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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 deg. F. the
maximum from 105 to 110 deg. 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 deg. 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 deg. 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 deg. 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 deg. 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 deg. 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 deg. to 140 deg. F. for ten minutes. When spores are
present, resistance is greatly increased, some forms being able to
withstand steam at 212 deg. 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 deg. F. An
exposure of twenty minutes, at a temperature of 230 deg. to 240 deg. F.
will kill all spores. Where heat is used in a dry state, it is much less
effective, a baking temperature of 260 deg. to 300 deg. 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.