cost of
the product. Pure, clean milk can be produced at a very slight
increase in cost over the regular expense of milk production, if the
right kind of attention is given to certain details of a practical
character. Improvement in our milk supplies must largely come from
this source, for any improvement to be permanent must be made to
pay, and it requires considerable education to secure the
co-operation of consumers and their willingness to pay for any
material increase in the quality of the product.
In the foregoing factors concerned in the contamination of milk, it
is of course impossible to measure accurately the influence of the
different sources of infection, as these are continually subject to
variation in every case. As a rule, the most important factors are
those pertaining to the utensils and the condition of the animal
herself. If these two factors are brought under reasonable control,
the major portion of contamination that ordinarily obtains is done
away with. The application of the remedial or preventive measures
heretofore mentioned will greatly reduce the germ content of the
milk.
=Cooling of milk on farm.= Bacterial growth is directly related to
temperature conditions, and with summer temperatures, such
development goes on apace, unless it is checked by early cooling.
The larger portion of bacteria that find their way into milk,
especially those that are previously in contact with the air, are in
a dormant condition, and are therefore not stimulated into immediate
growth, unless reasonably high temperatures prevail. In milk, which
comes from the animal at blood heat, this growth is greatly
stimulated. To counteract this effect, milk should be chilled as
soon after milking as possible. If the temperature is immediately
lowered to 50° F., or lower, actual cell development is greatly
retarded, and the rate of souring, and other fermentative changes
thereby diminished. In this country ice is liberally used in
accomplishing this result. In Europe, the use of ice is much less
common. The employment of such artificial means of refrigeration
makes possible the shipment of milk for long distances by rail. New
York city now receives milk that is produced in Canada and
northeastern Ohio.
[Illustration: Fig. 16.--Effect of Cooling Milk.]
=Aeration of milk=. The custom has been extensively recommended of
subjecting milk to the influence of air in the belief that such
exposure permits of the interchange of gases that would improve the
quality. In practice, this process, known as aeration, is carried on
in different ways. 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, falling in drops or tiny streams through the
air. Whenever this process is carried on at a temperature lower
than that of the milk, it results in more or less rapid cooling.
In earlier times, aeration was generally recommended and practiced,
especially in connection with the cheese industry, but carefully
controlled experiments fail to show that the process exerts any
material influence on the rate of germ development. If it is carried
out in an atmosphere more or less charged with bacteria, as in the
barn or stable, it is more than likely to add to the bacterial
content of the milk. While to some extent odors may be eliminated by
the process, the custom is not followed so generally now as it used
to be some years ago.
=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 absorbing volatile odors. 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 it 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.
Many kinds of feed consumed by the animal produce a more or less
pronounced taint or flavor in the milk. With some plants, such as
garlic, leeks, turnips, and cabbage, the odor is so pronounced as to
render the milk quite unfit for use. In some states along the
Atlantic seaboard, wild plants of this character in woodland
pastures may be so abundant as to make it impossible to pasture
milch animals. The difficulty in such cases is due to absorption of
the volatile principles into the circulation of the animal, and if
such feed is consumed shortly before milking, the characteristic
odors appear in the milk. If consumed immediately after the milk is
withdrawn from the animal, sufficient time may elapse so that the
peculiar odors are dissipated before the milk is again secreted. The
same principle applies in a lesser degree to the use of certain
green fodders that are more suitable for feed, such as rape, green
rye, or even silage. Silage produces a distinct, but not unpleasant
odor in milk, but newly pastured rye often confers so strong an odor
as to render the milk unusable.
Where certain drugs are employed in the treatment of animals, such
as belladonna, castor oil, sulfur, or turpentine, the peculiar odors
may reappear in the milk. Such mineral poisons as arsenic have been
known to persist for a period of three weeks before elimination.
On account of the elimination of many drugs, unchanged, from the
animal in the milk, the milk of any animal that is receiving
medicine should not be used for human food. When such milk is mixed
with that of a number of other animals and when it is used by
adults, no harm is likely to result, but when the dilution is not
great and the milk is used for young children it may affect them
through its content of the drug. The feed may not only affect the
quality of milk but its value as food. One of the most prominent of
American dairymen, who has for many years produced milk especially
for children's use, has said that he could feed his cows so as to
make ill every child receiving the milk.
=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 experiments of one of us (R), it has been 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 quite rapidly.
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 is
noted where the same is stored in a refrigerator with certain
fruits, meats, fish, etc.
=Distinguishing bacterial from other 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
by-products are most evident.
Moreover, if the difficulty is of bacterial origin, it can be
frequently produced in another lot of milk (heated or sterilized is
preferable) by inoculating the 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.
=Factory contamination.= As the time element is of importance in the
production of troubles due to bacteria, it follows that infection of
milk on the farm is fraught with more consequence than factory
contamination, as the organisms introduced would have a longer
period of development. Nevertheless, the conditions in the factory
are by no means to be ignored, as they not infrequently permit the
milk to become seeded with highly undesirable types. A much more
rigid control can be exercised in the factory, where steam is at
hand as an aid in the destruction of organisms. In the cleaning of
pumps and pipes, steam is absolutely necessary to keep such
apparatus in a sanitary condition.
The water supply of the factory is a matter of prime importance, as
water is used so extensively in all factory operations. 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 best be determined by making a curd test (p.
99) 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
cubic centimeter at most. The danger from ice is much less, for the
reason that good daily practice does not sanction using ice directly
in contact with milk or cream. Then, too, water is largely purified
in the process of freezing, although if secured from a polluted
source, reliance should not be placed in this method of
purification, for even freezing does not destroy all vegetating
bacteria.
The ordinary house fly is an important source of contamination in
creameries, cheese factories and city milk plants. They are of
importance not only in increasing the number of fermentative
bacteria in milk but they may serve to contaminate it with
disease-producing organisms. The windows of all places where milk is
handled, whether on the farm or elsewhere should be screened.
It should be kept in mind in the handling of milk and other dairy
products that human food is being prepared and that cleanliness is
desirable from every point of view, and that the methods of
handling and production should compare with those used in the
preparation of foods which like milk cannot be cleaned when once
polluted. Desirability, keeping quality, healthfulness and the value
of every product made from milk depends upon the extent and amount
of contamination.
CHAPTER IV.
INFECTION OF MILK WITH PATHOGENIC BACTERIA.
That the disease-producing, or pathogenic bacteria, are able to
infect milk supplies is shown by the fact that numerous epidemics of
contagious disease have been directly traced to milk infection. Milk
is generally consumed in a raw state, and as a considerable number
of this class of organisms are able not only to live but actually
grow in milk, which is such an ideal culture-medium for the
development of most bacteria, it is not surprising that disease
processes should be traced to this source. The organisms in milk
capable of causing disease do not alter or change its physical
properties sufficiently to enable their presence to be detected by a
physical examination.
=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 udder. 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 live a saprophytic
existence, growing in milk, as in any other nutrient medium, 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 milk supply is tuberculosis,
but in addition to this, foot and mouth disease (aphthous fever in
children), Malta fever, and acute enteric troubles have also been
traced to a similar source of infection.
The most important specific diseases that are disseminated through
subsequent infection 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 micro-organisms 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.
=Tuberculosis.= This disease is by far the most important bacterial
malady that affects man and beast. In man, it assumes a wide variety
of phases, ranging from consumption, tuberculosis of the lungs,
which is by far the most common type, to scrofulous glands in the
neck, cold abscesses, hip-joint, and bone diseases, as well as
affection of the bowels. These various manifestations are all
produced by the inroads of the specific organism, Bacillus
tuberculosis. The bovine, as well as swine, fowls, and other
warm-blooded animals, are also affected with similar diseases. In
man, the importance of the malady is recognized when it appears that
fully one-seventh of the human race die of this scourge. In cattle,
the disease is equally widespread, particularly in those countries
where live stock has been intensively developed. In the northern
countries of Europe, such as Denmark, Germany, England, France, and
the Netherlands, as well as in Canada, and this country, this
disease has been most widely disseminated. This has been occasioned,
in large measure, because of the exceedingly insidious nature of the
disease in cattle, thereby permitting interchange of such diseased
stock without the disease being recognized. Tuberculosis is found
more abundantly in this country in dairy than in beef stock. Dairy
cattle are, however, not more susceptible, but the closer
environment in which milch cattle are kept, and the fact that there
has been greater activity in the matter of introducing improved
strains, accounts for the larger percentage of affected animals.
It has been a disputed question for some years whether the organisms
producing bovine and human tuberculosis are identical or from the
practical standpoint, whether the bovine type of disease is
transmitted under natural conditions to man. The bacteriologist can
readily detect differences in appearance, in growth of cultures, and
in disease-producing properties between the two strains. Of the two,
the bovine is much the more virulent when inoculated into
experimental animals. In a considerable number of cases, record of
accidental infection from cattle to man has been observed. These
have occurred in persons making postmortem examination on
tuberculous animals, and the tubercular nature of the wound proven
by excision and inoculation.
More recently, since the agitation by Robert Koch of Germany, a
number of scientific commissions have studied particularly the
problem of transmission. It is now estimated that perhaps seven per
cent of the tuberculosis in man is of bovine origin. This is almost
wholly confined to children. The portions of the body that become
diseased, when the infection has resulted from the use of milk, are
the glands of the neck and of the abdomen.
=Manner of infection in man.= In the main, the source of the malady
may be traced either to air infection or to the food, if one
disregards the comparatively small number of cases of wound
infection. Air is frequently a medium by which the germ is
transferred from one person to another. The sputum is exceedingly
rich in tubercle bacilli and since this material is carelessly
distributed by tubercular people, the air of the cities, villages
and public buildings will frequently contain tubercle organisms.
Some of the organisms in the air find their way into the lungs,
there to develop and produce consumption. The organisms in the air
may be deposited in the nasal passages and throat, and ultimately
find their way into the tissues of the body by penetrating the walls
of the throat or of the intestine. It is probable that the tubercle
bacilli thus introduced may find their way to the lungs and there
develop without leaving any trace of their path.
Food may also possibly serve as a medium of infection. The
contamination of solid food from flies and other sources is, of
course, a possibility, but tuberculous meat from cattle and swine is
much more likely to occur, although it must be said that the
processes of preparing such food for use (roasting, frying, and
boiling) are sufficient to destroy the vitality of the causal
organism. The fact that most food products of this character are now
inspected renders this possibility less likely to occur.
Unquestionably, the likelihood of ingesting tubercle organisms is
much greater with milk than with any other food supply, as milk is
consumed usually in an uncooked state, and as microscopic and
physiologic tests indicate that not infrequently milk from
tuberculous animals contains these organisms.
=Distribution of the disease in animals.= As practically any organ of
the body may be affected with tuberculosis, it naturally follows
that the lesions of this disease are widely distributed. The disease
germ is introduced, in the main, through the lymph and not the blood
system; consequently, in the initial stages the evidence of
tuberculosis is often comparatively slight, and the lesion is
restricted in its development. Where such a condition obtains, it is
known as "closed," in contradistinction to "open" tuberculosis,
where the diseased tissue is more or less broken down and is
discharging into the circulation, or elsewhere. Manifestly, the
danger of spreading not only in the affected animal itself, but to
the outside, is much greater in the case of the open lesion.
Especially is this true where the disease is present in the lungs or
organs that have an exterior opening so that the material containing
the organisms is discharged from the body in the sputum, manure,
urine or milk. The intestines themselves are rarely affected, but
the lymph glands associated with the intestinal tract are not
infrequently involved.
=Infection of milk with tubercle bacilli.= In a small percentage of
cases, the udder itself becomes involved. Where this condition
obtains, one or more hard lumps are formed, which slowly increase in
size, usually being restricted to one quarter of the udder.
Sometimes the affected quarter may develop to an enormous size,
producing a hard, painless tumor. Not often does the affected tissue
break down into pus; consequently, no abnormal appearance is to be
noted in the milk secretion until the disease has made very extended
progress, in which case the percentage of fat generally diminishes.
Whenever the udder shows physical manifestation of this disease, the
milk almost invariably is rich in tubercle bacilli.
Tubercle organisms may also appear in milk of animals in which no
physical symptoms of the disease are to be found. This fact has been
demonstrated by microscopic and animal experiments, but it is also
abundantly confirmed by the frequent contraction of the disease by
calves and hogs when fed on factory by-products. This latter class
of animals is particularly dangerous, because there is no way in
which the danger can be recognized.
[Illustration: Fig. 17.--a Tuberculous Animal.
The animal appears perfectly healthy although she has had the
disease for five years.]
It has also been proven that milk may become infected through the
feces. In coughing up material from the lungs and associated glands,
the matter is swallowed, instead of expectorated, as in man. The
organisms retain their vitality in the intestine, and are voided in
the feces. Under ordinary conditions, the flanks and udder become
more or less polluted with such filth, and the evidence is
conclusive that infection of milk is not infrequently occasioned in
this way. The fact that hogs following tuberculous steers in the
feeding lots are very likely to acquire the disease is explained by
the presence of tubercle organisms in the manure of such animals.
[Illustration: Fig. 18.--a Tuberculous Animal.
The last stages of generalized tuberculosis. Note the emaciated
condition.]
It must be kept in mind that many animals may be infected with
tubercle bacilli and therefore have tuberculosis in the incipient
stages, without their being able to disseminate the disease to
others. In the early stages, they are bacillus-carriers without
being necessarily dangerous at that particular time, but the
possibility always exists, as the disease develops in the system,
that the trouble may assume a more formidable character, and that
slowly developing chronic lesions may become acute, and "open," in
which case, the affected animal becomes a positive menace to the
herd. As the time when the lesions change from the "closed" to the
"open" type and the animal becomes a source of danger cannot be
determined, the only safe way to do is to exclude the milk of all
tuberculous animals from the general supply, whether for direct
consumption, or for manufacture into dairy products and to look upon
every diseased animal as a menace to the herd. This is rendered all
the more necessary when the milk is used for the feeding of
children, who are relatively more susceptible to intestinal
infection than the adult. The early stages of the disease in cattle
are, however, so insidious that no reliance can be placed upon the
detection of the malady by physical means. Fortunately, in the
tuberculin test, a method is at hand, which in a simple, but
effective manner, enables the disease to be distinguished in even
the early stages, long before recognition is possible in any other
way.
=Tubercle bacilli in dairy products.= When infected milk is used for
the preparation of butter and cheese, the organisms inevitably are
incorporated in them. In the separation of milk a relatively large
part of the tubercle organisms in the milk appear in the cream. In
the making of cheese even more of the organisms are held in the
curd. In butter and cheese, as in milk, no growth of the organism
can take place; however, the vitality of the organism is retained
for a considerable number of months. It is not believed that these
products are of much importance in the spread of tuberculosis in the
human family, since they are not consumed by children to any extent.
Cream is to be considered as a means of distribution since it is
often used by children.
=Treatment of tuberculous milk.= It is easily possible to treat milk
or factory by-products so as to render them positively safe. The
process of pasteurization or sterilization is applicable to whole
milk, and when effectively done destroys entirely the vitality of
any tubercle bacilli. In making such exposure, care should be taken
to prevent the formation of the "scalded layer," as the resistance
of the organism toward heat is greatly increased under these
conditions. In a closed receptacle, 140° F. for 15 to 20 minutes has
been found thoroughly effective in destroying this organism. A
momentary exposure at 176° F. is likewise sufficient. This is the
method that is almost universally used in Denmark in the manufacture
of the finest butter.
In the treatment of factory by-products, heat should also be
employed. In Denmark, compulsory pasteurization at not less than
176° F. is required. This treatment prevents not only the
dissemination of tuberculosis among hogs and young cattle, but is
equally efficacious in preventing the spread of foot and mouth
disease.
The per cent of tuberculous milch cows varies widely in different
sections of the country, being greatest in the older dairy sections,
and in those supplying milk to the cities, on account of the
constant buying and selling of animals, thus giving more frequent
opportunity of introducing the disease into the herds. Throughout
the country at large, probably less than ten per cent of the cows
are tuberculous, and it is estimated that at least one per cent of
the diseased animals have tuberculous udders. It has been suggested
that the dilution of the milk of such animals with that of healthy
cows would remove a great part of the danger from milk. In the
case where the milk of a large number of herds is mixed, this may be
of some importance, but in no case is it safe to assume that
dilution of the milk of tuberculous cows is any guarantee of safety.
It has been shown that milk, perfectly normal in appearance, coming
from a tuberculous udder could be diluted a million times and still
produce the disease on inoculation into experimental animals. In the
case of swine, the susceptibility is so great that a single feeding
of infected milk, even in a very dilute condition, causes with
certainty the production of the disease.
Some observers maintain that the contamination of the milk with the
manure of tuberculous animals is of greater hygienic importance,
than that coming from diseased udders, since the number of animals
having tuberculosis of the lungs and intestines is far greater than
those with diseased udders.
=Economic aspects of bovine tuberculosis.= Not only is this disease
invested with much importance because of its inter-relation with the
human, but from an economic point of view alone, it is undoubtedly
the greatest scourge that affects the dairyman. Its insidiousness
makes it exceedingly difficult to recognize. The consequence is that
many fine herds become seriously involved before its presence is
recognized. In the main, the disease is introduced into a herd by
purchase, often by buying in pure-bred stock to improve the quality
of the herd. Where the disease has been established in a region for
some time, there is also danger that unheated factory by-products,
as skim milk and whey, may function in its spread. Where such
conditions prevail, the spread of the disease in the creamery
district is exceedingly rapid. When once introduced into a herd,
the disease sooner or later spreads from the originally affected
animal to others in the herd. Close contact, and close confinement
in ill ventilated stables facilitate the spread of the disease, and
sooner or later, other animals acquire the trouble. This may all
occur while all animals appear in a healthy condition.
The symptoms of the disease in the earlier stages are quite
indefinite. As the disease progresses, the nutritive functions
appear to be disturbed, and sooner or later, the body weight begins
to decline, and finally marked emaciation ensues. Accompanying this
condition, especially when the disease is in the lungs, is a cough,
which is generally aggravated with active exercise. While the
run-down condition permits frequently of the detection of the
disease in the advanced stages, it is wholly impossible with any
accuracy to diagnose the trouble in the incipient stages. It is at
this stage that the tuberculin test comes to the aid of the
stockman.
_Tuberculin test._ This test is made by the injecting beneath the
skin of the animal a small quantity (about 2 c. c.) of tuberculin,
and noting the temperature of the animal, before and after the
injection. Tuberculin, a product of the growth of the tubercle
bacillus, when injected into the body causes a marked rise in
temperature, in the case of an animal affected with the disease, and
no such elevation in the case of a healthy animal. The process of
preparing tuberculin makes it absolutely free from danger, so far as
liability of producing the disease, or in any way injuring the
animal, is concerned. Fig. 19 shows the temperature range of both
reacting and non-reacting animals. While the test is not
absolutely infallible, it is so far superior to any and all other
methods of diagnosis that it should take precedence over them.
=Miscellaneous diseases.= There are a number of diseases that affect
both human beings and cattle, the causal organisms of which may be
transmitted through the milk. Foot and mouth disease is one wide
spread in European countries but which has not yet gained a
permanent foothold in this country. The ingestion of the milk, which
always contains the causal organism, produces the disease in both
humans and cattle. In the human the disease is very similar to that
in cattle; it may end in death. Vesicles are produced in the mouth,
on the lips, nose and fingers. The causal organism, which has not
yet been demonstrated, may occur in butter or cheese. It is easily
destroyed by pasteurizing the milk.
[Illustration: Fig. 19.--Temperature Curves.
1, the temperature curve of a healthy animal after injection with
tuberculin; 2 and 3, the temperature curves of tuberculous animals
after injection with tuberculin. (After Moore.)]
Anthrax, actinomycosis (lumpy jaw), rabies, and malta fever are
diseases the organisms of which have been found in the milk of
affected animals. In case of the first three, while the possibility
exists of the infection of human beings by milk, it is improbable
that such infection does normally occur. Malta fever is becoming an
important disease in portions of southern Europe. It is produced in
man by the use of milk of goats suffering from the disease.
The organism causing contagious abortion in cattle is known to be
present in the milk of the infected animal at the time of its
withdrawal from the udder. It is not probable that the organism is
of any sanitary significance as far as man is concerned. It has been
shown that the organism is able to produce a disease in guinea pigs
on artificial inoculation that is very similar, so far as the
lesions are concerned, to tuberculosis. It is also probable that the
by-products of creameries and cheese factories may serve to spread
the disease from one herd to another.
Inflammation of the udder (garget) is a frequent trouble in every
herd. It is marked by the swelling of one or more quarters, by the
appearance of fever and changes in the appearance and composition of
the milk. The inflammation may be caused by cold or injury, or by
the invasion of the udder with pus-forming bacteria. In the first
case the trouble is not likely to persist for any length of time,
and does not spread to other members of the herd. The milk may be
more or less stringy, and may show a slimy flocculent sediment. It
cannot be asserted that such milk is harmful to man but it should be
rejected on general sanitary grounds, and because it cannot always
be differentiated from that coming from an udder in which the
inflammation is produced by bacteria.
Inflammation caused by the invasion of the udder with specific
bacteria is usually of greater severity, the entire gland often
becoming involved. The secretion of milk may cease and the function
of the diseased quarters may never be restored. The milk in the less
severe cases may not be abnormal in appearance, but with increasing
severity, the nature of the milk changes, until it may be a watery
liquid. The milk of any animal suffering from any form of garget
should be rejected, as it may cause trouble, especially in children.
There is some reason to believe that organisms coming from cases of
garget have been responsible for the extensive outbreaks of septic
sore throat that have occurred in some parts of the country.
The milk of animals suffering from indigestion, diarrhea, abscesses
on any part of the body, as from those which have retained the
afterbirth should be likewise rejected. In short only the milk of
healthy animals should be used for human food; that from any animal
suffering from any disease or which is receiving medical treatment
should not be so used.
=Typhoid fever=. The most important disease germ, distributed through
the medium of milk, that is unable to produce a diseased condition
in the cow is the organism of typhoid fever. This malady is an
intestinal affliction of man, and the germ causing the same is found
abundantly in the dejecta, both solid and liquid, as well as in the
blood in certain stages of the disease. While the causal organism
does not leave the body through the expired air, it is found
abundantly in both the urine and feces. Therefore, the dejecta, and
any articles that may be soiled with the same become a positive
menace.
Many different methods of transmitting the contagion exist, such as
water, food infected in various ways, contact with infected persons,
and through the medium of milk. Milk is not so frequently the cause
of dissemination as the other factors, but where milk supplies
become contaminated, epidemics of considerable magnitude are wont to
occur. The danger from milk is also aggravated by the fact that the
typhoid bacillus 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. In a
considerable percentage of cases, the disease is not sufficiently
severe to cause the patient to take to his bed. These so-called
"walking typhoid" cases are particularly dangerous, because they
serve to spread the disease organism more widely.
A very considerable proportion of the people that recover from
typhoid fever still continue to harbor the typhoid bacillus in their
urinary and gall bladders. This condition may obtain for years, and
since such individuals are in perfect health and are ignorant of
their own condition, and since they give off the organisms more or
less constantly, they are often the cause of extensive milk borne
epidemics. Such persons are known as "typhoid carriers" and
constitute one of the gravest problems the public official has to
contend with in his struggle to prevent the spread of typhoid fever.
Where outbreaks are caused by milk, they can readily be traced by
means of the milk route, as there are always a sufficient number of
susceptible persons, so that outbreaks of epidemic proportions
develop. In the Stamford, Conn., outbreak in 1895, 386 cases
developed on one milk route. In this case it was shown that the
carrying cans were thoroughly washed, but were later rinsed out with
_cold_ water from a polluted shallow well.
The mode of infection of milk varies, but in general, the original
pollution is occasioned by the use of infected water in washing the
utensils, or a case of "walking typhoid" or bacillus carrier, who
directly infects the milk. In case of sickness in rural families,
some member of the household may serve in the dual capacity of nurse
and milkmaid, thus establishing the necessary connection. Busey and
Kober report twenty-one outbreaks, in which dairy employees also
acted in the capacity of nurses. The fact that the urine of a
convalescent may retain the typhoid germ in large numbers for some
weeks renders the danger from this source in reality greater than
from feces, as, naturally, much less care is exercised in the
disposition of the urine.
The house fly is now regarded as one of the important means of
spreading typhoid fever, indeed it is often called the "typhoid
fly." The infectious material deposited in an open vault may serve
as a source from which the fly carries the organisms to milk and
other foods in the house or elsewhere. The protection of vaults and
the screening of every place where human food is handled or prepared
is the only protection.
It should be emphasized that in the case of the tubercle organism,
no growth ever occurs in milk, but with the typhoid bacillus growth
is possible. It thus needs but the contamination of the milk with
the smallest particle of material containing them to seed the
milk. By the time it is consumed it may contain myriads of the
disease-producing organisms.
=Diphtheria.= This is a highly infectious disease, affecting children
primarily and is characterized by the formation of membranous
exudates in the throat and air passages, which are teeming with the
causal organism, the diphtheria bacillus. This organism is capable
of forming highly toxic products, and it is to the effect of these
poisons that its fatal result is generally due. The organism is
thrown out from the body, in the main, through the mouth, the
surroundings of the patient being infected directly from the air,
and indirectly, by contact with polluted hands, lips, etc. Thus, the
germ deposited from the lips of a case of the disease, on the common
drinking cup, slate, lead pencils, toys, and the like, may easily
pass from child to child. Not infrequently, the causal organism
persists in the throat long after all evidence of membranous growth
has subsided, and so the child itself may act as a "bacillus
carrier."
Not so many epidemics of diphtheria as of typhoid have been traced
to milk, but the evidence is sufficient to indict milk as a
disseminator of contagion. In several cases, the diphtheria germ has
actually been isolated from infected milk supplies. Actual growth of
the diphtheria germ is said to take place in raw milk more rapidly
than in sterilized.
=Scarlet fever.= While the germ of scarlet fever has not yet been
isolated, and therefore its life history in relation to milk cannot
be depicted so accurately, yet milk-borne epidemics of this disease
are sufficiently abundant to leave no doubt but that this food
medium may sometimes serve as a means of disseminating such
troubles. Infection of the milk doubtless comes in the case of this
disease from direct contact with a person suffering from the malady.
=Cholera.= While this disease is of no practical importance in
America, owing to its relative infrequency, yet outbreaks of cholera
have been traced to milk, in spite of the fact that the causal
organism is more sensitive to the action of acids than most
disease-producing bacteria. In several outbreaks in India, milk has
been the medium through which the disease was spread. Generally,
infection of the milk has been traced to the use of polluted water.
=Children's diseases.= An exceedingly high mortality exists among
infants and young children in the more congested centers, especially
during the summer months. In the main, the cause of these troubles
is due to intestinal disturbances, and unquestionably, the character
of the food enters largely into the problem. As milk constitutes
such a large proportion of the diet of the young, and is so
susceptible to bacterial invasion, it would appear probable that
much of the trouble of this character is due to the condition of
this food supply. This is rendered more probable when it is
remembered that bottle-fed infants suffer a much higher mortality
than breast-fed children, due probably to the fact that the
lengthened period between the time the milk is drawn and consumed
permits of abundant bacterial growth. Much carelessness also
prevails among the poor in cities, relative to the care of utensils
used in feeding children. Nursing bottles often serve to infect the
milk. Where milk is pasteurized, or properly heated, it has been
found that the mortality rate has been greatly reduced, thus
indicating that the condition of the milk was directly responsible
for the death rate. In fact, the mortality from these indefinite
intestinal troubles probably exceeds that from all of the specific
infectious diseases combined. Improved care in handling this
sensitive food supply will do much to better conditions in this
direction.
=Ptomaine poisoning.= Acute poisoning affecting adults as well as
children, not infrequently occurs from the use of foods of various
kinds. Cases of poisoning arising from the use of shell fish, canned
meats, ice cream, cheese, and other dairy products, are from time to
time reported. These troubles are due to the production of toxic
compounds, in the main, probably caused by bacterial decompositions.
Often such troubles may affect a number of persons, as at banquets
and such gatherings, thereby giving the semblance of an epidemic.
While such troubles are doubtless to be ascribed to bacterial
activity, they are not transmissible from person to person.
In the case of troubles arising from ice cream and such confections,
the probable cause is due to the storage of milk or cream under
refrigerator conditions, where germ growth can go on in the product,
and yet the temperature be sufficiently low to prevent the usual
acid fermentations.
CHAPTER V.
FERMENTATIONS OF MILK.
Milk, under normal conditions, is always contaminated with bacteria
coming from the most varied sources. If it is produced under clean
conditions, the number of bacteria will be small, but in any case,
the number of kinds of bacteria that find their way into milk will
be large. Many of them find in milk at ordinary temperatures
suitable conditions for growth; they use a portion of some of the
constituents of the milk as food, producing certain other compounds
that are known as "by-products." These by-products impart to milk a
taste and odor that is not found in fresh milk. The effect of the
action of bacteria may also be made evident by the change in the
appearance of the milk. When these various changes become evident to
the senses, either by taste, smell or sight, the milk usually is so
modified as to be unfit for many ordinary purposes. The preservation
of milk, a subject to be treated later, is a study of the ways of
preventing or retarding the growth of bacteria in milk, and thus
delaying the time when evidences of their action first become
apparent.
Each class of bacteria produces more or less specific changes
in the milk as a result of their growth. Certain bacteria are
of the greatest benefit to the butter and cheese maker, while
others are distinctly harmful to the manufacturer of dairy
products. The changes produced by the different bacteria are called
"fermentations" of milk, each being most commonly named from the
most important by-product formed.
=Acid fermentation of milk.= Fresh milk has a sweet taste and little
or no odor, but if it is allowed to stand at ordinary temperatures,
it sours; the taste is no longer sweet because the sweetness of the
sugar of the milk is masked by the acid produced from the
decomposition of a portion of the sugar by the bacteria. The change
in odor and taste of milk is apparent long before the appearance is
altered and increases in intensity as the acid-fermentation
progresses. The first alteration in appearance is most usually one
of consistency; the liquid milk is transformed into a semi-solid
mass. The terms "curdling" and "sour" are usually synonymous. Milk
is, however, often said to be sour as soon as the acid fermentation
has progressed to a point where it is evident to taste or smell.
This process of souring, or the acid fermentation is so common a
change that raw milk which does not show this type of fermentation
is looked upon with suspicion, and, usually, justly so. The process
in the past was thought to be something inherent in the milk, a
natural and inevitable change. It is now known that this is not so,
but that it is due to certain kinds of bacteria, and that if these
are prevented from getting into milk, it will not sour, but will
undergo some other less desirable type of decomposition.
The acid-forming bacteria comprise but a very small part of the
total number of organisms that find their way into the milk during
its production on the farm, yet in sour milk scarcely any other
kinds of bacteria can be found. At ordinary air temperatures, the
acid-forming bacteria grow more rapidly in milk than do any other
forms, and the acid produced by them renders the milk an unfavorable
medium for the growth of other bacteria. This is the reason why milk
practically always undergoes the acid fermentation, although it is
contaminated with a host of other kinds of bacteria. If a mixture of
seeds is sown on low wet ground, certain kinds will grow best; if
the same mixture is sown on drier land, other types will find most
favorable conditions for growth, and the plants which appeared on
the low land will not appear. The same condition is found in milk
where the environment is most favorable for the acid-forming
bacteria.
=Amount of acid formed in milk.= In this country the acidity of milk
is expressed as so many per cent of lactic acid. A milk that shows
an acidity of one per cent should, theoretically, contain one pound
of lactic acid in each one hundred pounds of milk. The acid
determined does not actually represent lactic acid, as there are
other substances in milk which act as acids, with the reagents used
in the present methods of determining the acidity of milk. For
instance, perfectly fresh milk has an apparent acidity of 0.13 to
0.18 per cent, although no fermentation has occurred. Other acids
than lactic are formed in the acid fermentation, but the entire acid
content is referred to as lactic when speaking of the acidity of
milk. When the developing acidity of milk reaches 0.25 to 0.3 per
cent, a sour taste becomes evident and the milk will curdle on
heating. When the acidity increases to 0.6 to 0.7 per cent, the milk
curdles at ordinary temperatures. The acidity continues, however, to
increase until it reaches about 1 per cent, which is the maximum
amount that will be produced in milk by the ordinary acid-forming
bacteria. Milk contains about 4 per cent of milk sugar, all of which
is fermentable. If this were all decomposed by bacteria, the acidity
of the milk would actually exceed 4 per cent. It is thus evident
that the reason why more acid is not formed in milk is not because
of any lack of sugar. The bacteria, like all other kinds of living
things, are injured by their own by-products, unless these are
constantly removed in some way; in milk the bacteria cannot escape
the action of the acid which they themselves have formed,
consequently growth ceases. The amount of acid formed is dependent
on the kind of bacteria present and on the composition of the milk.
Certain bacteria will not produce enough acid to cause the curdling
of the milk; still others will form 2 or even 3 per cent. These
types, however, do not play any important part in the spontaneous
souring of milk.
In milk the acid first formed combines with the ash constituents and
the casein to form salts which do not seriously affect the growth of
the bacteria. Ultimately, the limit of the ash and casein to take up
acid is reached, and free lactic acid which is harmful to bacterial
growth appears. If the content of casein and ash constituents is
high, a higher degree of acidity will be reached than in a milk with
a lower content. If a large part of the volume of the milk is made
up of a compound that has no role whatever in the acid fermentation,
such as the butter fat in cream, the amount of acid formed per unit
volume of milk will be reduced, since in determining the acidity, a
definite volume of milk is taken, and the acidity is expressed, as
such a per cent of this amount.
=Types of acid-forming bacteria.= When substances undergo
decomposition, it is a common belief that compounds offensive to the
odor and taste are formed; but such is not necessarily the case. The
products of the decomposition may be as agreeable and as harmless as
the compounds decomposed. Whether the decomposition products of any
substance are offensive or not is dependent on the kinds of
micro-organisms acting on it. There are forms of acid-producing
bacteria that change milk in odor, taste, and appearance, yet the
sour milk is not offensive in any sense of the word. Other bacteria
also sour the milk, but produce offensive odors and a disagreeable
taste. Thus, the acid-forming bacteria may be divided into two main
groups, which may be designated as desirable and undesirable. This
division is of importance to the butter and cheese maker and to the
consumer of milk.
=Desirable acid-forming bacteria.= If milk is produced under clean
conditions, it is not likely to have a disagreeable odor or taste at
any time, even when it is sour; rather the taste is agreeable like
that of good butter milk. The curd is perfectly homogeneous, showing
no holes or rents, due to the development of gas, and there is but
little tendency for the whey to be expressed from the curd. This
type of fermentation is largely produced by the group of bacteria to
which has been given the name, _Bacillus lactis acidi_.
The main by-product of this group of bacteria is lactic acid; small
amounts of acetic acid and alcohol, with traces of other compounds,
are also formed. The agreeable odor and to some extent the flavor of
milk fermented by these bacteria is due to other by-products than
lactic acid, for this has no odor and only a sour taste. The acid
fermentation of milk is often called the lactic acid fermentation.
In reality only the fermentation produced by the desirable group in
which lactic acid is the most evident by-product should be thus
called.
[Illustration: Fig. 20.--Different Types of Curds.
On the left a solid, homogeneous curd produced by desirable
bacteria; on the right, the curd produced by harmful bacteria. Note
the gas holes and free whey.]
The bacteria of this group may enter the milk from the dust coming
from the coat of the cow. They are also found in the barn dust and
on cultivated plants. Under ordinary farm conditions, the larger
part of those found in milk come directly from the utensils. If the
milk is drawn under extremely clean conditions and care is taken to
sterilize the utensils, but few acid-forming bacteria of any kind
will enter the milk; under such conditions most of the acid-forming
bacteria will belong to the group in question. They find, however,
such favorable conditions for growth in milk that they develop more
rapidly than most other types with which milk becomes seeded;
consequently under normal conditions, they gain the ascendency and
so control the type of fermentation.
The desirable type of acid-forming bacteria do not form spores;
hence, are easily killed by heating the milk. They can grow in the
presence or in the absence of free oxygen. In the bottom of a can of
milk or in the middle of a cheese, there is no air, yet these
bacteria grow as well under these conditions, as in milk exposed to
the air. The range of temperature for growth varies from 50° to 100°
F. but development is most rapid at 90° to 95° F. and about 1 per
cent of acid is formed.
Another group of bacteria which may be classed among the desirable
acid-forming organisms is constantly found in milk. They have little
to do with the ordinary acid fermentation as they grow very slowly
at ordinary temperatures. If a sample of raw milk is placed at the
temperature of the animal body, the acidity will reach 1 per cent in
a few hours. Thereafter the acidity will increase slowly and may
reach three per cent or above. The continued increase in acid is due
to the growth of long rods of the _Bacillus Bulgaricus_ type,
which apparently enter the milk with the fecal matter. The nature of
the change produced by them in milk is very similar to that caused
by _Bact. lactis acidi_ in that lactic acid is the chief product; no
gas is produced and hence the curd is uniform in appearance.
Temperatures from 100° to 110° F. favor their development. Organisms
belonging to this group are used in the preparation of the fermented
milks now so widely sold in the cities.
These desirable, acid-forming bacteria are of the greatest service
in every branch of the dairy industry, whether in butter or in
cheese making, or in the sale of milk in the city. The dairy
industry is dependent upon fermentative activity, as much as the
manufacture of beer or wine, and the main basis of this is the acid
fermentation of the milk by these desirable types of bacteria.
Although milk contains a large amount of nitrogenous substances
(casein and albumen), it does not undergo putrid decomposition, as
do meat and eggs, not because it is not fitted for the growth of the
bacteria causing that type of change, but because the acid formed in
it stops the growth of the putrefactive bacteria. If a sample of
milk is placed in a stoppered bottle, it will have much the same
taste and odor at the end of several months as at the end of a few
days. The acid acts as a preservative, like the vinegar in pickles,
or the acid in silage and in sauerkraut. Meat placed in a stoppered
bottle which is then filled with milk will be preserved.
The products formed in the decomposition of meat and eggs are not
only offensive but may also be injurious to the health of the
consumer. Milk that has been fermented by the desirable kinds of
acid-forming bacteria is not harmful. It is consumed in a variety of
forms (buttermilk, cottage cheese) as a common article of food and
its use is rapidly increasing. The preparation of the pure culture
buttermilks or artificially soured milks that are now so frequently
recommended for digestive troubles rests upon an acid fermentation
of this type.
=Undesirable acid-forming bacteria.= Other types of bacteria capable
of forming substances that impart to milk an offensive odor and a
disagreeable taste not infrequently appear instead of the desirable
group. Instead of producing from the sugar of milk large quantities
of lactic acid, these types generate other acids, such as acetic and
formic, which impart a sharp taste to the milk. Besides the acids
the bacteria of this group form gases from the sugar of the milk.
Some produce small amounts of gas; others so much that the curd will
be spongy and will float on the surface of the whey. The
fermentation caused by them is often called a "gassy fermentation"
and is dreaded by butter and cheese makers since the gas is
indicative of bad flavors that will appear in the product. Gas may
also be produced in other types of fermentations to be discussed
later.
This class of bacteria enters the milk with the dust, dirt, and
manure, in which materials they are especially abundant. No spores
are formed; hence they are easily killed by heating the milk. They
grow both in the presence and in the absence of free oxygen. High
temperatures favor their growth, most rapid development taking
place at 100° to 103° F.
=Spontaneous fermentation of milk.= The normal souring of milk is due
to a mixture of these two groups of bacteria. The relative
proportions existing between the two in any sample of milk is
dependent on a number of factors, most important of which is the
degree of cleanliness exercised in the production of the milk. Where
careless conditions obtain under which dust and manure particles
find their way into milk, it becomes more abundantly seeded with
gas-generating bacteria, and consequently, the type of fermentation
is undesirable. If, however, the milk is drawn into clean utensils
and care is taken to exclude dirt, the pure lactic acid types are
able to control the character of the changes produced, and a clean,
pleasant tasting liquid results. It will be seen that things are
well arranged by nature; one of the most important food products
undergoes a type of decomposition that is not offensive and when
produced under clean conditions, the sour milk is as healthful a
food as is the fresh product. Thus there is every reason for
cleanliness in the production of milk, for cleanliness' sake and
because clean milk means better products, and greater returns to
everyone, producer and dealer.
There are other kinds of acid-forming bacteria in milk but they are
of small importance compared with those just discussed. Some of the
bacteria derived from the inside of the udder of the cow form acid,
but these forms grow very slowly in milk at ordinary temperatures,
and have no influence on the keeping quality.
[Illustration: Fig. 21.--Different Types of Curds.
The flask on the left shows the soft curd produced by the bacteria
that curdle the milk without the production of acid. The flask on
the right shows the gassy curd formed by butyric acid bacteria in
heated milk.]
=Sweet curdling fermentation of milk.= Samples of milk are sometimes
found that are curdled, but which do not taste sour, or have the
normal odor of sour milk. The curd is usually soft and the taste
bitter. It is evident that the curdling cannot be due to the same
factors as in the normal souring of milk. Such a change is similar
to the action of rennet which is used to curdle the milk in cheese
making. This ferment will curdle perfectly sweet milk, producing a
curd that looks like that formed in the acid fermentation of milk.
The cause of these sweet curdling milks, which appear from time to
time, is due to the introduction of certain bacteria which have the
power of secreting an enzyme resembling that found in rennet. In
such cases the milks curdle prematurely especially when warmed. The
curd may gradually disappear, for the bacteria also produce another
enzyme that digests the curd, and thus renders it soluble. When this
advanced phase becomes evident, it is often called the _digestive
fermentation_ of milk. This change is produced largely by
putrefactive bacteria of various kinds that find their way into milk
with dust and dirt. Many of them are spore formers; hence, are not
killed when milk is heated, as in pasteurization, while the
acid-formers are destroyed. Pasteurized milk is thus likely to
undergo the sweet-curdling fermentation, if it is kept for
any length of time. Raw milk rarely undergoes this type of
decomposition, since the rennet-forming bacteria under ordinary
conditions are unable to develop in competition with the
acid-forming bacteria.
=Butyric acid fermentation of milk.= A fermentation that is much less
frequently noted than the two previously discussed is known as the
butyric fermentation, since butyric acid is the principal
by-product. The causal bacteria cannot compete with the ordinary
acid-forming bacteria in raw milk; hence it is most frequently noted
in pasteurized milk, since the organisms produce spores and are not
killed by the heating. Pasteurized milk under the action of the
butyric acid bacteria undergoes a gassy fermentation, developing a
pronounced acidity and the disagreeable odor of butyric acid, which
resembles that of rancid butter. The butyric acid bacteria are
anaerobic, and thus can grow in butter and cheese away from the air.
=Slimy or ropy fermentation of milk.= A slimy or ropy condition of
milk is frequently noted on the farm and in the dairy. Several
causes for this abnormal condition exist. Sometimes the milk may be
slimy when milked from the cow. This occurs most frequently in the
case of inflammation of the udder which may or may not be due to
bacteria. The direct cause of the abnormal condition in milk is the
presence of fibrin and white corpuscles from the blood which form
masses of slimy material; in such cases the trouble does not
increase in intensity with age, nor can it be propogated by
transference to another sample of fresh milk.
[Illustration: Fig. 22.--Slimy Milk.
It does not mix with water when poured into it.]
Another type of slimy milk is produced by the growth of certain
types of bacteria which enter the milk after it is drawn from the
udder. These may come from various sources. The bacteria concerned
belong to two groups: (1) those that grow best in the air and do not
form acid; (2) those that grow in the absence of air, throughout the
entire mass of milk and which form acid. The slimy condition is
noted in the milk only after the milk has been stored for some time;
it usually increases with the age of the milk and can be produced
in a second sample by transferring a little of the slimy milk to it.
The fermentation produced by the aerobic bacteria is most often met
in bottled milk and cream during the warmer times of the year. On
account of their relation to oxygen, the growth is confined to the
surface of the milk and only the upper layer becomes slimy; thus
when the cream is removed, the abnormal condition is noted. The
sliminess is due to the mass of bacterial growth rather than to the
production of any specific substance in the milk. This trouble may
be of considerable economic importance to the dealer, as such
abnormal milk is objectionable for ordinary use, but as far as is
known, it is incapable of affecting the health of the consumer.
In numerous outbreaks of this trouble the source of contamination
has been traced to infection from well water or a stream, as the
organisms causing the trouble are found naturally in water. Keeping
the milk in a tank in the pump house sometimes permits of troubles
of this sort, the water used for cooling giving opportunity for
contamination. Cattle wading in a stream sometimes pollute their
udders and so indirectly infect the milk. Such outbreaks rarely
persist for any considerable length of time as the common acid
organisms soon regain the ascendency.
Creameries and cheese factories are sometimes troubled with
sliminess in starters. This seems to be due to some change which the
ordinary lactic acid bacteria undergo on long propagation rather
than to contamination of the starter. There are, however, types of
acid-producing bacteria that are able to form specific substances in
milk that are slimy in character. Two of these forms of slimy milk
are of economic importance. The slimy whey (lange Wei) of Holland
is added to milk in the manufacture of Edam cheese, apparently
serving the same purpose as the addition of the pure culture starter
in cheddar cheese making. In Norway, a sour, slimy milk
(taettemjolk) is used as food. It is produced by the addition of
some previously fermented milk. This beverage is also used in some
of the Norwegian settlements of Wisconsin, the original seed having
been brought from Norway, and the bacteria maintained by constant
propagation from one sample of milk to another. The milk has the
odor and taste of butter milk, but is not especially appetizing in
appearance to any one not accustomed to it; it is, however, as
harmless to health as is any other form of sour milk. It is not
known that any of these forms of slimy milk are distinctly harmful
to the quality of butter or cheese.
=Alcoholic fermentation of milk.= The bacteria as a class are
incapable of producing alcohol in appreciable amounts. The alcoholic
beverages, beer, wine, and cider, are produced by the growth of
yeast, in such sugar containing liquids as fruit juices, extracts of
grains, etc. The common types of yeasts are incapable of acting on
milk sugar, but they can ferment glucose, maltose, and cane sugar,
forming equal amounts of alcohol and carbonic acid gas, which causes
the effervescence of fermented and carbonated drinks. There are,
however, some types of yeasts found in milk and its products that
are able to ferment milk sugar.
All yeasts grow best in an acid medium, hence those fermenting milk
sugar find suitable conditions for growth in sour milk or whey. They
may at times become of economic importance in the cheese industry,
because of the contamination of the milk with large numbers of
them. The arrangement of the whey vat is often such that it cannot
be completely emptied and cleaned; the sour whey thus presents
favorable conditions for the growth of the lactose-fermenting
yeasts. The return of the whey to the farm in the milk can that is
often imperfectly cleaned may serve to contaminate the milk with the
yeast. In the making of Swiss cheese the whey is often so handled as
to favor especially the growth of such yeasts, and since this type
of cheese is prepared from sweet milk, the competition between the
yeast and the acid-forming bacteria is not so sharp as in the making
of cheddar cheese. The writers have found several instances where
considerable loss was occasioned in the Swiss cheese industry
through the development of gassy cheese due to this type of
fermentation.
The yeasty or alcoholic fermentation may also be of importance in
butter making. In many sections of the country the milk is separated
on the farm and the cream is forwarded to the creamery at more or
less infrequent intervals. It becomes sour and if it has become
contaminated with yeasts, they will find favorable conditions for
growth in the acid medium. A large amount of carbon dioxide gas is
produced. Cans of gathered cream often foam to such an extent as to
run over, and in some cases actual explosions have occurred on
account of the great pressure caused by the gas.
=Bitter fermentation of milk.= Bitterness in milk may be due to
bacteria that enter the milk after it is drawn from the cow, or it
may be caused by the feed consumed by the animal. It has been
previously shown that certain specific substances contained in the
food may be absorbed and reappear in the milk. If the animal eats
ragweed, lupines, or other plants containing bitter substances, the
milk is likely to have a bitter taste, which will be noticeable at
the time the milk is drawn. The milk of cows at certain advanced
stages of lactation may show a bitter taste, due to a change in the
ash constituents of the milk in which the lime salts are largely
replaced by salts of sodium.
There are many bacteria that will impart to milk a bitter taste.
Milk that has undergone the sweet-curdling fermentation is likely to
be bitter, as is the ease with pasteurized milk. Some of the
acid-forming bacteria are able to develop a bitter principle, the
milk retaining a pleasant odor and having the normal amount of acid,
while the taste is intensely bitter. One of the authors (H) found in
the case of a Wisconsin brick cheese factory, that the usual acid
organism was almost wholly replaced by a bitter type.
Storage of milk at very low temperatures is conducive to the
appearance of a bitter taste in milk, the explanation in this case
being that the acid-forming bacteria are unable to grow at a low
temperature, while some of the putrefactive forms can multiply and
develop these astringent or bitter by-products.
=Miscellaneous fermentations of milk.= There are a number of other
abnormal fermentations in milk that occur so rarely as to be of but
little economic importance. Some, as the colored milks, are however,
quite striking, and on this account have had much attention directed
to them in the past. There are bacteria that are able to produce
various colored substances, such as red, yellow, and blue. In case
milk becomes seeded with large numbers of any of these kinds, it is
very likely to be colored by the growth. Red milk may be due to
bacteria, but more frequently is caused by the actual presence of
blood in the milk, due to a wound in the udder, or the effect of a
severe case of inflammation of this gland. Such a condition may be
readily distinguished by allowing the milk to stand for a short
time, in which case, if due to blood, the red corpuscles will soon
settle to the bottom of the container, while bacterial troubles
producing a red coloration are more evident on the surface.
It is also claimed that certain bacteria may impart a soapy taste or
turnip flavor to milk.
=Cycle of fermentations in milk.= If a sample of milk is allowed to
stand, it will undergo a certain sequence of fermentations that well
illustrates the principle that one type of organisms is dependent on
some other type to furnish suitable conditions for its development.
This cycle of changes that normally occurs in milk is as follows:
(1) The bacteria that come from the interior of the udder are the
first to develop, but usually the change they produce is not
evident.
(2) Of the types that gain admission, subsequent to the milking, the
acid-producing species are able to adjust themselves most perfectly
to the conditions that obtain in milk. Within a few hours they
greatly predominate and soon the milk curdles under the production
of acid. Their growth, however, is soon stopped by the accumulation
of their own by-products.
(3) The semi-solid curdled milk, on account of its acid reaction
then becomes a favorable medium for the growth of molds; a prevalent
form, known as _Oidium lactis_ usually develops as a white velvety
layer. The molds in their growth form alkaline by-products, which
tend to neutralize the acid reaction, so that in the course of two
to three weeks, if the layer of the milk is not too deep (an inch or
less), the chemical reaction of the milk becomes neutral or
alkaline.
(4) The putrefactive bacteria which found their way into milk when
it was first drawn, and which have remained dormant in the sour
milk, now find favorable conditions for growth. As a result of their
activity, the milk soon undergoes a putrid decomposition, which is
marked by offensive odors.
If the milk is placed under such conditions as will exclude the
growth of the mold, such as where the air is excluded from the
surface, the sour milk will remain in that condition for an
indefinite period, since the putrefactive bacteria are inhibited in
their development by the acid, in a manner comparable to the
preservation of pickles in vinegar, or the keeping of silage because
of the acid that is produced as a result of the changes that the
plant tissue undergoes when excluded from the air. The preservative
effect of acids is of much importance in the case of certain dairy
products (see Chapter VIII).
=Fermented drinks from milk.= Within the last few years a great deal
of attention has been directed toward the preparation of various
kinds of drinks from milk. The use of such beverages has rapidly
increased. Butter milk is one which meets with the greatest
approval. The true butter milk from cream that has been soured by
the desirable acid-forming bacteria has a mild agreeable acid taste,
wholly free from any sharpness that is often noted in butter milk
made from cream in which considerable numbers of the undesirable
acid-forming bacteria have grown. Butter milk made from pasteurized
cream soured with pure cultures will have good keeping qualities and
is a most healthful drink for all classes of people, even for young
children.
Butter milk is also prepared by allowing milk to sour and then
breaking up the curd by stirring. If the type of fermentation is
controlled as may be done (see Chapter VII), such a form of
fermented milk is a most desirable drink. It is probably as
healthful and has all the therapeutic properties that are ascribed
to other forms of fermented milks such as the Bulgarian "Yoghurt."
This type of fermented milk is produced by an acid-forming organism
that can form large amounts of acid, 2.0 or 3.0 per cent. The casein
is dissolved to some extent and the remainder so changed, that it
will remain in suspension for a long time in a finely divided form,
after the curd has been broken up. Such milk is sold under various
names at home and abroad. One of the authors (H) has found such
organisms in practically all milks examined. If raw milk is kept
warm (98° to 100° F.) in a stoppered bottle which is filled full,
the acidity will be found to increase slowly from day to day,
reaching a maximum in ten to fourteen days. If the milk is then
examined, it will be found to contain large numbers of an
acid-forming organism very different in appearance from the bacteria
causing the rapid souring of milk at ordinary temperatures. This
organism is very similar if not identical with the one found in the
Bulgarian milk to which the name _B. Bulgaricus_ has been given. The
use of the milk fermented by this organism has spread rapidly
because it is claimed by certain European bacteriologists that it
has a favorable effect on the health of people, especially those
suffering from intestinal troubles. It is not at all certain
that ordinary sour milk or butter milk will not have the same
effect; in fact in many of the fermented milks sold in Europe, _B.
Bulgaricus_ has not been found, but only the ordinary lactic acid
bacteria.
Several alcoholic drinks made from milk, such as kefir and koumiss,
have been originated among the nomadic tribes of Western Asia. Kefir
is prepared from cow's milk by adding the kefir ferment in the form
of grains which contain a number of kinds of bacteria and a yeast.
The acid-forming bacteria impart a sour taste to the fermented milk,
while the yeast forms carbon dioxide and about two per cent of
alcohol. If the milk is allowed to ferment in stoppered bottles, the
resulting product will be an acid effervescing drink, which is
claimed to be more easily digested than sweet milk. This drink is
used frequently in the treatment of invalids but it is improbable
that it is more easily digested than ordinary soured milk or butter
milk. The grains are removed from the fermented milk, and are then
added to a quantity of fresh milk, or they may be dried and kept for
future use. When needed again, they are soaked in water, then added
to the milk.
Koumiss is made in Russia from mare's milk and has much the same
composition as kefir. In America and Europe it is made from cow's
milk, by adding cane sugar and compressed yeast. The yeast ferments
the cane sugar while the acid-forming bacteria ferment the milk
sugar. There is thus obtained a drink that is similar in composition
to the real koumiss, in which both the acid and the alcohol come
from the fermentation of the milk sugar. In koumiss and kefir the
curd is very finely divided and will remain in suspension for a long
time as with butter milk.
=Determination of the cause of taints in milk.= It is often of the
greatest importance to be able to locate the cause of abnormal
odors or tastes in milk, since methods for overcoming the trouble
can be intelligently applied only when the actual cause is known. An
abnormal condition may be caused either by the direct absorption of
odors before or after the milk is drawn from the animal, or it may
be due to bacteria. If the milk appears bad-flavored when first
drawn, and if such taint becomes less pronounced as the milk becomes
older, it is likely that the trouble is due to some characteristic
of the feed. Certain feeds, like green rye, rape, cabbage, and
certain of the root crops, like turnips, impart a strong odor to
milk, if the same are fed shortly before milking. If the tainted
condition appears only some time after the milk is drawn, it may be
due to the direct absorption of taints from the surroundings in
which the milk is kept, or it may be caused by bacteria. These
causes can often be differentiated, by noting whether the taint
tends to increase in intensity with age. If such is the case, it is
likely that the cause is of germ origin, but if the reverse is true,
it cannot be ascribed with certainty to bacteria and recourse must
be had to other methods, such as the transfer of a small quantity of
the tainted milk to a sample of perfectly fresh milk, or preferably
to some milk that has been heated to the boiling point and then
cooled. In the case of an odor due to direct physical absorption, it
will not appear in the inoculated sample, since the small amount
transferred is not sufficient to be noted. If it is due to living
organisms, the inoculation of the smallest quantity into a fresh
sample is likely to reproduce the same change as originally noted.
=Tests for the bacteriological condition of milk.= Within certain
limits milk can be indirectly examined as to its bacterial content
without any special equipment. Milk when drawn from the cow has an
apparent acidity ranging from 0.16 to 0.18 per cent. By the use of
any of the methods of determining acidity in milk, much can be told
concerning the number of bacteria in the milk, and hence concerning
its keeping quality. Milk that has an acidity of over 0.2 per cent
is certain to contain many bacteria, and consequently will keep
poorly. Such milk is of low value for market milk, but may not be
objectionable for butter or cheese making. If the acidity is below
0.2 per cent, but little can be told as to the numbers of bacteria,
since any increase in acid is always preceded by an enormous
increase in the numbers of acid-forming bacteria.
A more important test than the acid test, from the standpoint of the
butter and cheese maker, and even the milk dealer, is the
fermentation test. In its simplest form, it consists in placing a
sample of the milk to be tested in a warm place and noting the time
required to curdle and the type of curd formed. In this country the
fermentation test has been largely supplanted by the Wisconsin curd
test which possesses the advantage of detecting the presence of
bacteria harmful in cheese making, especially the gas forming
bacteria.
The curd test is helpful in detecting the source of an abnormal
condition in a milk supply coming from diverse sources. The milk
furnished by each patron can be tested separately and the trouble
located, perhaps in an individual herd; the offending herd
determined, the test may then be used on the milk of individual
cows. The cheese maker and the milk dealer should be able not only
to detect which of the patrons furnish him poor milk, but he should
be able to give the patron definite instructions how to avoid the
sources of such trouble. This information can be given only when
the source is positively known.
[Illustration: Fig. 23.--Curd Test.
A good curd obtained from milk containing no harmful bacteria but
many desirable acid-forming organisms.]
The Wisconsin curd test is made as follows: Samples of the milk to
be tested are placed in sterile pint fruit jars. The milk is warmed
to 90° F., ten drops of rennet are added to each sample, and as soon
as the curd is solid, it is cut into small pieces with a case knife
so as to facilitate the expulsion of the whey. As the curd settles
to the bottom of the vessel, the whey is poured off at intervals so
that a pat of firm curd is left. As the milk curdles the bacteria
are enmeshed and are carried with the curd. The jars are kept at a
temperature of 100° to 105° F., since this temperature is favorable
to growth of the bacteria that are sought, the gas-forming
organisms. At the end of ten to twelve hours, the jars are examined;
if the curd is solid, the texture firm, not mushy or slimy on the
surface, if the odor is agreeable, it indicates that the milk
contains few or none of the undesirable forms of bacteria. If the
curd is full of gas holes, it is apparent that undesirable bacteria
are present and under such circumstances the curd will not have an
agreeable odor. If the gas-forming bacteria are numerous, the curd
may even be spongy from the abundance of gas holes, and the
undesirable odor more pronounced. Such curds are tough and rubbery.
In some cases a bad flavor or odor is apparent even though the
texture of the curd is not open and full of holes. The curd, the
surface of which is slimy indicates undesirable organisms. A solid
curd of agreeable odor is indicative of the presence of the
desirable acid-forming bacteria. Such a milk is excellent from the
standpoint of the butter or cheese maker, but may not be so
desirable from the standpoint of the milk dealer on account of its
poor keeping qualities. On the other hand a milk suitable from the
standpoint of the milk dealer, on account of its low germ content,
and hence good keeping quality, may give a poor curd test. It is
certain to contain some bacteria, especially those from the interior
of the udder while it may contain none of the desirable acid-forming
organisms without which a curd of good texture and flavor can not be
obtained. The bacteria in the clean milk will grow rapidly at the
high temperatures at which the curds are kept and the changes they
will produce as to flavor and odor may be undesirable. The milk
might be judged as poor when in reality it might be a most excellent
sample, and if kept at the ordinary storage temperatures, it might
keep for days. The test when used for market milk should be
interpreted with this in mind.
[Illustration: Fig. 24.--Curd Test.
The curd obtained from milk containing many gas-forming bacteria.
The irregular, angular holes are mechanical, due to the imperfect
fusion of the pieces of curd.]
If the results are to be of any value, the test must be made with
care to avoid all sources of error; the tester must know that the
bacteria causing the gas and bad flavors in the sample were
originally present in the milk at the time the sample was taken, and
that they have not come from the containers used or from other
sources. To insure these conditions the jars must be thoroughly
cleaned and then sterilized just before use by placing them in cold
water and bringing them to the boiling point, or sterilized by a
thorough steaming. The sample of milk of a patron must be taken so
as to avoid contamination from the milk of the other patrons. This
can best be done by filling the jars as the milk is poured from the
patron's can into the weigh can. In cutting the curds, the knife
used must be dipped in hot water between each test to cleanse the
same. In short, the test should be carried out with great care so
that the tester is certain of the results obtained.
Other tests for the bacteriological condition of milk will be
described in Chapter IX.
=Overcoming abnormal fermentations.= The lactic acid bacteria are
often looked upon as normal to milk, and it is certain that they are
to be classed as harmful, only as they injure the keeping qualities
of milk. In milk designed for butter and cheese their presence is
necessary. At times these desirable forms of bacteria may disappear,
and be replaced by less desirable types. In one case it was observed
that the usual lactic bacteria had been replaced in a cheese factory
supply by an acid-forming organism that produced an intensely bitter
taste in the milk, thus rendering the cheese of no value. When such
harmful forms appear, they must be overcome, and the normal types of
bacteria replaced. A thorough cleaning of the milk utensils,
attention to the cattle and all places from which such bacteria may
find their way into the milk is often sufficient to cause a
disappearance of the trouble. If the acid-forming bacteria have
disappeared, the inoculation of the milk with cultures in ways later
to be discussed is often of advantage. At times more stringent
measures must be employed in order to destroy the harmful bacteria,
such as the use of strong disinfectants.
=Disinfection and disinfectants.= If any building or room becomes
infected with disease-producing bacteria, or if organisms causing
abnormal fermentations become established in a factory, the use of a
disinfectant that will destroy with great rapidity the life of
bacteria is necessary. The disinfection of all types of dairy
apparatus and utensils can be accomplished by thorough cleansing,
and by the use of steam or boiling water. The disinfection of rooms
and stables cannot be so readily accomplished.
Consideration must always be given to the resistance of the organism
it is desired to destroy. Those that form spores are very resistant
toward all chemical agents, while those that do not produce these
resistant bodies are easily killed. In the dairy and factory, it is
often necessary to destroy the organisms that develop in decomposing
organic matter. Here, as in all disinfection, a thorough cleaning
should precede the application of any disinfectant. Some chemicals
act as deodorants, _i.e._, destroy the offensive odor, without
removing the cause. It is impossible effectually to destroy bacteria
embedded in a mass of organic matter, and through the removal of the
material itself, the larger part of the bacteria will be removed.
The disinfectant then comes in direct contact with the surface to be
disinfected, consequently destroys the bacteria not removed in the
cleaning.
All places in which dairy work of any kind is done should be
provided with an abundance of light and air. The direct rays of
the sun have a powerful disinfecting action, and light makes evident
accumulations of dirt that in a darker room would be unnoticed.
Ventilation keeps the rooms dry and thus prevents the growth of mold
and the development of a musty odor.
Disinfectants are divided into two classes: (1) solid materials used
in suspension, or in watery solutions; (2) gaseous substances. The
latter are preferable for room disinfection when their use is
permissible, for the gas penetrates to every part of the space, even
into the cracks. Gaseous disinfectants can only be used when the
space is tightly closed, for the gas must be confined for several
hours in the room, in order to make the process effective. Such
disinfectants can often be used to advantage in the treatment of
refrigerators and cheese rooms to destroy mold spores. In less
tightly closed spaces, reliance must be placed on the use of the
solid or liquid disinfectants.
=Lime.= Quick lime or stone lime has a considerable disinfecting
action. On exposure to the air, quick lime becomes air slaked, and
then has no disinfecting action whatever. Water-slaked lime used in
the form of white wash, lime water, or the powder is effective.
Air-slaked and water-slaked lime are similar in appearance, but a
difference can be noted by placing a particle of each on the tongue;
the air-slaked tastes like chalk while the water-slaked material
causes the tongue to burn.
White wash is one of the most effective agents that can be used in
the disinfection of barns, milkrooms, etc. Besides being a fairly
strong disinfectant, it has a tendency to absorb odor, it encrusts
the walls and lightens the interior of rooms. It can be applied with
a brush or with a spray pump.
=Carbolic acid and cresol compounds.= These substances are among the
cheapest and best disinfectants, but their use in the dairy is not
advisable, on account of the penetrating and lasting odor. They can
be used to advantage on the farm. Some of the proprietary compounds,
as Zenoleum, Kresol, etc., are easily applied, since they mix
readily with water in all proportions, forming a milky-white
emulsion that can be easily applied. They are less caustic and less
poisonous than carbolic acid.
=Corrosive sublimate.= Corrosive sublimate is the most efficient
disinfectant under ordinary conditions. It is such an intense poison
that it must be used with caution in places to which stock have
access, or in the dairy. A solution of one part of the salt to a
thousand parts of water (half ounce to 4 gallons of water) is the
standard generally used.
For gutters, drains, and waste pipes in factories, ferrous sulphate
(green vitriol), and copper sulphate (blue vitriol), can be used to
advantage. They are to be classed as deodorants rather than as true
disinfectants. Since they have no odor of their own, they can be
used in any amount in the dairy.
=Sulphur= can be used to advantage in the destruction of mold spores
in cheese rooms, but the effect of the vapors of burning sulphur on
germ life is relatively slight, unless there is an abundant supply
of moisture in the air of the enclosed space, in which case
sulphurous acid is formed which has a much greater effect. To have
the desired effect sulphur should be burned at the rate of three
pounds to each one thousand cubic feet of space, and the room kept
sealed for at least twelve hours. If the sulphur is placed in an
iron kettle which is set in a vessel of water, danger from fire
will be avoided, and the heat generated by the burning sulphur will
evaporate sufficient water to increase the effect of the fumes.
=Formalin.= Another disinfectant that may be used as a liquid or as a
gas is formalin, which is a watery solution of the gas,
formaldehyde. It is much more powerful in its action than sulphur,
and has a great advantage over corrosive sublimate and other strong
disinfectants in that it is not so poisonous to animals as it is to
bacteria and fungi.
It can be used as a solution (one to five per cent) for the washing
of woodwork, or for the treatment of any object, since it has no
corrosive action. It can also be employed as a gaseous disinfectant
for the treatment of rooms. It is most conveniently applied by
suspending large cloths in the room and spraying them with the
solution, then closing the room for a number of hours.
=Bleaching powder.= Chloride of lime, or bleaching powder as it is
often called, is a good disinfectant, as well as a deodorant. It is
used as a wash in the proportion of four to six ounces to a gallon
of water. It must be used with care in factories since the free
chlorine that is given off has a penetrating odor.
CHAPTER VI.
PRESERVATION OF MILK.
It has been shown in a previous chapter that milk becomes
contaminated with a multitude of bacteria not only on the farm where
it is produced, but during the various stages prior to its use. Many
of the bacteria which find their way into milk are readily able to
develop, and by their growth, render the milk unfit, or even harmful
for human food. With the most stringent precautions that can
reasonably be taken, it is impossible to avoid all contamination;
hence, all grades of milk will soon spoil, unless some means of
preservation is employed. Indeed, of all the foods classed as
perishable, milk is the one that most rapidly deteriorates. Produced
under ordinary conditions, it is unfit for ordinary use in a few
hours if kept at 70° F.
There are three possible ways by which milk may be preserved: (1)
The removal of bacteria that have gained entrance to it; (2) The
prevention of growth of the contained bacteria; (3) The destruction
of the contained organisms. In practice at least two and sometimes
all of these methods are employed. The prevention of contamination,
a subject discussed in Chapter III is in reality one of the most
efficient means of preserving milk. In milk production, as
elsewhere, prevention is preferable to cure. Milk produced under
such conditions that its germ content is but a few thousand per
cubic centimeter will keep much longer than that handled in the
ordinary manner.
It might naturally be supposed that any method by which dirt is
removed from milk would improve the keeping quality of milk, due to
the reduction of bacteria, yet while the straining of the milk at
the time of milking removes dirt of various kinds, it does not
appreciably enhance the keeping quality, owing to the fact that the
bacteria adherent to the dirt particles are washed off in straining,
and pass through the pores of the strainer.
=Filtration of milk.= It is possible to remove all bacteria from water
and other fluids and thus render them sterile by passing through
filters of unglazed porcelain. This process can not be used with
milk for the fat globules are larger than the bacteria (see Fig. 6)
and any process that would remove the latter would also remove the
former. The term "filtration" is applied to a process used in some
European cities for the removal of the insoluble dirt that has been
introduced into the milk. Suitable containers are filled with layers
of coarse sand at the bottom and with finer sand at the top. The
milk is introduced at the bottom and is forced upward through the
sand. Such a filtering process is a very efficient means of removing
the dirt; but unless the filters are kept scrupulously clean, the
bacteria are likely to grow in the filtering material, so that the
number of organisms in the milk may actually be increased by the
filtering process. It is necessary to remove the sand daily and
thoroughly wash and sterilize the same. The extra care required in
keeping these sand filters in sanitary condition has been the great
objection to their employment in this country. Filters of other
material such as cellulose have been employed but with no marked
success.
=Clarifying milk.= A much more efficient and less troublesome means of
removing the insoluble foreign particles from milk is to pass it
through a cream separator, allowing the cream and skim milk to mix
in the same container. The slime that collects on the wall of the
separator bowl is made up of dirt, casein, bacteria, and the
cellular debris from the interior of the udder. The bacteria are
heavier than the milk serum, and would, therefore, be deposited on
the wall of the bowl were it not for other factors that in a measure
prevent this. The movement of the fat toward the center of the bowl
carries into the cream a considerable proportion of the bacteria in
the milk. The slime will always contain many more bacteria than the
milk, but the per cent of bacteria thus removed is relatively low,
due to the small amount of slime obtained from the milk, so that the
actual effect of clarification on the keeping quality of milk is
insignificant. The complete removal of all insoluble and therefore
visible dirt is, however, regarded of sufficient value to warrant
the use.
Machines designed especially for the clarification of milk are now
widely used. They differ from the cream separator in that the milk
is introduced at the outside of the bowl and hence there is no
separation of the fat from the serum. It is claimed that the removal
of the dirt, cells from the interior of the udder and bacteria is as
efficiently done as when the separator is used. The advantages
claimed for the machine are that it has no effect on the subsequent
gravity creaming of the milk and that less power is demanded than
for the separator.
From the standpoint of the consumer, all processes by which dirt is
removed from milk are objectionable, since they make the milk
appear cleaner and better than it really is, the harm having been
done when the dirt with the adherent bacteria found its way into the
milk. The removal of the foreign matter that has been introduced
into the milk will have but little effect in reducing the number of
bacteria, since a large part of the organisms will have been washed
off the insoluble material. All of these processes improve the
appearance of the milk but have little or no influence in increasing
its keeping quality or its healthfulness.
=Preservation by cold.= The only legitimate way of preventing the
growth of bacteria in milk is by holding it at temperatures at which
the ordinary forms of bacteria cannot thrive. Bacterial growth is
greatly checked at temperatures approximating 50° F., or below,
although certain types multiply at the freezing point or slightly
above. If food products are actually congealed, no germ growth
occurs, and they may be kept quite indefinitely, but this process
cannot be successfully applied to milk, as the fat and casein are
physically changed, so that a normal emulsion can not again be made
when the frozen milk is melted. The fat separates in visible masses
as though the milk had been partially churned. On account of this
fact milk must be stored at temperatures above the freezing point.
In Denmark efforts have been made to preserve milk, that is to be
shipped long distances, by freezing a portion of the milk, and
placing a block of the frozen milk in each can after cooling the
main mass of milk nearly to the freezing point. Even this method has
not proven practical, and at present reliance is placed on thorough
chilling of the milk. At 32° F., the lactic bacteria cannot grow,
but other types, such as certain of the putrefactive forms grow
slowly; the milk may, therefore, have no objectionable odor or taste
and yet be swarming with bacteria. In cities the practice is
followed of placing cream in cold-storage during the cooler periods
of summer in preparation for an increased demand, during hot weather
or on holidays. It seems probable that poisoning from ice cream may,
at times, be due to the use of such cream.
=Preservation by the use of antiseptics.= Many chemical substances
prevent the growth of bacteria when added to food supplies; such
substances thus used are called _preservatives_. In the past some of
these have been used in milk to a great extent, but at present, on
account of stringent pure food laws, they are employed only to a
slight extent. There is a great temptation for the small milk dealer
in the city to employ them to preserve the excess of milk from day
to day, as through the use of a few cents worth of some preparation,
many dollars worth of milk may be kept from spoiling until it can be
sold to the unsuspecting consumer.
Formalin has been most widely used in milk because it is a most
efficient preservative; it is cheap and cannot be detected by the
consumer, although it injures the digestibility of the casein. One
ounce will keep one thousand pounds of milk sweet for twenty-four to
forty-eight hours. Borax, boric acid, and salicylic acid have also
been used, but these substances must be employed in much larger
quantities than formalin. Bicarbonate of soda has sometimes been
used although it is not a true preservative. Its effect is based
upon the neutralization of the acid produced by bacterial growth.
The treated milk does not taste sour so quickly, and the curdling of
the milk is also delayed.
Many proprietary compounds for milk preservation have been placed on
the market in the past, but the use of all of these is illegal in
most states. The federal law also prohibits their use in all dairy
products that pass into interstate commerce.
Within recent years a method for the preservation of milk was
introduced by a Danish engineer, Budde, which consists of adding to
milk a very small amount of peroxid of hydrogen which is a very
efficient antiseptic. The peroxid is decomposed by some substance in
the milk; the products of decomposition being water and free oxygen.
The peroxid together with the application of heat at a comparatively
low temperature (122° F.) is sufficient to destroy the larger part
of the bacteria in the milk. Practical difficulties are encountered
in the commercial application, so that it is probable the process
will never be a commercial success.
For the preservation of composite samples of milk for analytical
purposes, such as the Babcock test, strong disinfectants, as
corrosive sublimate, are employed. This material is very poisonous,
and leaves the milk unchanged in appearance. Some coloring matter is
therefore usually mixed with the sublimate in making the
preservative tablets, so as to render their use more conspicuous.
Corrosive sublimate not only stops all bacterial growth, but quickly
destroys the life of the cells. Bichromate of potash is generally
employed in the preservation of composite samples for the Hart
casein test.
=Destruction of bacteria in milk.= Actual destruction of the life of
bacterial cells by heat is one of the most important ways for
preserving milk. Heat easily destroys the vegetating, growing
bacteria, while the spores, of which there are always a number in
milk, are very resistant. If, however, the growing organisms are
destroyed, the milk will keep much longer than if it had not been so
treated.
The process of pasteurization was first used by the French
bacteriologist, Pasteur, for the treatment of the wines of his
native district which were likely to undergo undesirable types of
fermentations due to bacteria. From the wine industry it was applied
in the brewing industry, and was later found to be of the greatest
service in the dairy industry. The process of pasteurization may be
briefly defined, as the heating of milk to temperatures, varying
from 140° F. and upward for a longer or shorter time, and
subsequently cooling to a low temperature, so as to prevent the
germination of the spores that are not destroyed by the heating.
=Effect of heat on milk.= When milk is heated it undergoes more or
less profound changes, depending on the temperature and time of
heating. Some of these changes are of practical importance, since
they are more or less evident, and objectionable to the consumer.
In raw milk the fat globules are largely found in larger or smaller
aggregates, rather than uniformly distributed throughout the serum.
The surface of a mass of fat globules is smaller in proportion to
the volume of the mass than is the case with single globules, hence
globule clusters encounter less resistance in their passage through
the serum, either as they rise to the surface in gravity creaming,
or in the separator bowl. If these clusters are broken up, so
that the globules are uniformly distributed, the milk will cream
much less rapidly and completely. In the process known as
"homogenization" of milk, the individual fat globules are broken
into such small globules, that they cannot overcome the viscosity
of the serum, and they remain distributed throughout the milk. In
such cases, no cream rises, and even the cream separator is unable
to remove the fat from such milk.
In selling bottled milk, it is highly desirable that the cream line
should show distinctly. In normal milk, this line forms in a few
hours, but where milk is heated to a high temperature, and agitated
at the same time, the clusters of fat globules are broken apart and
the creaming power injured. This physical change is dependent not
only on the temperature, but also on the time of exposure. A
momentary exposure at 160° F., or for 20 minutes at 145° F., is
about the maximum limit which can be applied to milk without
material injury to the creaming property.
[Illustration: Fig. 25.--Fat Globules in Raw Milk.
In raw milk the fat globules are in masses of varying sizes. These
rise to the surface quickly in gravity creaming.]
The body or consistency of pasteurized cream may be restored by
allowing the cream to stand for several days at low temperatures,
or by the addition of a small amount of sucrate of lime. This
substance, known to the dairy trade as "viscogen," is made by adding
to a thick solution of cane sugar, some freshly slaked lime. The
sugar solution permits of the dissolving of a much larger amount of
the lime than is possible in water. When the liquid is allowed to
settle, the clear solution is then decanted off and is used at the
rate of about one part to 100 to 150 parts of cream. The fat
globules are, by its action, brought into aggregates and the body of
the cream thus restored. Viscogen contains nothing that is at all
harmful, but milk and cream to which it is added must be sold under
some distinctive name as "visco-cream," since the laws of
practically all states do not allow the addition of any substance
whatever to milk or cream.
[Illustration: Fig. 26.--Fat Globules in Heated Milk.
When milk is heated the masses of globules are broken up and fat
globules are uniformly distributed throughout the milk.]
[Illustration: Fig. 27.--Creaming of Milk.
The cylinder on the left contains raw milk; that in the center, milk
heated to 140° F. for twenty minutes; on the right, milk heated to
160° F. for twenty minutes. The dark line indicates the depth of the
cream after twenty-four hours. The breaking up of the fat globule
clusters delays greatly the rising of the cream.]
Heated milk has a taste unlike that of raw milk; to one not
accustomed to it the taste is objectionable. This change is due to
some extent to the expulsion of the carbon dioxide from the milk.
The insipid taste of boiled water is, in part, due to its freedom
from carbon dioxide. The production of this cooked flavor is
dependent upon the time and temperature of exposure. It has been
claimed that heated milk is less digestible than raw, and a
considerable amount of experimental work has been done, both on
animals and children, in order to determine the relative
digestibility of heated and raw milk. The results obtained have been
contradictory. It is claimed that heated milk causes such diseases
as rickets, scurvy and marasmus in children. It is probably true
that milk heated to the boiling point is less fitted as food for the
young child than raw milk, but, on the other hand, it has not been
proven that properly pasteurized milk is an unsuitable food for
children. The best evidence has been accumulated in recent years, in
many of the large cities of this country and of Europe, where
pasteurized milk has been used with the greatest success in the
feeding of children of all ages.
The heated milk does not curdle readily when rennet is added due to
the precipitation of the lime salts by heat. The curdling power can
be restored by the addition of soluble lime salts or of acids.
=Purpose of pasteurization.= There are two reasons for the
pasteurization of milk: (1) To improve the keeping quality; (2) To
destroy any pathogenic bacteria it may contain. The first may be
called the economic reason; the second, the hygienic reason fur
pasteurization. In the selection of a proper pasteurizing
temperature, two factors must be taken into account: First, the
effect of heat on milk, and second, the temperature necessary to
destroy those forms of bacteria that are of the greatest importance,
as far as the keeping properties are concerned, and the pathogenic
bacteria that might possibly be present in the milk. The lactic
acid bacteria are non-spore-bearing and are not resistant to heat.
Most of them are destroyed when the milk is heated to 140° F. for
fifteen minutes or to 160° F. for a moment. To insure proper keeping
quality, somewhat higher temperatures must be employed, such as 145°
to 150° F. for fifteen to twenty minutes.
Milk pasteurized at these temperatures will, as a rule, undergo an
acid fermentation in much the same manner as will raw milk. The rate
with which the acid develops is of course much slower than in the
raw milk, due to the destruction of 95 to 99 per cent of the
acid-forming bacteria. If the milk has been pasteurized at higher
temperatures, the acid fermentation may not appear. The spores of
the spore-bearing organisms will be left; these may germinate and
cause their characteristic change in milk, which, as previously
noted, is usually a sweet-curdling or a digesting fermentation.
Since the changes they produce in the milk are not evident at first,
it might be used as food even though it was so far advanced in
decomposition as to be undesirable or even harmful as food. Indeed
one of the objections urged against pasteurization is that it
destroys the natural safe guard, the acid-forming bacteria. Many
people are so accustomed to use this as the indication of spoiled
milk that they will use milk long after it should be used if it does
not show an acid fermentation.
The butyric acid organisms are spore forming and may at times
produce their characteristic fermentation in pasteurized milk. The
milk shows gas formation and develops an objectionable odor.
The pathogenic bacteria most likely to be present in the milk are
the typhoid and the tubercle organisms. The typhoid bacillus is no
more resistant to heat than the ordinary acid-forming bacteria, and
all milk that has been heated, so as to impart to it satisfactory
keeping properties, will certainly be free from typhoid bacilli. It
has sometimes been asserted that the tubercle bacillus is very
resistant to heat; some claiming that it is necessary to heat milk
to 200° F. in order to destroy it. Other experimenters have asserted
that lower temperatures would suffice, but the temperatures were
still above those at which the milk is physically and chemically
changed by the heating process. More recent work has shown that not
all sources of error were avoided in the earlier attempts to
determine the thermal death point of the tubercle bacillus, as, for
example, it has been shown by the authors that the "scalded film"
that forms on the surface of milk when heated in an open vessel will
protect the bacteria imbedded in it. It has also been shown by the
authors that a temperature of 140° F., for twenty minutes or 160° F.
for one minute will destroy the tubercle bacilli in milk, in case
the heating is done with sufficient thoroughness to insure all
particles of the milk being heated to the same temperature for these
periods of time.
The pasteurization of milk can be done in such a manner as to impart
to it good keeping qualities and to insure its freedom from
pathogenic bacteria, and yet not impair its physical and chemical
properties, but much of the so-called pasteurized milk placed on the
market is not treated in accordance with proper hygienic methods.
[Illustration: Fig. 28.--The Pott's Discontinuous Pasteurizer.
The milk is placed in the inner compartment. For heating and
cooling, hot or cold water is passed between the jackets.]
=Methods of pasteurization.= In order to destroy the bacteria in milk,
it is necessary that the milk be heated for a varying time dependent
upon the temperature employed. A lower temperature for a
considerable period may exert the same effect on the bacteria as a
higher temperature for a shorter time. In practice, two types of
pasteurizing machines are employed, depending on the temperature at
which the milk is to be treated. The discontinuous machines or
intermittently operated pasteurizers are those in which the milk is
heated for any desired time at any temperature. Such machines
consist of jacketed containers the inner receptacle being filled
with milk, while the outer space between the walls is filled with
circulating hot water or steam. The milk is kept agitated by the
rotation of the machine. After it is heated, it is cooled in the
same container by replacing the hot water first with cold water,
then ice water. The disadvantage of this process is that the
capacity of the machine is limited which precludes its use in places
where large quantities of milk or cream are handled; for the
pasteurization of limited quantities, it is very successful, as
every particle of milk or cream is under the direct control of
the operator and may be thoroughly and efficiently treated.
As pasteurization was introduced for the treatment of market milk,
and for the preparation of cream for butter, machines have been
devised which permit large quantities, as thousands of pounds, to be
handled per hour. It is evident under these conditions that the milk
must be heated for only a short time, and hence a higher temperature
must be employed. These machines are called "continuous flow"
pasteurizers since the milk passes through them in a constant
stream. The period of exposure is very short, in some only a few
seconds; hence, they are sometimes called "flash" pasteurizers.
[Illustration: Fig. 29.--A Continuous Pasteurizer.
The milk is exposed but a short time since it flows through the
heater in a constant stream.]
All machines of this type possess the obvious disadvantage that it
is impossible to heat all of the milk for a uniform period. The milk
in contact with the walls of the machine flows much more slowly than
in the middle of the stream, just as the current near the bank is
less rapid than in mid-stream. In none of the machines yet devised
have the designers been able to overcome this disadvantage. In a
test of one of the most widely used pasteurizers of this type, it
was found that some of the milk passed through the machine in 15
seconds, while the larger part of it was held for about 30 seconds,
and some as long as forty-five to sixty seconds. If the temperature
employed had been such as to destroy the bacteria in that part of
the milk heated for the minimum time, hygienic safety would be
assured, but in order to avoid injuring the physical properties of
the milk, the tendency is to use as low a temperature as possible,
so that the milk heated for the minimum time may often contain
organisms that have passed through the machine uninjured.
Many devices have been proposed for the heating and cooling of the
milk. In many of the pasteurizers, the milk flows in a thin stream
over a metal surface, on the opposite side of which is the heating
agent, usually steam; while in others, the milk is allowed to flow
through a vat in which revolve a series of discs into which steam is
passed. The discs are of considerable size; thus, making a large
heating surface; the milk is thus heated quickly, and is constantly
stirred by the rotation of the heating discs. In other types the
milk passes into the bottom of a chamber in which a dasher revolves
at a rapid rate. This catches the milk, throwing it in a thin film
onto the wall of the chamber, which is heated with steam on the
opposite side. From such machines, of which the Fjord, the Jensen,
and the Reid machines are types, the milk may be forced to a
considerable height. These are widely used in this country for the
pasteurization of milk and cream for butter making.
Milk that has been heated must be cooled at once by the use of cold
water and ice. In order to economize in the use of both steam and
cooling agents, the so-called regenerative machines were devised.
The essential feature of these machines lies in the fact that the
cold milk inlet and the hot milk outlet are on opposite sides of a
single partition; thus the inflowing cold milk is partially heated
by means of the already treated hot milk which it is desired to
cool.
In order to avoid the disadvantages of the continuous machines,
viz., lack of control, an apparatus has recently been devised which
can handle large quantities of milk, heating the same to any
temperature for any desired time. In such a machine the milk is
first heated in a continuous heater, and is then passed into large
tanks in which it is allowed to remain for the desired time, and
from which it flows over the coolers. Such an apparatus is called a
"holding" machine, and is probably the most feasible type of
pasteurizer now on the market, when all factors are considered. In
some of the continuous machines, an attempt is made to accomplish
the same result, by building the machine so that the milk requires
fifteen to twenty minutes for passage through the machine, but in
all such cases the same disadvantage of variation in rate of flow,
as in other continuous flow type of machines obtains.
=Tests of pasteurizing machines.= It is possible for the operator to
test the rate of flow in a machine, so as to determine whether all
of the milk is heated for a uniform time. This is done most easily
in the following manner: The machine is first filled with water,
heating the same to the desired temperature, and regulating the rate
of flow as it would be if milk was used. The flow of water is then
turned off, and a stream of milk containing a known per cent of fat
admitted to the machine. The time elapsing between the admission of
milk to the machine, and that at which the first sign of turbidity
is noted at the outlet, will be the minimum period necessary for any
portion of the milk to flow through the machine. At frequent
intervals thereafter, samples of the outflowing liquid may be
collected, noting the time at which each sample is taken. The
percentage of fat in the various samples is determined by the
Babcock test; at the moment when all of the water has been removed,
the sample taken will show the same fat content as the milk used.
The samples taken previous to this will show a lower fat test,
dependent upon the relative amount of water and milk. In this
manner, the minimum, the maximum, and the average period of exposure
of milk in the machine tested, can be determined with exactness.
The accompanying table gives results that were obtained in the
testing of one of the continuous types of machines. The machine in
question required about three hundred pounds of milk to fill it and
was supposed to handle 1,000 pounds per hour. Thus theoretically it
should require twenty minutes for any portion of the milk to pass
through the machine. As will be seen from the data, some of the milk
passed through within seven minutes after the water was shut off and
the milk turned on. The figures also show that not all of the water
had been replaced by the milk in even 45 minutes. In actual practice
like results will be obtained, and a portion of the milk will be
heated to the temperature employed but a short time. In this, the
vegetating bacteria will not be wholly destroyed.
=========================================================
Trial | |Per cent of fat in milk coming from
| | machine at following times
|Per cent|---------------------------------------
| of fat| MINUTES
| in milk|---------------------------------------
| | 7 | 11 | 15 | 19 | 23 | 27 | 36 | 47
--------+--------+----+----+----+----+----+----+----+----
No. I | 4.0 | 0.2| 0.8| 1.6| 2.0| 2.4| 2.6| |
No. II | 3.8 | 0.2| 0.6| 1.5| 1.8| 2.2| 2.6| 3.0| 3.4
No. III | 3.5 | 0.7| 1.9| 2.4| 2.8| 2.8| 3.0| 3.4| 3.4
=========================================================
=Pasteurization of small quantities of milk.= It is often desirable to
treat a small quantity of milk for home use, in which case the
commercial types of pasteurizers are out of the question. This
treatment can be done in a number of ways, consideration always
being paid to the manner of heating which should be done under such
conditions, as have been shown to be necessary for efficient
pasteurization. Milk may be heated in tall, narrow cans which are
placed in hot water. In the household, milk may be treated by
placing the filled bottle in a pail having a false bottom so the
bottle shall not be broken when the pail is placed on the stove. The
pail should be filled with water so that its level is about the same
as that of the milk. The water is then heated to the desired
temperature, maintained for the requisite period of time, and is
then cooled as rapidly as possible. During the heating, the mouth of
the bottle should be covered, either with an inverted glass tumbler,
or the paper cap may be left in place, simply punching a small hole
through it so as to permit of the insertion of a thermometer.
[Illustration: Fig. 30.--A Pasteurizer for Use in the Home.
A milk bottle with a tumbler for a cover. The cover prevents the
formation of the "scalded layer" on the milk during the heating and
also protects the mouth of the bottle from dust.]
=Efficiency of pasteurizing.= It is easy to destroy over 99 per cent
of the bacteria present by the use of any of the modern types of
machines. The number remaining after treatment will be largely
dependent, other things being equal, upon the number of bacteria
before pasteurization. The pasteurizing process is not one by which
poor milk can be changed into good milk, nor is it legitimate to use
the process in place of cleanliness, as is sometimes done. There is
a legitimate field for the process in the handling of market milk,
as well as in the creamery; but it should be used to improve the
keeping quality, and to insure the freedom of the milk from
pathogenic bacteria, when other protective measures have been
carried as far as possible under the prevailing conditions.
=Details of process.= If the process is to be successful, due
attention must be given to certain details. In the treatment of
market milk, care should be taken to use only that in which the
acidity has not materially increased. A fair standard is about 0.2
per cent. High acid milk usually means old milk or dirty milk,
either of which is very likely to contain many more spore-bearing
bacteria than clean, fresh milk. The greater the number of spores,
the more rapidly will the pasteurized milk spoil. If it is possible
to exercise any selection of milk prior to pasteurization, the rapid
test for determination of acidity will prove of great advantage.
Care should be taken to prevent fluctuations in the temperature to
which the milk is heated. With varying steam pressure and variations
in the rate of flow of milk, these fluctuations may be very
considerable. Regulators are now made that will control the
temperature within narrow limits.
In all pasteurized milk as it flows from the machine, there will
remain some living bacteria. The spores will not be destroyed by any
pasteurizing process, and under commercial conditions, vegetating
bacteria are also present. If the milk is not quickly chilled after
heating, these forms will grow, and their development is
particularly hastened by the destruction of the lactic bacteria, the
acid of which would otherwise hold them in check. The result is
that, unless immediately chilled, pasteurized milk spoils almost as
rapidly as though it had not been heated at all. Efficient and rapid
cooling are, therefore, as essential a portion of the process as the
heating itself.
Care should also be taken to protect the milk from contamination
after treatment. Every utensil with which it comes in contact should
be sterilized. The bottles should be thoroughly washed and
sterilized and subsequently protected from dust until used.
=Sterilization of milk.= It is possible to render milk sterile by the
use of temperatures above the boiling point of water, where it is
heated in a closed vessel, in which steam under pressure is
generated. Such milk is often found in the European markets. In our
own country, the only milk of this kind is the so-called "evaporated
milk." In this process sweet fresh milk is evaporated in vacuum pans
to about one-third of the original volume. This is then placed in
tin cans, which are treated, as in the canning of such vegetables as
peas and corn, by heating the milk to 230° or 240° F. for a few
minutes. In this process, the bacteria (spores as well as vegetating
forms) are completely killed, and the milk acquires a brownish tint,
due to the caramelization of the sugar. The appearance of the
product is very similar to cream, and previous to the passage of
the pure food law, it was sold as evaporated cream.
Condensed milk is not wholly free from bacteria, but is sufficiently
thick, by reason of its treatment so that the contained bacteria
cannot grow. They remain dormant in the milk, but as soon as it is
diluted to a normal consistency, growth takes place, and the milk
rapidly spoils. Condensed milk is prepared by adding cane sugar to
fresh sweet milk, then evaporating the mixture to one-third the
original volume, forming a semi-solid product. Syrups owe their
keeping qualities to the same factor, as condensed milk, _i.e._, the
high consistency.
Milk is also preserved by wholly evaporating the water, thus leaving
a dry powder, which on being mixed with water again will have much
the same properties as the original milk. Various methods have been
devised for the preparation of these milk powders, all of which have
been patented by the inventors. If the powder is to be kept for long
periods, skim milk must be used, since the fat slowly undergoes
changes which cause it to have a rancid odor. These dry preparations
are largely used by bakers in place of fresh milk.
CHAPTER VII.
BACTERIA AND BUTTER MAKING.
In the making of butter it is necessary to concentrate the milk fat
into a small volume. This process, known as creaming, may be
accomplished by gravity, if the milk is allowed to stand
undisturbed, the fat globules rising slowly to the surface. Much
more rapid separation may be secured, by placing the milk in a
rapidly revolving container in which it is subjected to centrifugal
force, which causes the heavier parts of the milk to pass to the
outside of the bowl, while the lighter part, the fat, collects at
the center of the revolving bowl. There is an enormous number of fat
globules in milk, over 5,000,000,000 in each cubic centimeter, and
as these move through the milk serum, they carry with them many of
the bacteria. The cream is thus much richer in bacteria than is the
skim milk, or even the milk before separation. Besides the
mechanical separation in the manner described, the method of
creaming is of importance, in determining not only the number but
also the kind of bacteria in the cream.
=Methods of creaming.= In the shallow-pan method of creaming, the milk
is kept at ordinary room temperatures. These temperatures favor
especially the growth of the acid-forming bacteria. The milk is
usually sour by the time the cream is removed from it; consequently,
the bacterial content of the cream is high. Moreover, the cream is
exposed to air contamination, and is thus seeded with molds, and
those forms of bacteria that are always found in the air. The cream
obtained in this manner is likely to contain not only numerous
bacteria, but a great variety of forms, some of which undoubtedly
are the cause of the poor keeping qualities of butter made from such
cream.
In the more modern method of gravity creaming, in which the milk is
placed in deep narrow cans kept in cold water, the conditions are
not favorable for the growth of acid-forming bacteria. If the milk
is produced under clean conditions, and is placed in cold water at
once, the bacterial content of the cream will be low, and it will be
less likely to contain undesirable forms than the cream which is
obtained from the shallow pans.
In separator cream the bacteria will be represented by the kinds
present in the milk at time of separation. If this milk is quite
old, the cream will contain large numbers of bacteria; if, however,
early separation is made and the milk is clean, the bacterial
content of the cream will be low.
=Types of butter.= Butter may be divided into two types--acid or
sour-cream, and sweet-cream, depending upon whether the cream is
allowed to undergo the acid fermentation or not before it is
churned. In southern Europe, it is the custom to churn the cream as
sweet as possible, and the resulting product possesses only the
natural, or primary milk flavor. To one accustomed to butter made
from sour or ripened cream, this taste is flat, and if the butter is
free from salt, may remind one of grease. Sweet-cream butter has a
delicate flavor when it is made from good milk, and the taste for it
is rapidly acquired. In some centers, as in Paris, the market
demands this type of butter quite exclusively.
If the cream is allowed to undergo the acid fermentation before
churning, the butter has a much higher degree of flavor and one that
differs materially in kind. Under primitive methods, it was
difficult to keep the cream sweet until it could be churned. On the
small farm with gravity creaming in shallow vessels and infrequent
churning, the cream was certain to be sour when churned.
Undoubtedly, the making of butter from sour cream came into use
because of its greater convenience; people became accustomed to
sour-cream butter, and at the present time it is used in the greater
part of the world, and is the type made in all of the great dairy
countries.
=Ripening of cream.= In modern dairy practice the souring of the cream
is called the _ripening_ process, and is, where the best methods are
employed, largely under the control of the butter maker. The changes
that go on in the ripening process are the same as have been
discussed in the acid fermentation of milk. The increase in acid is
accompanied by an enormous increase in the number of bacteria; the
ripe cream will contain hundreds of millions of bacteria in each
cubic centimeter. The effect of this germ life is to improve or
injure the butter, depending upon the class of bacteria to which it
belongs. The problem of the modern butter maker is to control the
kinds of bacteria growing in the cream.
The temperature at which cream is held during the ripening process
is favorable to the growth of the acid-forming bacteria; hence, in
ripe cream, they are practically the only kind of bacteria to be
found. It must be remembered however, that there are different
classes of acid-forming organisms, some of which produce desirable
flavors, while others are distinctly harmful.
The intensity of flavor of butter is, in a general way, directly
related to the amount of acid that is formed in the cream. A low
acidity at time of churning is usually associated with a mild
flavor, while a higher degree of acidity, up to a certain point,
imparts a more pronounced flavor to the product. If cream is
over-ripened, the quality of the flavor is seriously impaired.
In determining the acidity of cream, a definite volume is taken, and
the acidity determined by titration, expressing the results as such
a per cent of lactic acid. Manifestly, the amount of fat in the
cream influences the apparent per cent of acidity. The acidity will
not usually exceed 0.5 to 0.7 per cent, but in reality the serum
will contain more than this, as the acid is formed in the serum, the
butter fat having no role whatever. In a very rich cream, 40 to 50
per cent fat, it is impossible to develop more than 0.4 to 0.5 per
cent of acidity, and the flavor of the butter will be low, because
of the relation between the amount of acid and fat, while in a thin
cream having the same acidity, the ratio between the amounts of fat
and acid will be very different. For example, in one hundred pounds
of 50 per cent cream of 0.5 per cent acidity there will be one-half
pound of acid and fifty pounds of fat; in the same quantity of cream
containing 20 per cent of fat and having an acidity of 0.5 per cent
there will be one-half pound of acid to twenty pounds of fat. The
flavor of the butter from the rich cream will be quite different in
intensity from that made from the thinner cream.
The acidity of cream cannot be determined with any degree of
accuracy by the taste or odor. Every butter maker should have some
method of determining the degree of acidity in his cream, so that he
may better control the flavor of his product. Several methods have
been devised for this purpose and the necessary apparatus is sold by
all dairy supply houses.
The effect of the ripening of the cream is shown not only in the
flavor of the product, but in a number of other ways. Sour cream
churns more easily, and more exhaustively than does sweet cream. It
is supposed that the fat globules are surrounded by a film of
albuminous material which prevents their coalescing readily. During
the ripening process, the action of the acid apparently dissolves
this enveloping substance, and the globules cohere more easily in
the churning process.
When raw cream is used the ripened-cream butter keeps better than
that made from sweet cream. In sweet cream there are few lactic
bacteria, the majority of the bacteria present being of various
kinds, many of which may be injurious, so far as the keeping quality
is concerned. In sour-cream butter the lactic bacteria make up over
99 per cent of the bacteria present, and their presence tends to
prevent the development of undesirable non-acid forms.
=Source of butter flavor.= The flavor of ripened-cream butter has been
shown to be directly connected with the acid-fermentation of the
cream. The amount of lactic acid formed from the sugar fermented is
dependent upon the kind of bacteria present. The acid-producing
organisms that are desirable from the standpoint of the butter maker
form comparatively small amounts of other by-products, but these
undoubtedly affect the flavor of the butter. As fats have the power
of absorbing odors, the butter fat absorbs some of the by-products
of the acid fermentation, thus acquiring a certain aroma and
flavor.
It is not necessary that the cream be ripened, in order to have the
fat acquire a flavor, for if sweet cream is churned with a
considerable proportion of sour milk, the butter will have much the
same flavor, both as to intensity and kind, as though the cream had
been allowed to sour naturally. A process of butter making known as
the LeClair method is based on this principle. The flavor-producing
substances can also be absorbed by the butter after it is churned,
by working the butter in contact with sour milk. Attempts have been
made to add pure lactic acid to the cream, instead of allowing the
acid to be formed by the bacteria, but while the physical effect on
the cream is the same, the flavor and aroma of the butter are
deficient, because the acid itself does not supply the necessary
aromatic products. This emphasizes the importance of the by-products
of the acid fermentation other than the lactic-acid.
In the past numerous attempts have been made to find organisms that
might be added to the cream, in order to produce the delicate flavor
characteristic of the best type of butter. Some bacteriologists have
claimed that the source of the flavor-giving substance was to be
found in the decomposition products of the nitrogenous constituents
of the milk. None of these attempts have stood the test of practical
use in creameries, and it has been demonstrated that the finest type
of butter can be made by the use of lactic bacteria alone. Formerly,
when butter was made wholly from cream soured under natural
conditions, a much higher degree of flavor was developed. Under
present market demands, a less pronounced flavor is desired, a
condition more readily met by the use of modern methods.
=Importance of butter flavor.= The importance of flavor in determining
the commercial value of butter is evidenced by the relatively high
value placed upon this factor in scoring, viz., flavor, 45 points;
body or texture, 25 points; color 15; salt 10; and package 5 points.
The factors on which butter is judged, are with the exception of
flavor, wholly under the control of the maker, but as the production
of flavor is dependent on the kind of bacteria present in the cream,
it is a far more difficult matter to control, and yet it is of the
utmost importance in determining the value of the product.
The flavor of the butter is dependent on the quality of the cream.
If this is dirty and sour, the maker has little control over the
type of fermentation, and hence, little control of the flavor of the
butter. This has led in some cases to the grading of the cream,
basing the division on the acidity, flavor, and fat content. Such
practice is entirely justifiable, as a better quality of butter can
be made from fresh, sweet cream than from that already fermented. It
is noteworthy that the quality of butter has not improved since the
introduction of the centralizer system, in which cream is shipped
for long distances.
=Control of the type of fermentation.= In the older methods of butter
making, there was little or no control of the type of fermentation
that took place in the cream. Where milk is produced under clean
conditions, and kept at ordinary temperatures, it will generally
undergo fermentation changes, due to the desirable type of
acid-forming organisms. In milk, which is less carefully handled,
the undesirable bacteria are more abundant and the quality of the
butter of lower grade. When butter was made on the farm, before the
development of the factory system, it was not a question of vital
importance whether the product was uniform from day to day, but
with the advent of the modern creamery, turning out thousands of
pounds of butter per day, and with the extension of the markets for
the product, the question of uniformity came to be of much
importance. A uniform product can be secured only by the control of
the type of fermentation in the cream, or by the control of the
kinds of bacteria that cause the souring of the cream. Modern
methods of butter making have been devised on the basis of an
improvement in the ripening process.
=Starters.= From the earliest practice of allowing the cream to stand
until sufficient quantity had accumulated for churning, it was only
a step, but a most important one, to the addition of sour milk, sour
cream, or butter milk, to hasten the ripening process. This was the
beginning of the modern starter. Experience demonstrated that the
addition of these already fermented liquids exercised a desirable
effect upon the production of butter flavor, even though, at that
time, the phenomenon of milk fermentation was not satisfactorily
understood, and the relation of bacterial by-products to the
production of flavor in butter was not recognized.
As a result of experience alone, improvements in the development of
the "home made" starter took place. By careful selection of clean
milk, and the natural fermentation of this under carefully
controlled conditions, as well as the control of the temperature of
the cream during the ripening, improvement in the technique of cream
ripening gradually developed. More and more attention was given to
the preparation of the starter, and its propagation from day to day,
under conditions which would prevent its deterioration. This method
of utilizing naturally fermented milk or cream was gradually
extended, until it became almost universal in the larger
butter-producing districts.
In 1890 a more refined and scientific process was introduced by the
Danish bacteriologist, Storch. Recognizing the fact that butter
flavor was attributable to the development of the bacteria present
in the ripening cream, he conceived the idea of isolating the
various types of organisms found in milk and testing them as to
their effect on the quality of flavor. Selection was then made of
the most favorable flavor-producing types, and these were propagated
in suitable culture media, such as skim milk, which was rendered
more or less perfectly sterile by pasteurization or sterilization.
Under such conditions the addition of a selected ferment could be
made to the fresh cream, and so control the type of fermentation
which occurred therein. An essential requisite in any organism used
for this purpose must be the ability to produce relatively large
amounts of acid rapidly at ordinary ripening temperatures, and also
to form sufficient quantities of the proper flavor-producing
substances to impart a suitable flavor to the butter fat. Such
starters are known as pure culture or commercial starters, and are
prepared in both liquid and dry form. At present they are used to a
greater or less extent in all of the leading dairy districts.
Liquid starters consist of a mass of sterile nutrient medium, milk
or beef broth, inoculated with the pure culture. The dry starters
are made by adding liquid cultures, containing the growing bacteria,
to some absorbing material, such as milk sugar, milk powder, or
starch, the whole mass being dried at low temperatures, so as not to
injure the bacteria. Under such conditions the bacteria, exist in a
dormant state, and are protected from their own by-products, to
which they would be exposed if maintained in liquid cultures. The
keeping quality, therefore, of dry cultures, is much better than
that of liquid cultures.
By the use of the pure-culture starters, the butter maker is able to
add to his cream the same kind of bacteria from day to day, and the
butter will be more uniform than when the less constant home-made
starter is employed. In cream to which the starter is added, there
are present a greater or less number of acid-forming bacteria,
depending upon the age of the cream, and upon the condition under
which it was produced. These will grow during the ripening process,
and the flavor of the product will be the result of the mixture of
the bacteria in the cream. The maker can not, therefore, be certain
that the addition of a pure culture to raw cream will effectively
control the type of fermentation. This can be secured only by first
destroying the existing bacteria in the cream, before the selected
culture is added. Heating the cream accomplishes this; and in cream
thus freed from the various kinds of bacteria, the butter maker can
insure the dominance of the desirable types, contained in the
pure-culture starter. If the cream can be obtained in a sweet
condition, the maker through this process of pasteurization, and the
use of pure cultures, secures almost perfect control over the type
of fermentation that occurs in the cream, and thus exercises control
over the degree and kind of flavor of the product. This most
scientific type of butter making is now used by the most progressive
butter makers in the leading butter-producing regions of the world.
Pasteurization of the cream also distinctly improves the keeping
quality of butter, a condition doubtless due to the freedom of the
same from organisms other than the lactic bacteria. This is a factor
of as much importance as uniformity, because under modern business
conditions, the surplus production must be kept in storage, and it
is essential that the quality should not deteriorate materially
during this time.
=Process of pasteurization for butter making.= In the pasteurization
of market milk, it is necessary to take into account the effect of
heating on the physical and chemical properties of the milk, and the
degree of heat that can be employed is limited. In pasteurizing
cream for butter, there is no such limitation, and the cream may be
heated to any temperature desired. In Denmark where the process of
pasteurization has been used most extensively, temperatures ranging
from 176° F. to 190° F. are used. The machines are of the
"continuous flow" type, and the cream rather than the whole milk is
treated. To prevent the spread of tuberculosis and other diseases,
the Danish government requires that all cream and milk be heated to
176° F., before the skim milk or butter milk is returned to the
farms.
The heating of the butter fat to high temperatures has an injurious
effect on the texture of the butter, unless the cream is cooled to
50° F., for a period of at least two hours previous to churning.
=Propagation of starters.= As has been previously shown, the quality
of butter depends on the kind of bacteria in the cream or in the
starter added. The commercial starters contain lactic acid bacteria
that have been selected with especial care; most of the starters now
sold contain but a single kind of bacteria; hence, are often called
pure-culture starters. The package purchased contains but a small
quantity, and before the starter can be used in the ripening of
cream, it must be increased in amount. It must also be propagated
from day to day so that a fresh starter shall be available daily for
addition to the cream. The propagation of the starter must be done
with especial reference to keeping it in good condition and in as
high a state of purity as possible.
In the past the starter was propagated, by adding the contents of
the bottle purchased to a small amount of milk that had been heated
and cooled; this, if kept in a warm place, would be curdled in
twenty-four hours, and could be used for the inoculation of a large
mass of milk, that had been treated in a like manner, and which,
when curdled, was added to the cream; a small amount was saved for
the purpose of again inoculating a mass of milk that had been heated
and cooled. Following this method it was very difficult to keep the
culture from becoming contaminated with other forms of bacteria.
More recently the most successful butter makers have propagated the
so-called "mother starters" in small vessels, and have used the
larger mass of starter for the inoculation of the cream alone.
Glass vessels are preferable for the propagation of the mother
starters since they are impervious and through the transparent wall
the condition of the ripened starter can be more easily determined
than in a metal or earthenware vessel. An ordinary milk bottle with
an inverted tumbler for a cover, to protect the starter from
contamination from the air, is a most convenient vessel.
The starters may be propagated either in whole or skim milk; the
former is preferable since, in most creameries, it can be more
easily selected. The quality of the milk used has much to do with
the quality of the starter; it should be as fresh and clean as it
is possible to obtain. The clean bottle should be filled half to
two-thirds full, covered and heated in some manner so that the milk
shall be at a temperature close to the boiling point for fifteen to
twenty minutes. The heating may be done by placing the bottles in
water, which is heated on a stove or by steam, or the bottles may be
subjected to streaming steam. The milk is cooled quickly and the
contents of the package purchased added and well mixed with the
milk. In the case of the dry starters, the mixing should be done
with especial care. The bottle is kept in a warm place and in
twenty-four to thirty-six hours, the milk should be curdled. A
second bottle must be treated as before and inoculated from the
first, and the process repeated daily since the bacteria must have
fresh food, if they are to be maintained in good condition.
In order to accomplish this, the maker must be able to maintain
constant conditions from day to day, especially with reference to
the amount of the ripened starter that is transferred to the fresh
bottle of milk, and the temperature at which the bottles are kept. A
spoon, arranged as shown in Fig. 31, enables one to carry a definite
amount of the ripened starter to the bottle of milk to be inoculated
and a constant temperature box (Fig. 32) permits of the maintenance
of the same temperature from day to day. Through careful supervision
of these points, and by taking care at every step to avoid the
introduction of contaminating organisms, the purity of the culture
can be maintained, and the bacteria kept in a healthy condition.
The starter is used because of the acid-forming bacteria it
contains; it is said to be ripe and in the best condition for use at
the time it contains the greatest number of living bacteria. It
has been found by experiment that this is at the time the milk
curdles at ordinary temperature, or when the acidity is about
0.6-0.7 per cent. If the acidity is allowed to increase to 0.8 or
0.9 per cent, the number of bacteria will be less and a larger
amount of the starter must be used in order to ripen a definite
amount of cream in the desired time. The use of an overripe starter
may also have an injurious effect on the flavor.
[Illustration: Fig. 31.--Bottle for Mother Starters.
A milk bottle with a tumbler for a cover and a spoon for inoculating
the other bottles enables the butter maker to propagate the starters
without contamination.]
The ripened starter should be perfectly homogeneous, showing no
bubbles of gas or free whey; the odor should be agreeable and the
acid taste mild; on shaking, the curd should break up into a smooth,
creamy liquid free from lumps. This is especially important in the
starter that is to be added to the cream, since otherwise the
starter cannot be uniformly mixed with it and white specks of
curdled casein will be noted in the butter.
[Illustration: Fig. 32.--An Incubating Chamber for Starters.
The inner compartment will hold a pail of water and the bottles for
the mother starters. The temperature can be kept at any desired
point by the use of warm or cold water. The four-inch space between
the walls is filled with hay or mineral wool.]
The firmness of the curd is not so dependent on the amount of acid
formed as upon other factors. If the curd shrinks to any extent and
the whey is expressed, it is certain to produce a starter that will
contain lumps that cannot be broken up. With a pure culture of
lactic bacteria, there is little difficulty in this regard, but as
soon as gas-forming bacteria are introduced, trouble is likely to
result.
In the propagation of starters, it is always to be remembered that
the bacteria, although invisible to the eye, are living things, and
unless conditions are favorable in every particular, it is
impossible to keep them in a healthy condition, so that growth in
the cream is rapid, producing the acid demanded for churning, and
imparting to the butter the desired flavor, both as to degree and
kind. No part of the daily routine of the butter maker should be
performed with more care than the preparation of the starters, both
the mother starters, and the larger one for addition to the cream.
The latter can best be made in one of the many forms of starter cans
now on the market, since by their use, the maker can heat and cool
the milk with little trouble, and can maintain the starter at any
desired temperature. Better starters cannot be made in them than by
the use of simple and improvised apparatus, but better results can
be obtained with the same expenditure of time and labor.
In the handling of the large starter, care should be used not to
overripen, since the larger quantity is more likely to "whey off"
than is the smaller starter. Skim milk rather than whole should be
used for this. It should be selected with care and heated to 200° F.
for thirty minutes. When it is impossible to secure fresh milk for
starter making purposes, either condensed skim milk or milk powder
may be used. The condensed milk is diluted with water until its
volume is about the same as the milk before concentration; the
mixture is then treated the same as fresh milk, being heated and
cooled before inoculation. In the case of milk powder, one part of
the powder is added to ten or twelve parts of water, allowed to
dissolve as far as possible, and the mixture heated and cooled.
Either of these liquids will give satisfactory starters; the cost
however is high, and in most places milk can be obtained more
cheaply. The inoculation and the temperature should be so
controlled, as to ripen the starter at the time it is to be needed.
These conditions must be determined by the maker for himself. It
should be remembered that the bacteria grow much more rapidly, as
the temperature is increased; and hence, the amount of inoculation
is dependent on the temperature at which the starter is to be kept.
When the starter is propagated under practical conditions, it sooner
or later deteriorates, either in acid production, or in flavor, and
a new pure culture must be procured from the manufacturer. It is
impossible to give a hard and fast rule as to the length of time a
starter can be kept in good condition. It will depend on how well
the maker satisfies the conditions necessary for maintaining its
purity and strength. The use of imperfectly sterilized milk, or
dirty utensils soon contaminates it; overripening is likely to
injure the flavor. One of the most frequent troubles encountered is
the appearance of a slimy or ropy condition in the starter, although
the acidity developed may be normal and the flavor desirable. It has
been found that this condition is not necessarily due to
contamination, as was considered true in the past, but rather to
some change in the lactic bacteria themselves. If the propagation is
continued, the slimy condition will often disappear.
=Starters in "process" butter and oleomargarine.= The advance which
has recently been made in the science and practice of cream ripening
and butter production is utilized most effectively in the treatment
of cream in the renovating process. Old, soured, and stale cream
is reduced in acidity by the addition of lime. The cream is then
pasteurized and aerated to expel the odors as much as possible. A
large amount of starter is then added and the cream immediately
churned. Under these conditions, the bad flavors are materially
reduced in intensity, and desirable flavors absorbed by the fat from
the selected starter used. It is thus possible to produce butter of
good quality from cream that would at first be regarded as quite
unsuitable for butter production.
In the manufacture of oleomargarine the same principle is utilized.
The butter aroma and flavor is imparted to the neutral oils and
tasteless fats by mixing the same with a properly prepared starter.
Renovated or process butter is given a desirable flavor in the same
way.
=Wash water.= It has been found that the purity of the water used in
washing the granular butter has a marked influence on the keeping
quality. If the water is from a shallow well into which surface
water finds its way, it is certain to contain large numbers of those
types of bacteria that are found in the soil, while if it comes from
a deep well that is properly protected from surface contamination,
the bacterial content of the water will be low and no injurious
effect on the butter will be noted. When it is impossible to obtain
pure water for washing purposes, a proper supply may be secured by
sterilizing the water. The most convenient way of heating the water
is by the direct injection of steam. It is necessary to use that
coming directly from the boilers and not the exhaust from the
engine, since the latter is likely to contain small amounts of oil
that will impart to the butter an objectionable flavor. After
cooling, the water is ready for use. It has been shown that the
cost of treating an impure water is more than covered by the
increased returns from the product.
A pure and healthful water supply should be one of the essential
things of every dairy, creamery, and cheese factory, not only for
the sake of the quality of the product, but also to avoid
contamination of products with disease-producing bacteria.
=Bacteria in butter.= The germ content of butter will depend on the
type of cream. Sweet-cream butter contains but few bacteria. In
sour-cream butter the content in bacteria will be greatly increased,
especially as to lactic organisms. Often, it may amount to several
millions of organisms per gram. The germ content of butter is said
to be greater on the outside of a package than within the mass, due
doubtless to the free access of air, thus favoring the growth of the
aerobic forms.
The composition of normal butter does not favor the growth of the
majority of kinds of bacteria that are contained in it. The washing
process removes much of the material suitable as food for the
bacteria, such as sugar and albumen. If considerable butter milk is
left in the butter, the growth of bacteria will be quite rapid, at
first, but does not continue for any considerable length of time.
The addition of salt also tends to restrain the growth of most kinds
of bacteria.
Butter is at its best when it is perfectly fresh. Deterioration
begins within a short time and the rapidity with which the changes
go on is dependent on the temperature at which the butter is stored.
The temperature of the butter rooms in the large cold storage plants
is kept below 0° F. The butter in such rooms will deteriorate very
slowly, but on removal from the cold rooms and in storage at
ordinary temperatures deterioration goes on more rapidly than would
have been the case when the butter was fresh. At the temperature of
an ordinary refrigerator the changes go on much more rapidly. This
fact has often been looked on as indicating that the factors causing
the changes are biological ones. The influence of temperature in
accelerating the changes would be the same if no biological factor
were active.
That biological factors are of importance is indicated by the fact
that the keeping quality of the product is profoundly affected by
the quality of the cream. Butter made from sweet, fresh cream, that
has been thoroughly pasteurized, has the best keeping quality, while
butter made from such cream, but not pasteurized, has the poorest
keeping quality, especially when no salt is added. Every process by
which the desirable lactic bacteria are increased in proportion to
other kinds has a marked effect in enhancing the keeping quality of
the butter. Thus, the use of pure cultures in raw cream, and
pasteurization together with the pure cultures, have a marked
beneficial effect.
The addition of preservatives exerts an effect on keeping quality.
Borax is the chemical most frequently employed for this purpose. Its
use is allowed in Australia and New Zealand in butter that is
shipped to England, but the use of all preservatives is forbidden in
the United States.
The size of the package also has an effect on the keeping quality;
the smaller the package, the greater is the surface exposed to the
air and the more rapidly the butter deteriorates. Butter used in the
United States Navy is packed in hermetically sealed cans so as to
exclude the air as far as possible.
From the fact that any condition which restrains or inhibits the
growth of micro-organisms has a tendency to improve the keeping
quality of butter, it would appear that the detrimental changes in
the quality of butter are due to biological causes. The most common
defect known is that usually referred to as rancidity. There are,
however, different types of changes that are probably included under
this head and it is very probable that different causes are
operative in their production. True rancidity is probably due to
biological causes; the so-called tallowy change, in which the butter
acquires the odor of tallow is probably due to the combined action
of light and air on the fat.
=Bacterial defects in butter.= There are a number of defects in butter
that are positively known to be due to the growth of bacteria in the
milk or cream, or in the butter itself. The lack of flavor is looked
upon as a defect in the case of ripened-cream butter. It may be due
to insufficient ripening of the cream, or to the lack of
acid-forming bacteria that produce the desirable flavor-forming
compounds. Not all acid-forming bacteria are able to produce
favorable, flavor-giving compounds; hence, sour cream butter may
sometimes be deficient in flavor by reason of this fact.
=Putrid butter.= This specific butter trouble has been observed in
Denmark, where it was first studied by Jensen. 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. It may be caused by a number of bacteria.
=Turnip flavored butter.= Butter sometimes acquires a flavor
resembling turnips. This trouble may be due to the feeding of such
roots, the aromatic substances peculiar to them being absorbed
directly by the milk and thus transferred to the butter. Weigmann
traced a similar flavor to certain bacteria that entered the milk
from barn filth.
=Cowy odor in butter.= There is sometimes to be noted an odor in
butter as in milk that resembles that of the cow stable. Usually
this defect has been ascribed to the absorption of these odors
directly by the milk. Organisms have also been described that impart
to the butter a very similar odor. Bitter butter may be due to the
feed that is consumed by the cow, or it may be due to those forms of
bacteria that produce a bitter fermentation of the milk.
=Other abnormal flavors.= Among the numerous abnormal flavors that
have been noted in butter is one of quite frequent appearance, the
so called "fishy" flavor. It is now believed by many that this
flavor is due to the presence of small amounts of iron or copper
salts that have been introduced into the milk from utensils from
which the protective coating of tin has been worn. If the milk or
cream stored in such utensils develops any marked degree of acidity,
the acid will dissolve a small amount of the iron or copper. The
fishy flavor has not been found in sweet-cream butter as would be
expected from the above explanation. In fresh butter a metallic
taste is sometimes present. It is believed by some that on storage
this flavor changes to the fishy flavor.
All utensils used for the storage of milk and cream should be kept
in good condition so as to prevent the acid milk or cream from
coming in contact with iron or copper.
[Illustration: Fig. 33.--Moldy Butter.
The mold grows on the paper in which the butter is wrapped rather
than on the butter. The print on the left was wrapped in the same
paper as the print on the right except that the parchment cover had
been steamed for a few moments.]
=Moldy butter.= A defect that causes a great amount of loss is the
development of mold on the surface of the butter, either in tubs
or in prints. This trouble is easily prevented. Butter is not well
suited to the growth of mold, but the paper used for lining the
tubs, or wrapping the prints is an excellent medium for mold growth.
The wood of the tub also furnishes ample food for this type of life,
especially where the wood contains any sap. One other essential
condition for mold growth is a supply of oxygen. The mold spores are
widely disseminated, and are always to be found on the butter tubs
and on the paper. The number is not likely to be sufficient to cause
trouble unless the tubs and paper have been kept under such
conditions, as to allow growth to take place on them before use.
During damp, hot weather, the amount of moisture absorbed by these
materials is often sufficient to allow molds to grow on them. This
trouble can be prevented by the storage of tubs and paper in a clean
dry place, or by a disinfecting treatment which will destroy the
mold spores. The most successful method of treatment of tubs is to
apply paraffin to the inner surface, which can be easily done by the
use of some one of the various machines now on the market. The
thin layer of paraffin excludes the moisture from the wood, and also
prevents the mold from obtaining a supply of oxygen for its growth.
The tubs may be steamed, treated with hot water, or filled with a
dilute solution of formaldehyde, and allowed to stand overnight.
Soaking in brine as is usually done in the creameries is of some
effect, but will not completely kill mold spores.
[Illustration: Fig. 34.--Moldy Butter.
The butter was placed in a paraffined tub, but the paper was not
treated so as to destroy the mold spores thereon.]
Butter may mold where the tubs have been thoroughly treated, because
of the mold spores on the paper used for the lining. One of the
black molds is able to thrive on parchment paper whenever the air is
damp. In the past but little attention has been paid to the paper as
a source of trouble. It is certain that it is often at fault, and
that as much attention should be paid to the paper as to the tub. A
most efficient way of treating paper, either for tub liners or print
wrappers is to place same in boiling water for a few minutes.
CHAPTER VIII.
BACTERIA AND CHEESE MAKING.
Butter, such as that of the sweet-cream type that is highly esteemed
in many parts of the world, may be made without the aid of bacteria,
but no important kind of cheese can be made under commercial
conditions without them.
=Types of cheese.= Cheese consists of the fat and the precipitated
casein of milk, together with a large amount of water and the salts
found in milk. The numerous types of cheese may be divided into two
groups, depending on the manner in which the curdling of the milk is
brought about. Sour-milk cheese is made from curd, formed as a
result of the acid fermentation of the milk. Thus, at the very first
stage in the making of this type, the importance of bacteria is
apparent.
The second type is that made from curd, which is precipitated by the
addition of rennet to the milk. This type may also be divided into
two groups, depending upon their texture; the hard cheese, and the
soft cheese. The ordinary cheddar, the common American type, is the
most important example of the hard cheese; Limburger, of the soft
cheese. Cheese are designated as hard or soft, depending upon the
amount of whey that is retained in them during the making process.
The moisture content has an important influence on the type and
amount of life that develops on and in the curd mass, and as will be
seen, the ripening and flavor of the cheese are dependent upon these
biological factors.
The two groups of hard and soft cheese have no sharply defined
limits, but merge into each other. The extreme types of the hard
cheese are so dry and firm that they can be cut only with
difficulty. Such cheese are used primarily as condiments to impart a
flavor to certain dishes, as macaroni, and for this purpose are
grated. The extreme type of soft cheese is a soft, pasty mass and
can be easily spread with a knife.
Hard cheese, because the ripening process goes on uniformly
throughout the entire mass of cheese, may be made of any size which
permits of commercial handling. They can also be kept for long
periods and preserve their good qualities. Soft cheese are made in
small sizes, since on account of their consistency, they could not
otherwise be handled, and also because of the manner of ripening.
The ripening is due to the action of organisms developing on the
surface, the by-products of which diffuse into the curd. If the
cheese are too large, the outer layers become overripe, while the
interior remains more or less unchanged, or insufficiently changed.
Soft cheese mature much more rapidly than hard cheese; consequently
they are short lived.
Although made from the same substance, milk, it is noteworthy that
there are over four hundred varieties of cheese produced. Most of
these find only a local market where made. Less than a dozen
varieties are to be regarded as general articles of commerce.
=Quality of milk.= In the making of butter there are a number of
processes that the maker can use when he finds himself obliged to
utilize poor milk. The milk can be pasteurized and the harmful
bacteria thus destroyed; desirable kinds can then be added in the
form of a pure-culture starter. Pasteurization also drives off some
of the volatile by-products of the first acid fermentation. By the
use of these means, the maker can prepare a very good product from
poor material.
In the making of most kinds of cheese, especially those of the
greatest commercial importance, the cheese maker can call to his
help no such aids, but must use the milk as it is brought to him. It
is possible to prepare certain kinds of soft cheese from pasteurized
milk that differ in no essential point from the same cheese made
from raw milk. Hard cheese are also made from pasteurized milk, but
in most cases such cheese differ, especially in the degree of
flavor, from that made from unheated milk. It is quite probable
that, as the factors concerned in the ripening of cheese become
better known, methods will be evolved for the successful production
of many kinds of cheese from pasteurized milk.
It has been shown that the quality of milk is almost wholly
dependent upon the number and kinds of bacteria it contains. These
bacteria pass into the cheese, and there produce the same products
as they would have done in the milk itself. In butter making,
practically all processes are under the control of the maker, until
the product is ready for the market; but cheese, on the other hand,
passes through a complicated series of changes after it has left the
maker's control. During the manipulation of the milk and the curd in
the vat, he can exert some influence on the quality of the product,
but he is much more dependent on the quality of the milk than is the
case in butter making.
Every effort should therefore be made to furnish to the cheese maker
the quality of milk from which he can prepare fine cheese. In other
words, the milk should be produced under clean conditions and
carefully cooled and handled until delivered to the maker. Poor milk
from a single farm may have such an effect upon the cheese made from
the milk of twenty farms as to depreciate the selling value of the
entire product several cents per pound.
The tests that have been previously described (p. 105) have been
devised especially for testing the quality of the milk for cheese
making purposes, and are of the greatest service to the maker in
tracing the source of poor milk.
=Cheddar cheese.= The first step in the making of cheddar cheese is
the "ripening" of the milk, or the development of a small amount of
acid. In this fermentation, the development of acid is preceded by
an enormous increase in the number of acid-forming bacteria. Milk
for cheese making should show an acidity of about 0.2 per cent or
slightly more than in fresh milk. In other words, the maker wishes
the milk to be in such condition, bacteriologically, that if kept at
a temperature favorable for the growth of the acid-forming bacteria,
the acidity will increase rapidly.
The curdling of the milk to precipitate the cheese solids is
produced by the addition of rennet, which is obtained by extracting
the fourth stomach of the young calf with a solution of common salt.
In the past the maker prepared his own rennet solution from the
dried stomachs ("rennets"), but at present, the extract is prepared
commercially, in a much more uniform manner. The rapidity of the
curdling is dependent upon the acidity of the milk. In order to
secure proper rennet action, a slight increase of acid over that
found in fresh milk is usually necessary; thus at the very beginning
of the process of making cheddar cheese, the bacteria are of
importance.
As the milk curdles, the bacteria are enclosed in the curd as are
the fat globules. The curd is cut into small fragments by means of
a curd knife, and as the mass is warmed, the acid develops, causing
the curd particles to shrink, thus expressing the whey. Within a
short time, the volume of the curd is not more than one-eighth that
of the milk, but in the curd are held over 75 per cent of the
bacteria of the milk. To secure rapid curdling in the vat, the milk
is warmed to 85° to 90° F., a temperature that is most favorable for
the growth of the lactic bacteria. Since there is a large number of
bacteria concentrated in a small volume, and the temperature, as
well as all other conditions, is favorable to growth, multiplication
of the bacteria goes on rapidly, and as a consequence, acid is
formed in large amounts, as is shown by the following figures given
by Publow for the manufacture of the export type of cheddar cheese:
Acidity of milk before adding rennet .2 to .21 per cent
Acidity of whey before heating curd .14 to .145 "
Acidity of whey before removing from curd .16 to .18 "
Acidity of whey coming from the curd after
removal of whey and curd is packed .24 to .30 "
Acidity of whey coming from curd before milling .65 to .75 "
Acidity of whey coming from curd before salting .90 to 1.10 "
If the milk had been kept at the same temperature as the curd, the
acidity would have increased much more slowly since the acid would
have been distributed through a larger volume. In the cheese curd
the same amount of acid is probably formed, as would have been
produced in the total amount of milk during the same interval.
The acid produced by this bacterial activity has a most marked
effect on the curd. At first the curd masses are tough and firm,
the particles showing no tendency to adhere to each other. As the
acid increases in amount, the curd becomes plastic, the outer
surface of the particles adhering or "matting," as the maker
expresses it. The result is a solid coalescent mass of curd, which
is cut into small pieces, _i.e._, "milled," before it is put to
press. The acid allows the blending of the pieces under the
influence of the pressure so that a cheese is one single mass. Under
certain abnormal conditions, the development of acid may be
interfered with and the particles of curd fail to mat, in which
case, the cheese will be crumbly when it is cut. The determination
of the proper time for pressing is made by the application of what
is known as the hot iron test. This is made by determining the
length of the "strings" or "threads" which can be drawn from a mass
of curd when it is brought in contact with a hot iron at a cherry
red heat, the length of the curd threads being a measure of the
amount of acid that has been formed in the curd.
The rate of acid formation within the curd particles is also
measured by determining the acidity of the whey as it comes from the
curd at different stages in the making. This test, which is often
used in place of the "hot iron" test is carried out in the same
manner, as in determining the acidity of milk or cream. The quality
of the cheese, both as to texture and flavor, is dependent to a
great degree upon the amount of acid that is formed during the
various stages in making; hence, the successful maker must follow
closely by some means the acid formation in the curd until it is put
to press.
It is very necessary that the milk shall contain a sufficient number
of acid-forming bacteria to produce the required amount of acid. If
a sufficient number of bacteria are not present in the milk as it is
received, as is the case with very sweet milk, they must be added
by the maker in the form of a starter, or the process of making will
be much prolonged.
[Illustration: Fig. 35.--Bacteria in Cheese.
A photomicrograph of curd just after curdling has taken place. Note
the few lactic acid bacteria embedded in the curd.]
=Starters in cheese making.= The starters used in cheese making, are
identical with those employed in butter making and the same
precautions should be observed in their propagation. It is important
that the starters should not be such as to form a hard curd that
cannot be mixed uniformly with the milk, since the curd particles
would appear as white specks in the cheese. The starter should be
added to the milk through a hair sieve, and well mixed with the
milk, so as to distribute the bacteria uniformly. Amounts varying
from 0.5 to 2 per cent are used. In butter making, it is essential
that the bacteria of the starter be able to form not only acid, but
sufficient flavor-forming substances to impart to the butter a
desirable flavor. In cheese making it is not probable that this
latter characteristic is of any particular importance.
[Illustration: Fig. 36.--Bacteria in Cheese.
A photomicrograph of curd at the time the salt is added. The lactic
acid bacteria have increased materially in numbers.]
It is desirable that the process of cheese making shall conform as
closely as possible to that which experience has shown to give the
best results. The rate at which acid is developed in the curd and
the rapidity with which the whey is expelled therefrom should bear a
certain ratio to each other. If the milk has too high a degree of
acidity, _i.e._, is overripe, the acidity developed in the curd will
be too high before the curd is sufficiently firm; with a very sweet
milk, the reverse may be true. It is desirable for the cheesemaker
to obtain as good an idea as possible of the condition of the milk
with reference to its bacterial content, since this will determine
the rate at which acid will be formed in the curd. If the milk is
too sweet, _i.e._, too low in acid-forming bacteria, a starter
should be added. The only methods by which this information can be
obtained by the maker is by determining the acidity by the usual
method or better by the use of the rennet test by which is
ascertained the time required for a given amount of rennet to curdle
a definite quantity of milk at a standard temperature. The varying
factor in the test will be the acidity of the milk. Very slight
differences influence profoundly the time of curdling. If, working
under standard conditions, it is found that the time of curdling of
one sample is 10 seconds and of another sample, 20 seconds, it is
proof that the acidity of the first is higher than that of the
second, that its bacterial content is greater and that acidity will
develop in the curd more rapidly. The first may need a small amount
of starter, the second a larger quantity. Working with milk from the
same source, the maker, from his experience, will know how much
starter should be added to milk that has given a certain result with
the rennet test in order that the acid shall be developed in the
curd at a desired rate.
=Ripening of cheese.= The curd at the time it is put to press is tough
and rubbery, and has none of the characteristic flavor of cheddar
cheese; it is also quite insoluble and indigestible. Before the
cheese is fit to eat it must pass through a complex series of
changes which are collectively known as _ripening_. In these changes
there is not only a breaking down of the casein into soluble
compounds, which process makes the cheese soft and plastic under
pressure, but the characteristic flavor is developed in greater or
less degree. A very considerable part of the cheese thus becomes
soluble in water, and it is much more easily digested than in an
unripened condition.
The different factors that are operative in the ripening changes are
not yet fully known, but in recent years as a result of scientific
study, material progress in the study of the changes has been made.
=Rennet.= The commercial rennet extract when in condition for use
contains very few bacteria. A preservative, boric acid, is added by
the manufacturer to restrain the bacteria, otherwise the extract
would soon be unfit for use. The bacteria in the commercial rennet
extract are too few to be of any importance whatever in the ripening
process.
Rennet extract contains an enzyme, rennin, that causes the milk to
curdle; also another enzyme, pepsin, that exerts a digestive action
on the curdled casein. Pepsin is always found in the stomach juices
of all animals, but no digestive action takes place, unless the
reaction is distinctly acid, as is the ease under normal conditions,
since hydrochloric acid is excreted by the walls of the stomach.
Outside of the stomach, the same conditions must obtain with
reference to the presence of acid, if pepsin is to exert a digestive
effect. In the cheese curd, the milk sugar is rapidly changed into
lactic acid by the action of the bacteria. This gives the proper
chemical reaction for peptic action, and the enzyme is then able to
act on the paracasein, the nitrogenous part of the cheese. If milk
contains no acid-forming bacteria, conditions will not permit of
peptic action, and as a consequence, the ripening processes do not
take place. If the sugar is fermented by some organism that does
not form acid, as the lactose-fermenting yeasts, the cheese does not
ripen. The lactic bacteria are therefore an essential factor in
inaugurating the ripening changes in all types of rennet cheese.
=Preservative action of acid.= In a previous chapter it was shown that
raw milk does not undergo putrefaction because of the restraining
effect of the acid formed by the lactic bacteria on the putrefactive
organisms. This same phenomenon is noted in cheese. Milk always
contains putrefactive bacteria which pass into the cheese, but they
cannot grow therein because of the high acidity. In the absence of
the acid-forming organisms in the cheese, the cheese may remain
tough and rubbery, on account of the lack of suitable conditions for
the action of the pepsin of the rennet extract, or when the milk
contains large numbers of digesting organisms, the cheese may
develop a putrefactive condition, as noted by the offensive odor and
soft pasty texture.
=Other factors concerned in cheese ripening.= There are other factors
that are also concerned in the complex series of ripening changes
noted in cheddar cheese. All animal fluids and tissues, if kept
under perfectly sterile conditions at ordinary temperatures, will
undergo a certain amount of decomposition, due apparently to their
content in enzymes that have a digestive action. Meat kept in
storage becomes more tender due to the softening of the connective
tissue. Milk, derived as it is from actively secreting cell tissue,
gives certain reactions that are common to living material. If
chloroform, which restrains the action of bacteria, but does not
prevent the activity of enzymes, is added to it, it will curdle in
the course of a few weeks and will become partially digested. This
digesting ferment found in milk is known as _galactase_. Compounds
are formed in milk thus preserved that are similar to those found in
a ripe cheddar cheese. Many experiments have been made with
pasteurized milk, but it has not been possible to produce typical,
normal cheese from thoroughly pasteurized milk. Such cheese are
markedly deficient in the typical flavor of cheddar cheese. From
this fact it is believed that the inherent enzymes of milk are a
factor of some importance in the ripening of this type of cheese at
least, if not of all types.
In the past, other factors have been thought to be of importance.
Duclaux, a French bacteriologist, considered that the enzymes formed
by the digesting bacteria are responsible for the ripening. It is
now known that they can have but little if any part in the process,
since they are not present in all cheese in sufficient numbers to
have any marked effect, and since the acidity of the cheese mass
will not permit of their development.
Other types of bacteria have been considered by bacteriologists to
be of importance in the ripening process, but it is certain that the
purely digestive change in the mass of the cheese can be accounted
for through the action of the factors already noted.
=Flavor production.= The flavor of any type of cheese is the most
important characteristic, just as it is in butter, for it is largely
the flavor that determines the selling value of the product, and is
the most difficult thing to control. It has been thought that the
flavor-producing substances were derived from the paracasein of the
curd and were produced by the factors that are concerned in the
digestion of the paracasein. It has been shown that a cheese may be
thoroughly ripened as far as its physical properties are concerned;
that it may contain the end products of casein digestion, and yet
be low in flavor. From recent researches it seems probable that the
production of flavor is connected with the change that the sugar
undergoes in the acid fermentation, as volatile acids, acetic,
formic, etc., as well as alcohols and esters are formed in
increasing amounts as the ripening progresses. These may have come
from the decomposition of the milk sugar, or from a secondary change
in the products of the lactic fermentation. There are organisms in
both milk and cheese that do not grow on the ordinary culture media
used by the bacteriologist, and it may well be that some of these
are of importance in flavor production. Their destruction in
pasteurization is likely to be one of the reasons for the failure of
cheese made from pasteurized milk to develop typical flavor.
=Effect of temperature on ripening.= The temperature at which the
ripening cheese is kept has been found to be of the greatest
importance in determining the quality of the product. If the cheese
is kept at high temperatures, the ripening proceeds rapidly; the
cheese is short lived, and has 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 likely to undergo undesirable fermentations
when ripened at high temperatures.
Within recent years it has been found possible to ripen cheese at
temperatures that were previously thought to be certain to spoil the
product. Much of the cheese is now ripened at temperatures below 50°
F. The ripening goes on more slowly than at higher temperatures, but
the flavor of the cheese is clean and entirely devoid of the sharp
undesirable tang that is so frequently noted in old cheese, and the
texture is solid and meaty. Ripening at low temperatures, when the
milk is not of the best quality, is certain to result in a much
better product than when higher temperatures are employed.
=Abnormal fermentations in cheese.= As has been previously shown, it
is necessary to have an abundant supply of acid-forming bacteria in
the milk from which cheese is to be made. If these bacteria are
supplanted by other kinds, the product will be more or less abnormal
either in texture or in flavor, or possibly in both. Many of these
abnormal fermentations have been studied and the organisms concerned
in the changes found.
If the milk is handled carelessly, it will contain many bacteria
able to form acid and gas. As noted previously, these organisms form
products in milk that have an offensive odor and a disagreeable
taste. In cheese the gases cause the formation of holes, more or
less numerous, depending on the number of the gas-forming bacteria
in the milk. Where these bacteria are abundant, gas may appear while
the curd is in the vat, causing it to float in the whey, when it is
known as a "floater." Again, the gas may not become evident until
the cheese is in the press or on the curing shelf, when it becomes
apparent by the swelling or bulging of the cheese. Such cheese is
termed "huffed" or "swelled." The internal pressure may be so great
as to cause the cheese to crack and to force out some of the curd.
The presence of gas holes is indicative of a poor cheese, because
the formation of gas is always accompanied by the presence of other
undesirable compounds.
Pure culture starters are often used to overcome gassy
fermentations. In cheese a certain amount of acid can be produced by
the acid-forming bacteria. When the pure lactic bacteria alone are
present, the cheese is very likely to be of good quality. If the
sugar is fermented by gas-forming organisms, the curd will be full
of holes and the flavor poor, while if the sugar is fermented by a
mixture of the desirable and undesirable bacteria, the quality of
the product will depend on the relation of the two types. If through
the addition of a pure-culture starter, the proportion of desirable
bacteria is increased, the gas will be lessened in amount and the
cheese improved. It was formerly supposed that the lactic bacteria
had an injurious effect on the gas-forming organisms. There is no
good reason to believe that this is the case, but that both grow in
the milk and cheese, but since only a certain amount of acid can be
produced, it is important to have as much of it formed by the lactic
bacteria as possible, since the amount of injurious products in the
cheese will thus be limited.
[Illustration: Fig. 37.--Gassy Cheese.
Such a cheese is worthless on account of its poor flavor. The
irregular holes are mechanical. The crack on the upper side is due
to the pressure of the gas which has caused the cheese to bulge at
this point.]
The gas formed in the curd before the cheese is put to press can be
gotten rid of by proper manipulation of the curd. While this
treatment may improve the appearance of the cheese, it does not
eliminate the substances that impart to the cheese undesirable
qualities.
Gassy curds have also been treated by washing the curd with cold
water. Care must be taken in applying this method for the removal of
too much of the sugar and acid from the curd by the washing will
permit the growth of injurious forms of bacteria. The addition of
salt or of saltpeter has also been made to the milk in order to
overcome gassy conditions in the milk. In the handling of gassy
milk, the usual practice has been to develop a larger amount of acid
before drawing the whey than in the case of good milk. This was done
with the idea that acid suppressed gas formation. It has been shown
previously that this is not the case. It has also been shown by
Doane that the development of too much acid before drawing the whey
is likely to result in undesirable flavors, producing what is known
as "high-acid" or "sour" cheese.
The gas-forming bacteria grow best at high temperatures; hence,
cheese kept under these conditions are more likely to be affected by
this trouble than are those kept at lower temperatures.
The most successful method of preventing trouble with gassy milk in
cheese making is to eliminate undesirable milk by frequent testing
of the supply of the different patrons by means of the Wisconsin
curd test.
Not only gas-forming bacteria may be the cause of gassy cheese, but
the lactose-fermenting yeasts may cause similar trouble. If these
are abundant in the milk, a considerable part of the sugar may be
fermented by them, in which case, carbon dioxide gas is abundantly
formed. The cheese thus rendered gassy will present the same
appearance to the eye as where the gas is formed by bacteria, but
will have a different flavor. The odor of alcohol may be evident,
and if most of the sugar has been fermented by the yeast, the
acidity of the cheese may not be sufficient for the pepsin to exert
its digestive action.
Milk containing many gas-forming bacteria occurs most frequently in
summer. It is claimed by some that the milk of cattle pastured on
low lands is more likely to contain the gas-forming organisms than
that from cattle running on higher lands. If this is true, it must
be due to the bacterial content of the soil; the udders of the
animals become soiled as they lie on the ground, and during the
milking, the dust finds its way into the pail. Many cheese makers
think that the milk from an animal suffering from a garget may be
the cause of the huffing of cheese. This belief is undoubtedly well
founded, as some of the bacteria known to be the cause of garget are
gas-forming.
=Bitter cheese.= In a previous chapter the bitter fermentation of milk
has been discussed. If milk containing large numbers of such
organisms is made into cheese, the bitterness is very likely to be
noted in it. Cheese made from milk containing few or no lactic
bacteria is likely to develop a bitter taste, due to the growth of
the digestive bacteria that are able to grow through the lack of
acid in the cheese.
If the milk contains considerable numbers of yeasts, a sweet or
fruity flavor is apt to develop, due to the products of the
fermentation of the sugar by the yeast. This flavor resembles that
of fermented fruit, or the bouquet of certain kinds of wine.
=Putrid cheese.= In the absence of acid-forming bacteria, the cheese
may develop a putrid or rotten odor, due to the growth of some types
of putrefactive or digesting bacteria. This trouble is very
infrequent in cheddar cheese, since this is made from ripened milk,
but occurs more frequently in those types in which no acid is
developed.
Bacteria develop in the cheese in colonies or masses, just as they
do in the plate cultures of the bacteriologist, made with
transparent media, such as gelatin. Cheese is opaque; therefore, the
growing colonies cannot be readily discovered, but when
pigment-forming bacteria grow in the cheese, their presence is
likely to be noted, because of the colored spots that are formed.
=Rusty spot.= The "rusty spot" that has been encountered in New York
and Canada is due to one of the colored bacteria which produces an
orange or yellowish-red pigment. Various other pigment-forming
organisms have been met in cheese, each producing its colored colony
which differentiates itself from the mass of the cheese. If the
pigment is produced in considerable quantities, and is soluble in
any of the constituents of the cheese, the color will not appear in
spots but will be more diffuse, or may impart a color to the entire
mass.
Cases of acute poisoning arising from the ingestion of cheese are
not infrequently reported; similar instances result from the use of
ice cream. In both cases it is believed that poisonous products have
been formed by bacteria, probably by some of the putrefactive forms.
From what has been said with reference to the abnormal fermentations
of cheese, it will be seen that they are always due to the lack of
acid-forming bacteria, or to their partial replacement by other
types. In order to prevent such troubles, it is necessary to insure
that the milk has been produced under clean conditions, from healthy
cows, and has been handled in such a manner as to reach the maker in
as sweet and fresh condition as possible. The maker can, by the use
of proper starters, control the kinds of bacteria essential for the
ripening process. A well trained maker should be able to prepare
from such milk a uniform product of the highest quality. The effort
of cheese makers at the present time is to handle milk of more or
less objectionable quality so as to secure from it as good cheese as
is possible. But cheese is so sensitive as to character of milk used
that greater effort should be spent in securing an improved supply.
=Moldy cheese.= In the case of the cheddar cheese and other types of
hard cheese, it is essential that their surfaces be kept clean, and
not discolored by the growth of molds, which find favorable
conditions for growth on the surface of the cheese in the moist
atmosphere of the curing room. The molding of cheddar cheese can be
prevented by covering the cheese with a layer of paraffin which
stops the development of the mold spores, by shutting off the
necessary supply of oxygen. For this purpose the cheese are dipped
in melted paraffin when a few days old.
In the case of types of cheese which are salted by applying the salt
to the surface, or with soft cheese which ripen from the outside,
other methods of mold prevention are employed, such as rubbing and
washing the cheese. The curing room itself may be freed from the
mold spores by the use of such standard disinfectants as formalin or
sulphur.
=Swiss cheese.= One of the most important kinds of hard cheese, is the
Swiss or Emmenthaler, so named, from the country and valley in which
the cheese was first made. In America, this type was introduced by
Swiss immigrants, and is being made in constantly increasing
quantities in Ohio and Wisconsin.
Swiss cheese is a hard firm type, appearing in the markets in the
form of the flat circular "drum" cheese, two to three feet in
diameter, and six to eight inches thick, or in the smaller "block"
form. In this country the cheese is prepared twice a day, since it
is necessary to work up the milk while it is perfectly sweet.
Indeed, the milk is received at the factories while it is still
warm, and within five or six hours after it is drawn from the cow
the cheese is on the press. If the attempt is made to prepare Swiss
cheese from the kind of milk that is best suited for cheddar
purposes, _i.e._, milk in which the acidity has increased to some
extent, the flavor of the resulting product is likely to approximate
a cheddar cheese rather than that of a Swiss.
In the salting process, the salt is not mixed with the curd before
it is pressed, but is applied by immersing the cheese for a few days
in a saturated brine, and then rubbing salt over the surface of the
cheese. In this way the salt gradually diffuses quite uniformly
through the cheese. The method of salting has apparently a marked
influence on the ripening process, since if the salt is added in the
same way, and in amounts used in the cheddar process, the flavor
will not be that of a Swiss cheese but will resemble a cheddar.
In cheddar cheese, the whey is expelled from the curd by means of
the acid which is developed in the curd, and by heating the curd to
a temperature of 95° F. to 100° F. In Swiss cheese the development
of acid during the making process is prevented, because of the
smaller number of acid-forming bacteria in the milk; other factors
must therefore contribute to the expulsion of the whey to secure a
firm curd. This is accomplished by cutting the curd into very small
pieces and by briskly stirring it during the making, heating it
during this process for a period of 20 to 30 minutes at 130° to 140°
F. It might be thought that this high temperature, which is
approximately that used in pasteurization would destroy the
acid-forming bacteria, but these are apparently protected as they
are within the curd. During the time the cheese is being pressed,
the contained bacteria begin to grow and the whey coming from the
cheese toward the end of the pressing shows a high acidity. If it
does not show such a development of acid, the maker has reason to
believe that the cheese may never ripen in a typical manner.
It has been mentioned that the milk contains but few acid-forming
bacteria. The maker, however, attempts to insure the presence of a
sufficient number by the use of "home-made" rennet. This is prepared
by placing a piece of dried rennet, _i.e._, the stomach of the calf,
in whey, keeping the same in a warm place for twenty-four to
thirty-six hours. As the rennet contains acid-forming organisms,
these grow rapidly in the warm whey, so that by adding this sour
whey to the milk, the maker is not only adding rennet, that is to
curdle the milk, but also a small starter of lactic bacteria. If the
rennet thus prepared contains no harmful bacteria and the milk is of
good quality, the cheese is likely to ripen in a normal manner. The
rennet should be prepared with due regard to bacteriological
principles, a condition that is rarely met in Swiss factories in
this country.
Swiss cheese has two striking characteristics, the flavor and the
presence of holes or "eyes." The flavor is sweetish rather than the
sharp and pungent flavor of cheddar cheese. The bacteria concerned
in its production are not known, but it is certain that specific
organisms play some role, since if the flora of the cheese is
changed by salting the curd or by the use of milk containing large
numbers of lactic bacteria, the flavor will also be changed. This
role of the acid-forming bacteria in Swiss is the same as in
cheddar, _i.e._, through the acid, conditions are established for
peptic action, the curd being partially digested while at the same
time the curd mass is protected from putrefactive processes.
In Swiss cheese during the ripening process, holes about the size of
a large cherry develop which should be quite uniformly distributed
throughout the cheese. The inner surface of the hole is glistening
and, in a well-ripened cheese, a small quantity of clear brine,
_i.e._, "tears" may be noted. These holes or "eyes" may be called
the trade mark of the Swiss cheese, since without them the product
has a lessened commercial value, even if it possesses the typical
flavor. The "eyes" are caused by bacteria that ferment the lactic
acid produced by the lactic bacteria, forming from it propionic
acid and carbon dioxide, the latter gas being the cause of the hole
or "eye."
[Illustration: Fig. 38.--Swiss Cheese.
Normal development of "eyes" in a Swiss cheese. The eyes are
generally as large as a cherry.]
The "eye"-forming organisms cannot grow in the presence of any
amount of salt, hence, if salt is added directly to the curd, the
cheese is likely to be "blind" or free from holes. The eyes are
formed not at the time gas holes are produced in a cheddar cheese,
_i.e._, early in the ripening process, but after a lapse of three or
four weeks. They are most abundant in the middle of the cheese since
the manner of salting is such as to inhibit their formation near the
surface. The eye-forming bacteria may have some effect on the flavor
of the cheese.
The Swiss maker encounters the same troubles as does the cheddar
maker. Gassy cheese is more prevalent in the Swiss than in the
cheddar industry, since the maker cannot call to his aid the methods
used by the cheddar maker, viz., the addition of a heavy starter,
the washing of the curd, etc. It is especially important that the
quality of the milk be first class in every respect, and yet
customs prevail in the Swiss industry that are directly inimical to
the production of good milk. The grossest carelessness prevails at
the factories in the matter of handling the whey. It is often kept
in individual barrels for each patron. (See Fig. 8.) These are not
kept thoroughly clean and the result is that the whey taken back to
the farm in the cans that are used to bring the fresh milk is often
in an advanced stage of fermentation.
There are many other kinds of hard cheese; but in each, so far as is
known, the role of the acid-forming bacteria is identical with that
noted in cheddar and Swiss cheese, viz, in preparing conditions
favorable for peptic action, and preventing the development of
putrefactive bacteria present in the curd.
=Roquefort cheese.= Among the more important foreign types of cheese
that are characterized by the development of mold is Roquefort, so
named from the district in France in which it is made. This cheese
is made from sheep's milk, in much the same manner as cheddar. The
characteristic process in its preparation is the inoculation of the
curd, at the time it is put to press, with the spores of a
particular kind of mold, a type closely related to the ordinary
green mold of bread and cheese. The mold for inoculation is grown on
bread, the whole mass being dried so that it can be powdered; then
the ground-up material is sprinkled on the curd as it is placed in
the press hoops. The first stage in the ripening of Roquefort is
probably identical with that of the types of hard cheeses already
considered, the breaking-down of the curd being due to the pepsin of
the rennet used, which action is made possible by the acid formed by
the bacteria.
The second stage in ripening, and one in which the characteristic
flavor of the cheese is developed, is due to the growth of the mold
with which the cheese is seeded. Molds can grow only in the presence
of air, and in order to provide this condition, the cheese are run
through a machine having a series of needle-like projections which
fills the cheese with fine holes. This allows the air to penetrate
the cheese and the mold to grow, the fruiting of which develops the
characteristic flavor. The changes produced by the mold are not well
understood, but the flavor is evidently connected with its
development since in the absence of mold, it does not appear. The
cheese must be cured under carefully controlled conditions, as to
temperature and moisture; in France these are secured by curing the
cheese in limestone caves that are highly saturated with moisture.
Attempts have been made to make Roquefort cheese in other parts of
the world, but they have never been successful, due undoubtedly to
the fact that the proper environment and conditions for the
development of the various types of organisms necessary in the
ripening process have not been met. This cheese is sold for 50 to 75
cents per pound in the markets of the world.
There are two other kinds of cheese that are closely related to
Roquefort, as to the manner of ripening, viz., the Gorgonzola of
Italy and the Stilton of England, both of which possess their
characteristic flavors by reason of the development of molds. In
Stilton cheese the mold is not intentionally added, the maker
relying on the contamination that comes from the factory for the
usual seeding. If this does not develop, it is sometimes inoculated
by exchanging plugs with a well-ripened Stilton. This method is not
so certain as in the inoculation of Roquefort.
=Camembert cheese.= A typical example of soft cheese is one of the
French types, known as Camembert. This cheese is prepared from cow's
milk which is curdled by rennet. The curd is not cut but is dipped
into the forms, which condition, taken with the absence of pressure
in forming the cheese, accounts for the large quantity of whey in
it. The finished cheese are about one inch in thickness and three
inches in diameter. In the ripening, the moisture and temperature of
the curing room are very carefully regulated.
The first stage in the ripening is due to the rennet and the lactic
bacteria. Later there appears on the surface of the moist cheese, a
moldy growth. In this, there are at least two kinds of molds, the
ordinary mold that appears on sour milk, _Oidium lactis_, and
another that is related to the bread mold but which has a white
instead of a green fruiting stage. These molds are confined to the
surface of the cheese but the enzymes which they produce diffuse
into the substance, changing the color from a dull, opaque white to
a translucent yellow. The acid that has been formed by the lactic
bacteria is gradually used up by the growth of the mold, and
conditions then become favorable for the growth of putrefactive
bacteria which digest the curd. The cheese is ready for use when the
action of the mold has penetrated to the center of the cheese, and
before any pronounced putrefaction has taken place. The production
of the typical flavor is dependent upon there being a definite
relation between the growth of the molds and bacteria. This relation
is dependent largely upon the moisture and temperature of the curing
room. These cannot always be regulated with exactness; and hence,
much of this type of cheese is not of first quality, and must be
sold for a low price. While such fancy cheeses, as Camembert, bring
fifty cents and upward per pound, and the yield from the milk is
much greater than with the hard type of cheese, yet the difficulties
of successful manufacture are such as to make success less easily
attained than with the other types.
There are many other kinds of soft cheese that depend for their
ripening upon factors similar to those concerned in the ripening of
Camembert; most of them are, however, of small importance from a
commercial standpoint.
=Limburger cheese.= A very famous cheese is one originally made in
Germany to which the name Limburger is given. It is classed as a
soft cheese although it is much firmer than Camembert. This cheese
is made from cow's milk and is pressed very lightly or not at all,
which condition accounts for its high per cent (50 per cent) of
moisture. The surface is kept moist by repeated washing of the
cheese, and by keeping the air of the curing room very moist. A
yellowish, slimy, bacterial layer soon develops on the surface under
these conditions. The enzymes produced by this external growth
gradually diffuse to the center of the cheese, when it is regarded
as ripe. The odor of the matured product is somewhat putrefactive,
but is not so offensive as is usually supposed.
Definite knowledge concerning the types of organisms concerned in
the surface layer is very limited. It is not certain whether the
same kinds of organisms must always be present. Limburger is much
easier to make than Camembert, due possibly to the fact that there
are not needed definite forms of life and that the balance between
them is not so delicate.
A cheese known as brick is closely related to Limburger in its
method of making and of ripening but is less pronounced in flavor.
In the manufacture of all of these types of cheese, troubles are
likely to develop, due to an abnormal bacterial condition of the
milk.
It will be seen from what has been said that the bacteria are
essential factors in cheese ripening, and that the cheese industry,
like the butter industry, may be called a true fermentation
industry. Close co-operation must exist between the milk producer,
and the maker so that the type of fermentation that goes on in the
milk can be controlled. A recognition of the fundamental principles
governing these fermentations, both normal as well as abnormal, is
now regarded as an essential part of the training of the dairy
manufacturer of today.
CHAPTER IX.
BACTERIA IN MARKET MILK.
Within the last decade attention has been especially directed toward
the quality of milk that is furnished to the people in the cities.
This has come about, in part, in connection with the demands made
for better and purer food of every kind. These demands are reflected
in the pure-food laws enacted by the federal government, and by the
various states and municipalities. Another factor that has focused
attention on the milk supplies has been the belief that it plays an
important role in the production and distribution of disease,
especially among children. The rapid growth of cities in all of the
great countries of the world, the higher standard of living, and the
greater demand for milk and other dairy products, has, of necessity,
widened the zone from which the milk supply of any particular city
must be drawn. Milk is now an article of export and of import; some
of the great cities draw a portion of their supply from farms
hundreds of miles away. This means that a longer time must elapse
between the time of production and consumption, necessitating the
exercise of greater care in production and handling in order to
preserve the milk until it reaches the consumer.
In the past in the cities, as in the smaller towns at present, the
supply was largely furnished by the producer directly to the
consumer. This direct contact afforded the consumer the opportunity
of informing himself of the conditions under which his milk supply
was produced if he desired. The advent of the middleman in the
business, and the gathering of the milk from many hundreds of farms,
and its redistribution to thousands of homes has made it impossible
for the individual consumer to learn anything of the conditions
surrounding production. When the individual cannot protect himself
against fraud and unhealthful conditions, it is the duty of the
government to protect him. This is the theory underlying the modern
control of food supplies, water supplies, and of living conditions
in general. Acting on this basis the cities are seeking to control,
to an increasing degree, the healthfulness and cleanliness of the
milk supply.
Formerly such control as was given was largely with reference to the
composition of the milk, the regulations providing that it should
contain not less than a minimum amount of fat and other solids, and
be free from preservatives. The more modern regulations are much
more complex and touch every phase of production and handling that
can, in any way, affect the value of the milk as human food.
=Municipal regulations.= The different cities vary widely in the
methods employed to secure a satisfactory milk supply. Rules and
regulations are adopted to which the producer and dealer must
conform. In order to ascertain whether the regulations are being
obeyed, two types of examinations may be made: first the inspection
of the farms and of the plants of the dealers; second the
examination of the milk itself with reference to its chemical
composition, bacterial content and temperature.
The city of New York is doing more to safeguard and to improve its
milk supply than any other large city in this country. A brief
summary of its regulations and methods follow. A copy of the rules
is furnished to each dairyman and is supposed to be posted in the
stable.
The Cows.
1. The cows must be kept clean, and manure must not be permitted to
collect upon the tail, sides, udder and belly of any milch cow.
2. The cows should be groomed daily, and all collections of manure,
mud or other filth must not be allowed to remain upon their flanks,
udders or bellies during milking.
3. The clipping of long hairs from the udder and flanks of the cows
is of assistance in preventing the collection of filth which may
drop into the milk. The hair on the tails should be cut, so that the
brush will be well above the ground.
4. The udders and teats of the cow should be thoroughly cleaned
before milking; this to be done by thorough brushing and the use of
a cloth and warm water.
5. To prevent the cows from lying down and getting dirty between
cleaning and milking, a throat latch of rope or chain should be
fastened across the stanchions under the cow's neck.
6. Only feed which is of good quality and only grain and coarse
fodders which are free from dirt and mould should be used.
Distillery waste or any substance in a state of fermentation or
putrefaction must not be fed.
7. Cows which are not in good flesh and condition should be
immediately removed and their milk kept separate until their health
has been passed upon by a veterinarian.
8. An examination by a veterinary surgeon should be made at least
once a year.
The Stable.
9. No stagnant water, hog-pen, privy or uncovered cesspool or manure
pit should be maintained within 100 feet of the cow stable.
10. The cow stable should be provided with some adequate means of
ventilation, either by the construction of sufficient air chutes
extending from the room in which the cows are kept to the outside
air, or by the installation of muslin stretched over the window
openings.
11. Windows should be installed in the cow barn to provide
sufficient light (2 sq. feet of window light to each 600 cubic feet
of air space the minimum) and the window panes be washed and kept
clean.
12. There should be at least 600 cubic feet of air space for each
cow.
13. Milch Cows should be kept in a place which is used for no other
purpose.
14. Stable floors should be made water-tight, be properly graded and
well drained, and be of some non-absorbent material. Cement or brick
floors are the best, as they can be more easily kept clean than
those of wood or earth.
15. The feeding troughs and platforms should be well lighted and
kept clean at all times.
16. The ceiling should be thoroughly swept down and kept free from
hanging straw, dirt and cobwebs.
17. The ceiling must be so constructed that dust and dirt therefrom
shall not readily fall to the floor or into the milk. If the space
over the cows is used for storage of hay, the ceiling should be made
tight to prevent chaff and dust from falling through.
18. The walls and ledges should be thoroughly swept down and kept
free from dust, dirt, manure or cobwebs, and the floors and
premises be kept free from dirt, rubbish and decaying animal or
vegetable matter at all times.
19. The cow beds should be so graded and kept that they will be
clean and sanitary at all times.
20. Stables should be whitewashed at least twice a year unless the
walls are painted or are of smooth cement.
21. Manure must be removed from the stalls and gutters at least
twice daily. This must not be done during milking, nor within one
hour prior thereto.
22. Manure should be taken from the barn, preferably drawn to the
field. When the weather is such that this cannot be done, it should
be stored not nearer than 200 ft. from the stable and the manure
pile should be so located that the cows cannot get at it.
23. The liquid matter should be absorbed and removed daily and at no
time be allowed to overflow or saturate the ground under or around
the cow barn.
24. Manure gutters should be from six to eight inches deep, and
constructed of concrete, stone or some non-absorbent material.
25. The use of land plaster or lime is recommended upon the floors
and gutters.
26. Only bedding which is clean, dry and absorbent should be used,
preferably sawdust, shavings, dried leaves or straw. No horse manure
should be used as bedding.
27. The flooring where the cows stand should be so constructed that
all manure may drop into the gutter and not upon the floor itself.
28. The floor should be swept daily. This must not be done within
one hour prior to milking time.
29. If individual drinking basins are used for the cows, they should
be frequently drained and cleaned.
30. All live stock other than cows should be excluded from the room
in which the milch cows are kept. (Calf or bull pens may be allowed
in the same room if kept in the same clean and sanitary manner as
the cow beds.)
31. The barnyard should be well drained and dry, and should be as
much sheltered as possible from the wind and cold. Manure should not
be allowed to collect therein.
32. A suitable place in some separate building should be provided
for the use of the cows when sick, and separate quarters must be
provided for the cows when calving.
33. There should be no direct opening from any silo or grain pit
into the room in which the milch cows are kept.
The Milk House.
34. A milk house must be provided which is separated from the stable
and dwelling. It should be located on elevated ground, with no
hog-pen, privy or manure pile within 100 feet.
35. It must be kept clean and not used for any purpose except the
handling of milk.
36. The milk house should be provided with sufficient light and
ventilation, with floors properly graded and made water-tight.
37. It should be provided with adjustable sashes to furnish
sufficient light and some proper method of ventilation should be
installed.
38. The milk house should be provided with an ample supply of clean
water for cooling the milk, and if it is not a running supply, the
water should be changed twice daily. Also a supply of clean ice
should be provided to be used for cooling the milk to 50 degrees
within two hours after milking.
39. Suitable means should be provided within the milk house, to
expose the milk pails, cans and utensils to the sun or to live
steam.
40. Facilities consisting of wash basins, soap and towel should be
provided for the use of milkers before and during milking. During
the Summer Months the Milk House should be properly screened to
exclude flies.
The Milkers and Milking.
41. Any person having any communicable or infectious disease, or one
caring for persons having such disease, must not be allowed to
handle the milk or milk utensils.
42. The hands of the milkers must be thoroughly washed with soap and
water, and carefully dried on a clean towel before milking.
43. Clean overalls and jumpers should be worn during the milking of
the cows. They should be used for no other purpose, and when not in
use should be kept in a clean place protected from dust.
44. The hands and teats should be kept dry during milking. The
practice of moistening the hands with milk is to be condemned.
45. The milking stools should be at all times kept clean, and iron
stools are recommended.
46. The first streams from each teat should be rejected, as this
fore milk contains more bacteria than the rest of the milk.
47. All milk drawn from the cows 15 days before, or 5 days after
parturition should be rejected.
48. The pails in which the milk is drawn should have as small an
opening at the top as can be used in milking; top opening preferably
not to exceed 8 inches in diameter. This lessens the contamination
by dust and dirt during milking.
49. The milking should be done rapidly and quietly, and the cows
should be treated kindly.
50. Dry fodder should not be fed to the cows during or just before
milking, as dust therefrom may fall into the milk.
51. All milk utensils, including pails, cans, strainers, and
dippers, must be kept thoroughly clean and must be washed and
scalded after each using, and all seams in these utensils should be
cleaned, scraped and soldered flush.
The Milk.
52. Milk from diseased cows must not be shipped.
53. The milk must not be in any way adulterated.
54. The milk as soon as drawn should be removed to the milk house
and immediately strained and cooled to the proper temperature.
55. All milk must be cooled to a temperature below 50 degrees F.,
within two hours after being drawn, and kept thereafter below that
until delivered to the creamery.
56. The milk should be strained into cans which are standing in ice
water which reaches the neck of the can. The more rapidly the milk
is cooled, the safer it is, and longer it will keep sweet. Ice
should be used in cooling milk, as very few springs are cold enough
for the purpose.
57. If aerators are used, they should stand where the air is free
from dust or odors, and on no account should they be used in the
stable or out of doors.
58. Milk strainers should be kept clean; scalded a second time just
before using, and if cloth strainers are used, several of them
should be provided, in order that they may be frequently changed
during the straining of the milk.
59. The use of any preservative or coloring matter is adulteration,
and its use by a producer or shipper will be a sufficient cause for
the exclusion of his product from the City of New York.
Water Supply.
60. The water supply used in the dairy and for washing utensils
should be absolutely free from any contamination, sufficiently
abundant for all purposes, and easy to access.
61. This supply should be protected against flood or surface
drainage.
62. The privy should be located not nearer than 100 feet of the
source of the water supply, or else be provided with a water-tight
box that can be readily removed and cleaned, and so constructed that
at no time will the contents overflow or saturate the surrounding
ground.
63. The source of the water supply should be rendered safe against
contamination by having no stable, barnyard, pile of manure or other
source of contamination located within 200 feet of it.
In order that the farm inspection shall be as effective as possible,
and to make the work of the several inspectors as uniform as may be,
the dairies are scored. A copy of the score card follows.
DEPARTMENT OF HEALTH
The City of New York
=Division of General=
=Sanitary Inspection= =Dairy Report=
Inspection No. ... Time...... A. P. M. Date......191..
1 =Dairyman=.................. =Owner= .....................
2 =P. O. Address=............. =P.O. Address=.......State...
3 =County=..... State..... =Party Interviewed=............
4 Milk delivered to Creamery at.......... Formerly at.........
5 Operated by.................. Address.......................
6 Distance of farm from Creamery..... Occupied farm since.....
7 No. Cows....... No. Milking...... No. Qts. Produced.........
8 All persons in the households of those engaged in producing
or handling milk are............free from all infectious
disease. Weekly reports are..................being filed
..........................................................
9 Date and nature of last case on farm........................
10 =WATER SUPPLY= for utensils is from a............... located
.......... feet deep and apparently is............ pure and
wholesome............ State any possible contamination
located within 200 feet of source of water supply or
if water supply is not protected against surface drainage
...........................................................
...........................................................
11 Water supply on this farm analyzed.... 191.. Result........
12 Style of Cow Barn.... Length.... ft. Width.... ft. Height
of ceiling.... ft.
13 =Dairy Rules= of the Department of Health are........ posted
..................
14 =Dairy Herd= examined by............. on.............. 191..
Report............
=================================================================
|Perfect| Allow |
-----------------------------------------------------------------
EQUIPMENT | | |
| | |
15 =COW STABLE= is.......located on elevated | | |
ground with no stagnant water, hog-pen, | | |
privy, uncovered cesspool or manure pit | | |
within 100 feet | 1 | ..... |
| | |
16 =FLOORS=, other than cow beds, are | | |
of concrete or some non-absorbent material | 2 | ..... |
| | |
17 Floors are...properly graded and water-tight | 2 | ..... |
| | |
18 =Cow beds are=...of concrete or planks | | |
laid on concrete | 2 | ..... |
| | |
19 =DROPS= are.....constructed of concrete, | | |
stone or some non-absorbent material | 2 | ..... |
| | |
20 Drops are......water-tight and space beneath | | |
is clean and dry. | 2 | ..... |
| | |
21 =CEILING= is constructed of.......and is | | |
tight and dust proof | 2 | ..... |
| | |
22 =WINDOWS= No.......total square feet | | |
there is...........2 square feet of window | | |
light for each 600 cu. ft. air space (1 | | |
sq. ft. per each 600 cu. ft.--1) | 2 | ..... |
| | |
23 =VENTILATION= consists of ......sq. ft. muslin| | |
in ceiling or..........which is sufficient | | |
3, fair 2, poor 1, insufficient 0 | 3 | ..... |
| | |
24 =AIR SPACE= is......cu. ft. per cow (600 and | | |
over--3) (500 to 600--2) (400 to 500--1) | | |
(under 400--0) | 3 | ..... |
| | |
25 =LIVE STOCK=, other than cows, are....excluded| | |
from rooms in which milch cows | | |
are kept | 2 | ..... |
| | |
26 There is..........direct opening from stable | | |
into silo or grain pit | 1 | ..... |
| | |
27 Separate quarters are...........provided for | | |
cows when calving or sick | 1 | ..... |
| | |
28 =COW YARD= is..............properly graded and| | |
drained | 2 | ..... |
| | |
29 =WATER SUPPLY= for cows is..........unpolluted| | |
and plentiful | 1 | ..... |
| | |
30 =MILK HOUSE= has...........direct opening into| | |
cow barn or other building | 1 | ..... |
| | |
31 Milk house has..........sufficient light and | | |
ventilation | 1 | ..... |
| | |
32 Floor is.................properly graded and | | |
water-tight | 1 | ..... |
| | |
33 Milk house is...........properly screened to | | |
exclude flies | 1 | ..... |
| | |
34 Milk pails are............of smoothly tinned | | |
metal in good repair | 1 | ..... |
| | |
35 =MILK PAILS= have...........all seams soldered| | |
flush | 2 | ..... |
| | |
36 Milk pails are..........of the small mouthed | | |
design, top opening not exceeding 8 inches | | |
in diameter. Diameter | 2 | ..... |
| | |
37 =Racks are=........provided to hold milk pails| | |
and cans when not in use | 2 | ..... |
| | |
38 =Special milking suits= are......provided | 1 | ..... |
|-------|-------|
| 40 | |
| | |
=METHODS= | | |
| | |
39 =STABLE INTERIOR= painted or whitewashed | | |
on.......which is satisfactory 3, fair 2, | | |
unsatisfactory 1, never 0 | 3 | ..... |
| | |
40 =FEEDING TROUGHS=, platforms or cribs are | | |
......well lighted and clean | 1 | ..... |
| | |
41 =Celling= is..........free from hanging straw,| | |
dirt or cobwebs | 3 | ..... |
| | |
42 =Window panes= are.............washed and kept| | |
clean | 1 | ..... |
| | |
43 =WALLS AND LEDGES= are...............free from| | |
dirt, dust, manure or cobwebs | 2 | ..... |
| | |
44 =FLOORS AND PREMISES= are.......free from | | |
from dirt, rubbish or decayed animal or | | |
vegetable matter | 2 | ..... |
| | |
45 =COW BEDS= are.........clean, dry and no horse| | |
manure used thereon | 2 | ..... |
| | |
46 =Manure= is.......removed to field daily 4, | | |
to at least 100 feet from barn 2, stored | | |
less than 100 feet or where cows can get | | |
at it 0 | 4 | ..... |
| | |
47 =Liquid Matter= is....... allowed to saturate | | |
ground under or around cow barn | 2 | ..... |
| | |
48 =Milking stools= are.......clean | 1 | ..... |
| | |
49 =Cow Yard= is.......clean and free from | | |
manure | 2 | ..... |
| | |
50 =COWS= have......been tuberculin tested and | | |
all tuberculous cows removed | 7 | ..... |
| | |
51 Cows are.....all in good flesh and condition | | |
at time of inspection | 2 | ..... |
| | |
52 Cows are.....all free from clinging | | |
manure and dirt. (No. dirty.....) | 4 | ..... |
| | |
53 =LONG HAIRS= are.....kept short on belly, | | |
flanks, udder and tail | 1 | ..... |
| | |
54 =UDDER AND TEATS= of cows are...... | | |
thoroughly brushed and wiped with a | | |
clean damp cloth before milking | 3 | ..... |
| | |
55 =ALL FEED= is.....of good quality and | | |
distillery waste or any substance in a state | | |
of putrefaction is......fed | 2 | ..... |
| | |
56 =MILKING= is.....done with dry hands | 2 | ..... |
| | |
57 =FORE MILK= or first few streams from each | | |
teat is.....discarded | 2 | ..... |
| | |
58 =Clothing= of milkers is.....clean | 1 | ..... |
| | |
59 Facilities for washing hands of milkers are | | |
......provided in cow barn or milk | | |
house | 2 | ..... |
| | |
60 =Milk= is strained at.....and.....in | | |
clean atmosphere | 1 | ..... |
| | |
61 Milk is.....cooled within two hours after | | |
milking to 50 degrees F. 3, to 55 degrees | | |
F. 2 to 60 degrees F. 1 | 3 | ..... |
| | |
62 Ice is.....used for cooling milk | 1 | ..... |
| | |
63 =MILK HOUSE= is.....free from dirt, rubbish | | |
and all material not used in the | | |
handling and storage of milk | 1 | ..... |
| | |
64 =Milk utensils= are.....rinsed with cold | | |
water immediately after using and washed | | |
clean with hot water and washing solution | 2 | ..... |
| | |
65 Utensils are.....sterilized by steam or | | |
boiling water after each using | 2 | ..... |
| | |
66 =Privy= is.....in sanitary condition, with | | |
vault and seats.....covered and protected | 1 | ..... |
| | |
|-------|-------|
| 60 | |
Remarks
Equipment 40 per cent. Score .... per cent
Methods 60 per cent. Score .... per cent
Perfect Dairy 100 per cent. Score .... per cent
A copy of the completed report is left with the dairyman.
Before the farm inspection is carried out the creameries to which
the milk is delivered by the farmers are inspected at the time the
milk is being delivered. The temperature of the milk and its
cleanliness are noted. In the creamery the straining, cooling and
handling of the milk are observed as well as the washing of the milk
cans and other utensils, and the construction and condition of the
creamery, the opportunity for the water supply to become
contaminated, and the presence of infectious diseases among the
employees.
=Grades of milk.= Three grades of milk have been established. Each
dealer is required to state which grade or grades he expects to
handle. The specifications for the different grades are as follows.
_Grade A. Guaranteed Milk._ Guaranteed milk is that produced at
farms holding permits therefor from the Department of Health and
produced and handled in accordance with the following minimum
requirements, rules and regulations:
1. Only such cows shall be admitted to the herd as have not re-acted
to a diagnostic injection of tuberculin.
2. All cows shall be annually tested with tuberculin, and all
re-acting animals shall be excluded from the herd.
3. No milk from re-acting animals shall be shipped to the City of
New York for any purpose whatever.
4. The milk shall not contain more than 30,000 bacteria per c. c.
when delivered to the consumer, or at any time prior to such
delivery.
5. The milk shall be delivered to the consumer only in sealed
bottles, which have been sealed at the dairy.
6. The milk shall be delivered to the consumer within 30 hours of
the time at which it was drawn.
_Grade A. Certified Milk._ Certified milk is milk certified by a
milk commission appointed by the Medical Society of the County of
New York, or the Medical Society of the County of Kings, as being
produced under the supervision and in conformity with the
requirements of that commission as laid down for certified milk, and
sold under a permit therefor issued by the Board of Health.
No milk shall be held, kept, offered for sale, or sold and delivered
as certified milk in the City of New York which is produced under
requirements less than those for guaranteed milk.
_Grade A. Inspected Milk--Raw._ Inspected milk (raw) is milk
produced at farms holding permits therefor from the Board of Health,
and produced and handled in accordance with the following minimum
requirements, rules and regulations:
1. Only such cows shall be admitted to the herd as have not re-acted
to a diagnostic injection of tuberculin.
2. All cows shall be tested annually with tuberculin, and all
re-acting animals shall be excluded from the herd.
3. No milk from re-acting animals shall be shipped to the City of
New York for any purpose whatsoever.
4. The farms at which the milk is produced must obtain at least 75
points in an official score of the Department of Health. These 75
points shall be made up as follows: A minimum of 25 points for
equipment, and 50 points for method.
5. The milk shall not contain more than an average of 60,000
bacteria per c. c. when delivered to the consumer, or at any time
prior thereto.
6. Unless otherwise specified in the permit, the milk shall be
delivered to the consumer only in bottles.
_Grade A. Selected Milk--Pasteurized._ Selected milk (pasteurized)
is milk produced at farms holding permits therefor from the Board of
Health, and produced and handled in accordance with the following
requirements, rules and regulations:
1. The farms at which the milk is produced must obtain at least 60
points in an official score of the Department of Health. Of these 60
points, a minimum of 20 points shall be required for equipment and a
minimum of 40 points for method.
2. All milk of this grade shall be pasteurized, and said
pasteurization shall be carried on under a special permit issued
therefor by the Board of Health, in addition to the permit for
"Selected Milk (Pasteurized.)"
3. The milk shall not contain more than an average of 50,000
bacteria per c. c. when delivered to the consumer, or at any time
after pasteurization and prior to such delivery.
4. Unless otherwise specified in the permit, the milk shall be
delivered to the consumer only in bottles.
5. All containers in which pasteurized milk is delivered to the
consumer shall be plainly labeled "Pasteurized." Labels must also
bear the date and hour when pasteurization was completed, the place
where pasteurization was performed, and the name of the person, firm
or corporation performing the pasteurization.
6. The milk must be delivered to the consumers within 30 hours after
the completion of the process of pasteurization.
7. No milk shall be pasteurized more than once.
8. No milk containing in excess of 200,000 bacteria per c. c. shall
be pasteurized.
_General Regulations for Grade A_--
1. The caps of all bottles containing milk of Grade A shall be
white, and shall contain the words "Grade A" in black letters, in
large type.
2. If cans are used for the delivery of milk for Grade A, the said
cans shall have affixed to them white tags, with the words "Grade A"
printed thereon in black letters, in large type, together with the
designation "Inspected Milk (Raw)" or "Selected Milk (Pasteurized),"
as the quality of the contents may require.
_Grade B. Selected Milk--Raw._ Selected milk (raw) is milk produced
at farms holding permits therefor from the Board of Health, and
produced and handled in accordance with the following minimum
requirements, rules and regulations:
1. Only such cows shall be admitted to the herd as have been
physically examined by a regularly qualified veterinarian and
declared by him to be healthy, and free from tuberculosis in so far
as a physical examination may determine that fact.
2. The farms at which the milk is produced must obtain at least 68
points in an official score of the Department of Health. These 68
points shall be made up as follows: A minimum of 25 points for
equipment, and a minimum of 43 points for method.
3. The milk shall not contain an excessive number of bacteria when
delivered to the consumer, or at any time prior thereto.
_Grade B. Pasteurized Milk._ Pasteurized milk (Grade B) is milk
produced under a permit issued therefor by the Board of Health,
and produced and handled in accordance with the following minimum
requirements, rules and regulations and in further accordance with
the special rules and regulations relating to the pasteurization of
milk.
1. The milk after pasteurization must be at once cooled and placed
in sterilized containers, and the containers immediately closed.
2. All containers in which pasteurized milk is delivered to the
consumer shall be plainly labeled "Pasteurized". Labels must also
bear the date and hour when the pasteurization was completed, the
place where pasteurization was performed, and the name of the
person, firm or corporation performing the pasteurization.
3. The milk must be delivered to the consumer within 36 hours after
the completion of the process of pasteurization.
4. No milk shall be pasteurized more than once.
5. No milk containing an excessive number of bacteria shall be
pasteurized.
_General Regulations for Grade B_--
1. Caps of bottles containing milk of grade B shall be white and
marked "Grade B" in bright green letters of large type.
2. The necks and shoulders of cans containing grade B milk shall be
painted bright green, and a metal tag shall be attached to each can
with the words "Grade B" in large type, and the words of the
subdivision to which the quality of the milk in said can conforms.
_Grade C._ Grade C is to be used for cooking and manufacturing
purposes only. It includes all raw milk that does not conform to the
requirements of any of the subdivisions of grade A or grade B.
1. The caps of all bottles containing milk of grade C shall be white
and shall contain in red the words "Grade C" in large type and "for
cooking" in plainly visible type.
2. Cans containing milk of grade C shall be painted red on necks and
shoulders and shall have in red the words "Grade C" in large type
and the words "for cooking" in plainly visible type affixed to each
can.
All creameries handling milk of different grades will be required to
demonstrate to the Department of Health that they are capable of
keeping the grades separate, and must keep records satisfactory to
the Department of Health concerning the amount of milk of each grade
handled each day.
It is to be noted that the grades of milk are based on the bacterial
content of the milk and on the opportunity for the milk to become
contaminated with pathogenic organisms. From the statements made in
a previous chapter it is evident that the number of bacteria in any
sample of milk is dependent upon (1) the original amount of
contamination, (2) the age of the milk, and (3) the temperature at
which it has been held. A high bacterial content is indicative of
poor milk, while a low bacterial content can be obtained, in the
case of raw milk, only where due attention is paid to cleanliness
and cooling. This relation between the quality of milk and its
bacterial content has led many cities to adopt numerical bacterial
standards, even when grades of milk have not been established.
Boston requires that the milk shall not contain more than 500,000
bacteria per cubic centimeter. Rochester, N. Y., has a standard of
100,000 per cubic centimeter, while Chicago requires that the milk
on arrival in the city shall not contain more than 1,000,000 per
cubic centimeter from May first to September thirtieth, and not over
500,000 between October first and April thirtieth. The sale of milk
containing more than 3,000,000 bacteria per cubic centimeter is
prohibited.
It has been urged that bacterial standards are not of value since
the healthfulness of milk depends on the kind of bacteria present
rather than on the number. It is well recognized that milk
containing millions of acid-forming organisms, butter milk, is a
healthful food, while that containing many less bacteria may contain
some disease-producing organisms. It has been urged that a
qualitative standard should supplant the quantitative. The consumer
desires milk that has been produced under clean conditions, and
which has good keeping qualities. The harmless forms of bacteria
exert the greatest influence on the keeping quality. Experience has
shown that the quantitative examination of the milk supply as
it comes from the farm is the most feasible method of determining,
in the laboratory, whether the farmer has obeyed the rules
with reference to cleanliness and cooling of the milk. The
bacteriological examination also gives an indication as to whether
the large number of bacteria is due to gross contamination of the
milk with mud and manure, or actual growth of bacteria as in old
milk. In the latter case the ordinary acid-forming bacteria will
usually predominate in the milk, while in the former, the number of
kinds of bacteria and the proportion between the kinds will be
changed. It is of course evident that the quantitative standards
should be applied with judgment.
It is also claimed that the delay in securing the results in the
quantitative examination of milk is an objection to the bacterial
standard, since the milk is consumed before the laboratory findings
can be obtained. It is true that it does not protect the community
as far as the particular sample is concerned, but it is also true
that the examination is not made for the purpose of determining the
condition of the particular sample, so much as it is to determine
the methods that are employed on any particular farm, and these do
not vary widely from day to day. Thus, if a number of samples give
high results, it is evident that conditions surrounding production
need investigation.
If the milk is well cooled on the farm, and kept cold while being
shipped, the growth of bacteria will be slow, and the condition of
the milk as far as keeping quality is concerned, much better than if
less care is used. Some cities have temperature standards; New York
requires that the milk shall be cooled to 50° F. on the farm, and
shall not be above 50° F. on arrival in the city. Others require
that it shall not be above 50° F. on delivery to the consumer.
=Certified milk.= In many cities the Medical Societies have appointed
Milk Commissions, that adopt rules and regulations, concerning the
production of milk that shall receive the certificate of the
commission. Producers, who desire to have their milk thus certified,
must satisfy the commission that they are able to conform to the
rules. The commission appoints a physician to examine the personnel
of the farm, a veterinarian to make frequent examinations of the
herd, a chemist to examine the milk as to its contents in fat and
other solids, and a bacteriologist to determine the bacterial
content of the milk. The rules are very stringent and cover every
point that may influence, in any way, the value of the milk as human
food. In order to conform to these requirements, a heavy
expenditure must be incurred, and the business must pay for such
expert service; hence, certified milk must be sold at high prices,
twelve to twenty-five cents per quart. This price makes it a special
product and its use is confined mainly to infant feeding.
The bacterial standard for certified milk is usually 10,000 bacteria
per cubic centimeter. It is only by the exercise of the greatest
care at every point that the bacterial content can be kept below
this maximum.
The term "certified milk" has been registered by Mr. Francisco of
New Jersey, who was the first to engage in the production of such
milk under the direction of the Medical Milk Commission of Essex
County, New Jersey. The use of the term is allowed when the milk is
produced under the regulation of any Medical Milk Commission.
Most certified milk is now produced on fancy dairy farms conducted
by wealthy men. The barns and other equipment are the best that can
be obtained, and the methods employed, as far as cleanliness is
concerned, are extreme. In some of the dairies the bacterial content
is reduced to a few hundred per cubic centimeter, or to that which
is derived from the interior of the udder. Such milk will, when well
refrigerated, keep for long periods of time. It is a not uncommon
thing for such milk to keep perfectly sweet for ten to fifteen days.
=Tests for the quality of milk.= At the milk depot and elsewhere, it
is frequently desired to determine the bacterial condition of the
milk in a less refined manner than by the plate cultures of the
bacteriologist, which require a large amount of time for their
preparation and do not yield any positive information for at least
twenty-four hours. There are a number of such tests that may be
applied.
[Illustration: Fig. 39.--Sediment Testers.
In the use of the apparatus on the right, increased air pressure is
used to hasten the filtering process; the same is accomplished in
the apparatus shown in the center by warming the milk by the
injection of steam between the walls of the double jacket.]
1. _Dirt or sediment test._ This is made by filtering a pint of the
mixed milk through a small disc of absorbent cotton. The insoluble
dirt is retained and imparts a color to the cotton, the shade of
which is dependent on the amount of dirt (P. 45). Since it is
impossible to have dirt without bacteria, it is evident that milks
containing a large amount of dirt will be high in bacteria. The
reverse, however, is not necessarily true.
[Illustration: Fig. 40.--Good Milk.
A plate culture inoculated with 1/100 of a cubic centimeter of milk
containing 67 colonies, which equals 6,700 bacteria per cubic
centimeter of milk. Such milk will keep well.]
2. _Acidity test._ The acidity of the milk is also an indication of
its bacterial content. If the acidity has increased, above the
normal for fresh milk, the bacterial content is certain to be high,
and the keeping quality poor. An acidity above 0.2 per cent in
market milk is to be avoided, as an increase in acidity is always
preceded by a great increase of bacteria.
Whether the acidity is above or below this point can be rapidly and
easily determined at the receiving station by a modification of
the Farrington acid test. Dissolve one alkaline tablet in an ounce
of water. A unit volume of this solution added to a unit volume of
milk is equal to 0.1 per cent of acidity. If two measures are
provided,--one for the alkaline solution holding just twice as much
as that used for the milk, the approximate acidity can be quickly
determined by mixing a measure of each in a common white cup. If the
acidity is above 0.2 per cent the color will remain white; if a pink
color develops, it indicates an acidity less than this amount.
This test is also useful in the selection of milk or cream that is
to be used for special purposes, such as pasteurization.
[Illustration: Fig. 41.--Poor Milk.
A plate culture inoculated with 1/1000 of a cubic centimeter of
market milk containing 1,680 colonies, which equals 1,680,000
bacteria per cubic centimeter. Such milk has poor keeping
qualities.]
3. _Alcohol test._ A test giving similar information is made by
adding two parts of 70 per cent alcohol to one part of milk, and
noting whether curdling occurs.
4. _Curd test._ The curd test described on p. 100 gives no
indication of the number of bacteria present, only concerning the
types present. It has been proposed to combine the fermentation test
with the reduction test referred to below and thus gain some idea
of, not only the number, but the kinds of bacteria present.
5. _Reduction test._ The reduction test is made by adding to twenty
cubic centimeters of milk, one-half cubic centimeter of a solution
of methylene blue, a coal tar dye. A saturated solution of the dye
is made in alcohol, and 2.5 per cent of this solution added to
water. The time required for the reduction of the dye or the change
of the color from blue to white when the samples are placed in tubes
and kept at 98 to 100° F., is dependent upon the number of bacteria
present. By allowing the tubes to stand until curdling occurs, and
noting the nature of the curd, whether the solid curd of the
desirable acid-forming bacteria or the gassy curd of the harmful
types is produced, knowledge is gained of the kinds of bacteria
present.
According to Barthel, milks that reduce the methylene blue within
fifteen minutes contain hundreds of thousands of bacteria per cubic
centimeter. Those that require from fifteen minutes to one hour for
the disappearance of the color are also high in bacteria, and are to
be classed as a poor grade of market milk. If one to three hours is
required, the milk is comparatively low in bacteria, and is to be
classed as a good grade of market milk. When more than three hours
elapse before the disappearance of the blue color, the bacterial
content is low and the milk is to be placed in the highest grade.
The time of reduction is only a rough index of the number of
bacteria present, but it gives a good idea of the keeping quality of
the milk, and of the conditions of production and handling. Of the
above tests the sediment and acid tests are more frequently used.
=Examination of milk sediments.= In the modern municipal laboratory,
efforts are made to determine, as far as possible, the conditions of
production on the farms, by an examination of the milk in the
laboratory. The samples of milk are sedimented in a small
centrifuge, and an examination of the sediment made with the
microscope. The types of bacteria and the number of body cells found
is an indication as to whether any of the animals of the herd are
suffering from inflammation of the udder. The test also gives
information similar to the dirt test since the insoluble dirt will
be thrown down and will impart a color to the sediment.
=Pasteurization of market milk.=.The spread of the pasteurizing
process as applied to market milk has been rapid. This has been due
to the recognition of the fact that only by this process can a safe
milk _i.e._, one free from pathogenic bacteria, be obtained. As
previously mentioned a small proportion of all human beings that
have suffered from typhoid fever become bacillus carriers. It is
impossible to examine all persons who may be concerned in the
handling of milk in order to ascertain whether they belong to this
dangerous and unfortunate class of people.
The larger cities have also recognized the impossibility of
requiring the tuberculin test of all cattle furnishing milk.
Pasteurization remains the only safeguard, and it is probable that
within a short time all the larger cities will require the
pasteurization of all milk, except that produced under strict
supervision.
As previously mentioned heating causes certain changes in milk. In
the treatment of market milk it is desirable to use as low
temperatures as will suffice to destroy the disease-producing
bacteria. It is fortunate that temperatures that will insure this
result have little effect on the milk. The temperatures now
recommended for pasteurization are as follows:
158 degrees F. for 3 minutes.
155 degrees F. for 5 minutes.
152 degrees F. for 10 minutes.
148 degrees F. for 15 minutes.
145 degrees F. for 18 minutes.
140 degrees F. for 20 minutes.
In actual practice the milk is heated to 145 degrees for 25 to 30
minutes. The acid-forming bacteria are not completely destroyed and
the pasteurized milk as a rule will undergo the same type of
fermentation as raw milk. It is, however, deemed essential that all
pasteurized milk be sold as such; that it be delivered to the
consumer within twenty-four hours after pasteurization and that no
milk be pasteurized a second time.
The continuous pasteurizing machines have the disadvantage that a
small portion of the milk passes through so quickly that all
pathogenic bacteria therein might not be destroyed, (p. 131). This
has led to the use of the "holding" process in which the milk is
heated to the desired temperature and then placed in tanks where it
remains at this temperature for any desired time. Every portion is
thus treated in a uniform manner.
If the milk is bottled after pasteurization, there remains
opportunity for reinfection, possibly with typhoid bacilli.
Pasteurization in the final container, the bottle, is being
recommended. This is possible only when a special bottle is used
with a metal cap lined with paper.
=Milk distribution.= Until within recent years in the cities and at
present in smaller towns, milk is largely retailed from cans which
are carried on the wagons or are kept in stores. This exposes the
milk to contamination from street dust and from the container
furnished by the consumer. It is well recognized that every utensil
with which milk is brought in contact adds more or less bacteria to
it, and the less milk is handled, the better will be its condition
when it reaches the consumer. Milk is now largely retailed in glass
bottles which are closed with pulp caps. In some cities the bottling
is mainly done in the country at the bottling station to which the
milk is brought by the farmers; or it may be shipped by the producer
to a distributing company, and all subsequent treatment, as
pasteurization and bottling done in the city.
Milk plants are now generally equipped for the rapid and economical
handling of large quantities of milk in a most sanitary manner. The
bottles as they are returned from the consumer are washed in a
continuously-acting automatic washer which washes, rinses and
sterilizes the bottles without their being removed from the cases in
which they are carried on the wagons. These machines are effective,
if not run at too rapid a rate, so that the bottles are not exposed
for a sufficiently long period of time to sterilize them. The
bottles are then filled and the paper caps inserted by machinery.
The caps can now be obtained from the manufacturers in sealed
tubes in which they have been sterilized so that the contamination
from this source is avoided. The shipping cans are washed and
sterilized with live steam, and in many plants are thoroughly dried,
by passing hot air into them. Under these conditions they then reach
the farmer with none of the musty and disagreeable odor that
frequently is present when the can contains a small quantity of
water, condensed from steam.
The top of the milk bottle over which the milk is poured is exposed
to contamination from the hands of the deliveryman. Trouble from
this source can be avoided if the consumer cleans the lip of the
bottle before removing the cap. The better grades of milk are
dispensed in bottles, the top of which is protected by an additional
cover of paper or tin foil which reaches to the neck of the bottle
and is held in place by a crimped metal band.
=Milk supply of the small cities.= It is true that the quality of milk
supplied to the large cities by the great milk companies is
generally much superior to that sold in the smaller cities and
villages. Many of the smaller places are however, attempting in
various ways to improve their supply. It is evident that methods
will be successful here that can not be employed in the larger
places. A detailed and careful farm inspection by a tactful, capable
inspector, coupled with proper publicity will do much to improve
conditions. The publication of the scores of the different farms,
and the demonstration of the sediment test as applied to their
product attracts favorable attention to the good dairies and
unfavorable attention to the poor. This usually has an effect on
the trade sufficient to cause the negligent producer and dealer to
improve.
It is also becoming recognized that high grade milk can be produced
with very simple equipment. In fact the small farm is often more
successful in producing high grade milk than is the large farm on
which the work must be done by hired help for here the personality
of the owner can not make itself felt as where the producer is doing
a portion of the work about the barn and dairy himself. It is
becoming more and more evident that the chief factor in the
production of clean milk is the personality of the producer; he
should be one who gets enjoyment out of his clean stables and cows
and his high grade product.
The man who is producing milk for the city market is but one of many
and his individual efforts can not make themselves felt. The
dairyman who is marketing his own product is in a position where his
efforts to produce a fine product should prove of distinct advantage
to him in enabling him to sell it for a higher price than that
obtained for ordinary milk.
It should be remembered that the production of clean, healthful milk
is not a question of equipment, but of methods and of additional
work. The cows must be fed, the stables must be cleaned, the cows
milked, and the milk delivered to the consumer. If beyond this
unavoidable labor a small additional amount is expended, the
improvement in the product will be great. It is necessary that the
additional work be placed where it will do the most good, in keeping
the cows clean both summer and winter so that little need be done in
cleaning them before milking, the pails and other utensils kept
clean and sterilized, and the milk cooled as soon as possible and
kept cold until delivered to the consumer. The delivery should be
made within the shortest practicable time after the milk is drawn.
In order that the healthfulness of the milk may be beyond question,
the herd must be kept free from tuberculosis and some attention
should be paid to the health of the men, especially with reference
to whether they may be typhoid carriers or not. The necessary labor
should not increase the cost of the milk over one cent per quart. It
has been shown in many cases that such a product can be marketed at
a price that will more than compensate for the additional cost.
Clean, fresh, rich milk is being sold in villages and small cities
located in the great butter and cheese producing sections of the
country for eight to ten cents per quart.
=The duty of the consumer.= The educational campaign that has been
carried on by the health departments with reference to farm
conditions and methods of handling has been most effective in
improving the milk supply. Many cities are now extending this to the
consumer, recognizing that as much harm may be done in the home as
on the farm. The importance of keeping the milk cold, of not
allowing it to stand exposed in open vessels, of thoroughly cleaning
the vessel in which it is kept, or the milk bottle before returning
it to the milkman are especially emphasized.
Moreover, it must be impressed upon the consumer that all of these
improvements, not only on the farm where the milk is produced, but
in the hands of the distributing companies in the cities, involve
much expense, and cannot be carried out, unless the consumer is
willing to pay their cost. More objection seems to be raised over
an increase in the price of milk than any other food stuff. The
consumer therefore needs education along the line of higher prices
for milk. Dairy products of all types have increased much in value
in recent years, so that at present prices milk, sold directly as
milk, is relatively cheaper than in any form, when prevailing prices
are compared with those that obtained a decade ago.
INDEX.
Abnormal fermentations, overcoming of, 108.
Abortion, contagious, 75.
Acid, amount of formed in milk, 84.
Acidity test, 211.
Actinomycosis, 75.
Aeration of milk, 55.
Aerobic bacteria, 13.
Air, contamination of milk from, 51.
Alcohol test, 213.
Alcoholic fermentation, 96.
Anaerobic bacteria, 13.
Animal, contamination of milk from, 42.
Anthrax, 75.
Antiseptics, 16, 117.
B.
Bacillus Bulgaricus, 89, 101.
Bacillus lactis acidi, 86.
Bacteria, aerobic, 13;
anaerobic, 13;
culture media for, 20;
desirable acid-forming, 86;
determining number of, 22;
distribution of, 18;
effect of cold on, 14;
effect of heat on, 15;
food of, 12;
forms of, 8;
manner of growth of, 9;
movement of, 11;
nature of, 8;
parasitic, 11;
products of, 17;
pure cultures of, 25;
rate of growth of, 13;
relation to air, 13;
relation to chemicals, 16;
relation to drying, 15;
relation to light, 16;
relation to temperature, 12;
size of, 9;
saprophytic, 11;
spores of, 10;
types of acid-forming, 86;
undesirable acid-forming, 90.
Bedding, 47.
Bitter fermentation, 97.
Bleaching powder, 112.
Bloody milk, 99.
Butter, bacteria in, 154;
bacterial defects in, 156;
cowy odor in, 157;
deterioration of, 155;
fishy, 157;
metallic, 157;
molding of, 157;
preservatives in, 155;
putrid, 156;
source of flavor, 140;
turnip flavored, 156;
types of, 137.
Butter-milk, 100.
Butyric fermentation, 93.
Boric acid, 117.
Borax, 117.
C.
Carbolic acid, 111.
Cheese, abnormal fermentations of, 174;
bitter, 177;
Camembert, 186;
Cheddar, 164;
colored, 178;
flavor production in, 172;
gassy, 174;
Gorgonzola, 185;
Limburger, 187;
moldy, 179;
preservation of by acid, 171;
putrid, 178;
quality of milk for, 162;
ripening of, 169;
Roquefort, 184;
Stilton, 185;
Swiss, 180;
temperature of ripening, 173;
types of, 161.
Children, diseases of, 80.
Chloride of lime, 112.
Cholera, 80.
Cleaning utensils, 39.
Clean milk, production of, 53.
Cold, effect of, on bacteria, 14.
Colored milk, 98.
Condensed milk, 135.
Contagious abortion, 75.
Contamination of milk, from milking machine, 50;
in factory, 59.
Cooling of milk, 54.
Corrosive sublimate, 111.
Cream, control of fermentation of, 142;
pasteurization of, 146;
ripening of, 138;
separators, 36.
Cresol, 111.
Curd test, 104.
Cycle of fermentations, 99.
D.
Deodorants, 109.
Digestive fermentation, 93.
Diphtheria, 79.
Dirt, exclusion of, 44;
removal of from milk, 53.
Disinfectants, 16, 109.
Disinfection, 109.
Distribution of bacteria, 18.
Dried milk, 135.
Drugs, excretion of in milk, 58.
Drying, effect of on bacteria, 15.
E.
Emmenthaler cheese, 180.
Evaporated milk, 134.
F.
Factory by-products, 36;
treatment of, 38.
Feeds, effect of on milk, 57.
Fermentation test, 104.
Fermented milks, 100.
Fly, contamination of milk by, 60;
means of spreading typhoid fever, 78.
Foot and mouth disease, 74.
Fore milk, 31; rejection of, 34.
Formalin, 112.
G.
Galactase, 172.
Garget, 75.
Germicidal action of milk, 33.
Gorgonzola cheese, 185.
H.
Hairs, bacteria on, 43.
Heat, effect on bacteria, 15.
Heated milk, detection of, 39.
Hydrogen peroxide, 118.
K.
Kefir, 102.
Koumiss, 102.
L.
Lange Wei, 95.
Light, effect on bacteria, 16.
Limburger cheese, 187.
Lime, 110.
Lumpy jaw, 75.
M.
Malta fever, 75.
Market milk, municipal regulations concerning, 190;
pasteurization of, 214.
Milk, acid fermentation of, 83;
aeration of, 55;
affected by feed, 57;
alcoholic fermentation of, 96;
bacterial standards for, 206;
bitter fermentation of, 93;
certified, 202;
butyric fermentation of, 93;
certified, 202, 208;
clarifying of, 115;
condition of when secreted, 29;
contamination of from animal, 42;
from by-products, 36;
from utensils, 34;
contamination of with tubercle bacilli, 67;
cooling of, 54;
creaming of, 136;
culture medium for bacteria, 28;
cycle of fermentation in, 99;
distribution of, 216;
digestive fermentation of, 93;
dirt in, 44;
effect of heat on, 119;
filtration of, 114;
germicidal action of, 33;
grades of, 201;
guaranteed, 201;
inspected, 202;
miscellaneous fermentations of, 98;
pasteurization of, 120;
pasteurization of in home, 131;
preservation of by antiseptics, 117;
preservation of by cold, 116;
relation to children's diseases, 80;
ropy fermentation, 94;
sediments, examination of, 214;
selected, 203;
slimy, 94;
spontaneous fermentation of, 91;
sterilization of, 134;
straining of, 153;
supply of small cities, 217;
sweet curdling fermentation of, 92;
tainted, 56, 58;
temperature standards for, 208;
tests for quality of, 209.
Milk pails, sanitary, 48;
small topped, 48.
Milker, factor in contamination of milk, 51.
Milking-machines, 36, 50.
Mold on butter, 177;
on cheese, 179.
O.
Odors, absorption of, 56, 58.
Oidium lactis, 186.
Oleomargarine, 152.
P.
Pasteurization, 120;
efficiency of, 133;
purpose of, 123;
methods of, 125.
Pasteurized milk, fermentations in, 124.
Pasteurizing machines, tests of, 130.
Process butter, 152.
Ptomaine poisoning, 81.
Pure cultures, 25.
R.
Rabies, 75.
Reduction test, 213.
Rennet, 170.
Ropy fermentation, 94.
Roquefort cheese, 184.
Rusty spot in cheese, 178.
S.
Salicylic acid, 117.
Scarlet fever, 79.
Score card for dairies, 198.
Sediment test, 210.
Skim milk, heating of, 38.
Slimy fermentation, 94.
Spores of bacteria, 10.
Stalls, 46.
Starters, 143;
for cheese, 167;
propagation of, 146.
Sterilization, 21, 134.
Stilton cheese, 185.
Storch test, 39.
Straining of milk, 33.
Sulphur, 111.
Sweet curdling of milk, 92.
Swiss cheese, 180.
T.
Taints, determination of cause of, 58, 103.
Temperature effect on growth, 12.
Tubercle bacilli, destruction of, 71;
in butter, 70;
in cheese, 70;
in milk, 67.
Tuberculin test, 73.
Tuberculosis, 64;
closed, 70;
distribution of disease in animal, 66;
economic aspects of, 72;
open, 70.
Typhoid fever, 76.
U.
Udder, inflammation, 75;
invasion of by bacteria, 30;
number and kind of bacteria from, 32;
structure of, 30;
washing of, 47;
cleaning of, 39.
Utensils, contamination from, 34.
W.
Water, effect on butter, 153;
supply, 59;
testing of, 60.
Whey, heating of, 38.
Wisconsin curd test, 104.
Y.
Yeast fermentation, 96.
Yoghurt, 101.
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