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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
EIGHTH EDITION
THOROUGHLY REVISED
MADISON, WISCONSIN
H. L. RUSSELL
1907
Copyrighted 1905
BY
H. L. RUSSELL
STATE JOURNAL PRINTING COMPANY,
Printers And Stereotypers,
Madison, Wis.
Transcriber's note:
For Text: A word surrounded by a cedilla such as ~this~ signifies that
the word is bolded in the text. A word surrounded by underscores like
_this_ signifies the word is italics in the text. For numbers and
equations, underscores before bracketed numbers in equations denote a
subscript.
Minor typos have been corrected.
PREFACE.
Knowledge in dairying, like all other technical industries, has grown
mainly out of experience. Many facts have been learned by observation,
but the _why_ of each is frequently shrouded in mystery.
Modern dairying is attempting to build its more accurate knowledge upon
a broader and surer foundation, and in doing this is seeking to
ascertain the cause of well-established processes. In this, bacteriology
is playing an important role. Indeed, it may be safely predicted that
future progress in dairying will, to a large extent, depend upon
bacteriological research. As Fleischmann, the eminent German dairy
scientist, says: "The gradual abolition of uncertainty surrounding dairy
manufacture is the present important duty which lies before us, and its
solution can only be effected by bacteriology."
It is therefore natural that the subject of Dairy Bacteriology has come
to occupy an important place in the curriculum of almost every Dairy
School. An exposition of its principles is now recognized as an integral
part of dairy science, for modern dairy practice is rapidly adopting the
methods that have been developed as the result of bacteriological study.
The rapid development of the subject has necessitated a frequent
revision of this work, and it is gratifying to the writer that the
attempt which has been made to keep these Outlines abreast of
bacteriological advance has been appreciated by students of dairying.
While the text is prepared more especially for the practical dairy
operator who wishes to understand the principles and reasons underlying
his art, numerous references to original investigations have been added
to aid the dairy investigator who wishes to work up the subject more
thoroughly.
My acknowledgments are due to the following for the loan of
illustrations: Wisconsin Agricultural Experiment Station; Creamery
Package Mfg. Co., Chicago, Ill.; and A. H. Reid, Philadelphia, Pa.
H. L. Russell.
University of Wisconsin.
CONTENTS.
CHAPTER I. Structure of the bacteria and conditions governing
their development and distribution 1
CHAPTER II. Methods of studying bacteria 13
CHAPTER III. Contamination of milk 19
CHAPTER IV. Fermentations in milk and their treatment 62
CHAPTER V. Relation of disease-bacteria to milk 82
Diseases transmissible from animal to man
through diseased milk 84
Diseases transmissible to man through infection
of milk after withdrawal 94
CHAPTER VI. Preservation of milk for commercial purposes 102
CHAPTER VII. Bacteria and butter making 134
Bacterial defects in butter 156
CHAPTER VIII. Bacteria in cheese 160
Influence Of bacteria in normal cheese processes 160
Influence of bacteria in abnormal cheese processes 182
CHAPTER I.
STRUCTURE OF THE BACTERIA AND CONDITIONS GOVERNING THEIR DEVELOPMENT AND
DISTRIBUTION.
Before one can gain any intelligent conception of the manner in which
bacteria affect dairying, it is first necessary to know something of the
life history of these organisms in general, how they live, move and
react toward their environment.
~Nature of Bacteria.~ Toadstools, smuts, rusts and mildews are known to
even the casual observer, because they are of evident size. Their
plant-like nature can be more readily understood from their general
structure and habits of life. The bacteria, however, are so small, that
under ordinary conditions, they only become evident to our unaided
senses by the by-products of their activity.
When Leeuwenhoek (pronounced Lave-en-hake) in 1675 first discovered
these tiny, rapidly-moving organisms he thought they were animals.
Indeed, under a microscope, many of them bear a close resemblance to
those minute worms found in vinegar that are known as "vinegar-eels."
The idea that they belonged to the animal kingdom continued to hold
ground until after the middle of the nineteenth century; but with the
improvement in microscopes, a more thorough study of these tiny
structures was made possible, and their vegetable nature demonstrated.
The bacteria as a class are separated from the fungi mainly by their
method of growth; from the lower algae by the absence of chlorophyll,
the green coloring matter of vegetable organisms.
~Structure of bacteria.~ So far as structure is concerned the bacteria
stand on the lowest plane of vegetable life. The single individual is
composed of but a single cell, the structure of which does not differ
essentially from that of many of the higher types of plant life. It is
composed of a protoplasmic body which is surrounded by a thin membrane
that separates it from neighboring cells that are alike in form and
size.
~Form and size.~ When a plant is composed of a single cell but little
difference in form is to be expected. While there are intermediate
stages that grade insensibly into each other, the bacteria may be
grouped into three main types, so far as form is concerned. These are
spherical, elongated, and spiral, and to these different types are given
the names, respectively, _coccus_, _bacillus_ and _spirillum_ (plural,
_cocci_, _bacilli_, _spirilla_) (fig. 1). A ball, a short rod, and a
corkscrew serve as convenient models to illustrate these different
forms.
[Illustration: FIG. 1. Different forms of bacteria. _a_, _b_, _c_,
represent different types as to form: _a_, coccus, _b_, bacillus, _c_,
spirillum; _d_, diplococcus or twin coccus; _e_, staphylococcus or
cluster coccus; _f_ and _g_, different forms of bacilli, _g_ shows
internal endospores within cell; _h_ and _i_, bacilli with motile organs
(cilia).]
In size, the bacteria are the smallest organisms that are known to
exist. Relatively there is considerable difference in size between the
different species, yet in absolute amount this is so slight as to
require the highest powers of the microscope to detect it. As an average
diameter, one thirty-thousandth of an inch may be taken. It is difficult
to comprehend such minute measurements, but if a hundred individual
germs could be placed side by side, their total thickness would not
equal that of a single sheet of paper upon which this page is printed.
~Manner of Growth.~ As the cell increases in size as a result of growth,
it elongates in one direction, and finally a new cell wall is formed,
dividing the so-called mother-cell into two, equal-sized daughter-cells.
This process of cell division, known as _fission_, is continued until
growth ceases and is especially characteristic of bacteria.
~Cell Arrangement.~ If fission goes on in the same plane continually, it
results in the formation of a cell-row. A coccus forming such a chain of
cells is called _strepto-coccus_ (chain-coccus). If only two cells
cohere, it is called a _diplo-coccus_ (twin-coccus). If the second cell
division plane is formed at right angles to the first, a _cell surface_
or _tetrad_ is formed. If growth takes place in three dimensions of
space, a _cell mass_ or _sarcina_ is produced. Frequently, these cell
aggregates cohere so tenaciously that this arrangement is of value in
distinguishing different species.
~Spores.~ Some bacteria possess the property of forming _spores_ within
the mother cell (called _endospores_, fig. 1g) that are analogous in
function to the seeds of higher plants and spores of fungi. By means of
these structures which are endowed with greater powers of resistance
than the vegetating cell, the organism is able to protect itself from
the effect of an unfavorable environment. Many of the bacilli form
endospores but the cocci do not. It is these spore forms that make it
so difficult to thoroughly sterilize milk.
~Movement.~ Many bacteria are unable to move from place to place. They
have, however, a vibrating movement known as the _Brownian_ motion that
is purely physical. Many other kinds are endowed with powers of
locomotion. Motion is produced by means of fine thread-like processes of
protoplasm known as _cilia_ (sing. _cilium_) that are developed on the
outer surface of the cell. By means of the rapid vibration of these
organs, the cell is propelled through the medium. Nearly all cocci are
immotile, while the bacilli may or may not be. These cilia are so
delicate that it requires special treatment to demonstrate their
presence.
~Classification.~ In classifying or arranging the different members of any
group of living objects, certain similarities and dissimilarities must
be considered. These are usually those that pertain to the structure and
form, as such are regarded as most constant. With the bacteria these
differences are so slight that they alone do not suffice to distinguish
distinctly one species from another. As far as these characters can be
used, they are taken, but in addition, many characteristics of a
physiological nature are added. The way that the organism grows in
different kinds of cultures, the by-products produced in different
media, and effect on the animal body when injected into the same are
also used as data in distinguishing one species from another.
~Conditions favoring bacterial growth.~ The bacteria, in common with all
other living organisms are affected by external conditions, either
favorably or unfavorably. Certain conditions must prevail before
development can occur. Thus, the organism must be supplied with an
adequate and suitable food supply and with moisture. The temperature
must also range between certain limits, and finally, the oxygen
requirements of the organism must be considered.
~Food supply.~ Most bacteria are capable of living on dead, inert, organic
matter, such as meats, milk and vegetable material, in which case, they
are known as _saprophytes_. In contradistinction to this class is a
smaller group known as _parasites_, which derive their nourishment from
the living tissues of animals or plants. The first group comprise by far
the larger number of known organisms which are concerned for the most
part in the decomposition of organic matter. The parasitic group
includes those which are the cause of various communicable diseases.
Between these two groups there is no sharp line of division, and in some
cases, certain species possess the faculty of growing either as
parasites or saprophytes, in which case they are known as _facultative_
parasites or saprophytes.
The great majority of bacteria of interest in dairying belong to the
saprophytic class; only those species capable of infecting milk through
the development of disease in the animal are parasites in the strict
sense of the term. Most disease-producing species, as diphtheria or
typhoid fever, while parasitic in man lead a saprophytic method of life
so far as their relation to milk is concerned.
Bacteria require for their growth, nitrogen, hydrogen, carbon, oxygen,
together with a limited amount of mineral matter. The nitrogen and
carbon are most available in the form of organic compounds, such as
albuminous material. Carbon in the form of carbohydrates, as sugar or
starch, is most readily attacked by bacteria.
Inasmuch as the bacteria are plant-cells, they must imbibe their food
from material in solution. They are capable of living on solid
substances, but in such cases, the food elements must be rendered
soluble, before they can be appropriated. If nutritive liquids are too
highly concentrated, as in the case of syrups and condensed milk,
bacteria cannot grow therein, although all the necessary ingredients may
be present. Generally, bacteria prefer a neutral or slightly alkaline
medium, rather than one of acid reaction; but there are numerous
exceptions to this general rule, especially among the bacteria found in
milk.
~Temperature.~ Growth of bacteria can only occur within certain
temperature limits, the extremes of which are designated as the
_minimum_ and _maximum_. Below and above these respective limits, life
may be retained in the cell for a time, but actual cell-multiplication
is stopped. Somewhere between these two cardinal temperature points, and
generally nearer the maximum limit is the most favorable temperature for
growth, known as the _optimum_. The temperature zone of most dairy
bacteria in which growth occurs ranges from 40 deg.-45 deg. F. to somewhat
above blood-heat, 105 deg.-110 deg. F., the optimum being from 80 deg.-95
deg. F. Many parasitic species, because of their adaptation to the bodies
of warm-blooded animals, generally have a narrower range, and a higher
optimum, usually approximating the blood heat (98 deg.-99 deg. F). The
broader growth limits of bacteria in comparison with other kinds of life
explain why these organisms are so widely distributed in nature.
~Air supply.~ Most bacteria require as do the green plants and animal
life, the free oxygen of the air for their respiration. These are called
_aerobic_. Some species, however, and some yeasts as well possess the
peculiar property of taking the oxygen which they need from organic
compounds such as sugar, etc., and are therefore able to live and grow
under conditions where the atmospheric air is excluded. These are known
as _anaerobic_. While some species grow strictly under one condition or
the other, and hence are _obligate_ aerobes or anaerobes, others possess
the ability of growing under either condition and are known as
_facultative_ or optional forms. The great majority of milk bacteria are
either obligate or facultative aerobes.
~Rate of growth.~ The rate of bacterial development is naturally very much
affected by external conditions, food supply and temperature exerting
the most influence. In the neighborhood of the freezing point but little
growth occurs. The rate increases with a rise in temperature until at
the _optimum_ point, which is generally near the blood heat or slightly
below (90 deg.-98 deg. F.), a single cell will form two cells in 20 to 30
minutes. If temperature rises much above blood heat rate of growth is
lessened and finally ceases. Under ideal conditions, rapidity of growth
is astounding, but this initially rapid rate of development cannot be
maintained indefinitely, for growth is soon limited by the accumulation
of by-products of cell activity. Thus, milk sours rapidly at ordinary
temperatures until the accumulation of acid checks its development.
~Detrimental effect of external conditions.~ Environmental influences of a
detrimental character are constantly at work on bacteria, tending to
repress their development or destroy them. These act much more readily
on the vegetating cell than on the more resistant spore. A thorough
knowledge of the effect of these antagonistic forces is essential, for
it is often by their means that undesirable bacteria may be killed out.
~Effect of cold.~ While it is true that chilling largely prevents
fermentative action, and actual freezing stops all growth processes,
still it does not follow that exposure to low temperatures will
effectually destroy the vitality of bacteria, even in the vegetative
condition. Numerous non-spore-bearing species remain alive in ice for a
prolonged period, and recent experiments with liquid air show that even
a temperature of -310 deg. F. for hours does not effectually kill all
exposed cells.
~Effect of heat.~ High temperatures, on the other hand, will destroy any
form of life, whether in the vegetative or latent stage. The temperature
at which the vitality of the cell is lost is known as the _thermal death
point_. This limit is not only dependent upon the nature of the
organism, but varies with the time of exposure and the condition in
which the heat is applied. In a moist atmosphere the penetrating power
of heat is great; consequently cell-death occurs at a lower temperature
than in a dry atmosphere. An increase in time of exposure lowers the
temperature point at which death occurs.
For vegetating forms the thermal death point of most bacteria ranges
from 130 deg.-140 deg. F. where the exposure is made for ten minutes which
is the standard arbitrarily selected. In the spore stage resistance is
greatly increased, some forms being able to withstand steam at 210
deg.-212 deg. F. from one to three hours. If dry heat is employed, 260
deg.-300 deg. F. for an hour is necessary to kill spores. Where steam is
confined under pressure, a temperature of 230 deg.-240 deg. F. for 15-20
minutes suffices to kill all spores.
~Drying.~ Spore-bearing bacteria like anthrax withstand drying with
impunity; even tuberculous material, although not possessing spores
retains its infectious properties for many months. Most of the dairy
bacteria do not produce spores, and yet in a dry condition, they retain
their vitality unimpaired for considerable periods, if they are not
subjected to other detrimental influences.
~Light.~ Bright sunlight exerts on many species a powerful disinfecting
action, a few hours being sufficient to destroy all cells that are
reached by the sun's rays. Even diffused light has a similar effect,
although naturally less marked. The active rays in this disinfecting
action are those of the chemical or violet end of the spectrum, and not
the heat or red rays.
~Influence of chemical substances.~ A great many chemical substances exert
a more or less powerful toxic action of various kinds of life. Many of
these are of great service in destroying or holding bacterial growth in
check. Those that are toxic and result in the death of the cell are
known as _disinfectants_; those that merely inhibit, or retard growth
are known as _antiseptics_. All disinfectants must of necessity be
antiseptic in their action, but not all antiseptics are disinfectants
even when used in strong doses. Disinfectants have no place in dairy
work, except to destroy disease bacteria, or preserve milk for
analytical purposes. Corrosive sublimate or potassium bichromate are
most frequently used for these purposes. The so-called chemical
preservatives used to "keep" milk depend for their effect on the
inhibition of bacterial growth. With a substance so violently toxic as
formaldehyde (known as formalin, freezene) antiseptic doses are likely
to be exceeded. In this country most states prohibit the use of these
substances in milk. Their only function in the dairy should be to check
fermentative or putrefactive processes outside of milk and so keep the
air free from taints.
~Products of growth.~ All bacteria in their development form certain more
or less characteristic by-products. With most dairy bacteria, these
products are formed from the decomposition of the medium in which the
bacteria may happen to live. Such changes are known, collectively, as
fermentations, and are characterised by the production of a large amount
of by-products, as a result of the development of a relatively small
amount of cell-life. The souring of milk, the formation of butyric acid,
the making of vinegar from cider, are all examples of fermentative
changes.
With many bacteria, especially those that affect proteid matter,
foul-smelling gases are formed. These are known as putrefactive changes.
All organic matter, under the action of various organisms, sooner or
later undergoes decay, and in different stages of these processes,
acids, alkalies, gases and numerous other products are formed. Many of
these changes in organic matter occur only when such material is brought
in direct contact with the living bacterial cell.
In other instances, soluble, non-vital ferments known as _enzyms_ are
produced by the living cell, which are able to act on organic matter, in
a medium free from live cells, or under conditions where the activity of
the cell is wholly suspended. These enzyms are not confined to bacteria
but are found throughout the animal and plant world, especially in those
processes that are concerned in digestion. Among the better known of
these non-vital ferments are rennet, the milk-curdling enzym; diastase
or ptyalin of the saliva, the starch-converting enzym; pepsin and
trypsin, the digestive ferments of the animal body.
Enzyms of these types are frequently found among the bacteria and yeasts
and it is by virtue of this characteristic that these organisms are
able to break down such enormous quantities of organic matter. Most of
these enzyms react toward heat, cold and chemical poisons in a manner
quite similar to the living cells. In one respect they are readily
differentiated, and that is, that practically all of them are capable of
producing their characteristic chemical transformations under
anaesthetic conditions, as in a saturated ether or chloroform
atmosphere.
~Distribution of bacteria.~ As bacteria possess greater powers of
resistance than most other forms of life, they are to be found more
widely distributed than any other type. At the surface of the earth,
where conditions permit of their growth, they are found everywhere,
except in the healthy tissues of animals and plants. In the superficial
soil layers, they exist in myriads, as here they have abundance of
nourishment. At the depth of several feet however, they diminish rapidly
in numbers, and in the deeper soil layers, from six to ten feet or more,
they are not present, because of the unsuitable growth conditions.
The bacteria are found in the air because of their development in the
soil below. They are unable to grow even in a moist atmosphere, but are
so readily dislodged by wind currents that over land areas the lower
strata of the air always contain them. They are more numerous in summer
than in winter; city air contains larger numbers than country air.
Wherever dried fecal matter is present, as in barns, the air contains
many forms.
Water contains generally enough organic matter in solution, so that
certain types of bacterial life find favorable growth conditions. Water
in contact with the soil surface takes up many impurities, and is of
necessity rich in microbes. As the rain water percolates into the soil,
it loses its germ content, so that the normal ground water, like the
deeper soil layers, contains practically no bacterial life. Springs
therefore are relatively deficient in germ life, except as they become
infected with soil organisms, as the water issues from the soil. Water
may serve to disseminate certain infectious diseases as typhoid fever
and cholera among human beings, and a number of animal maladies.
While the inner tissues of healthy animals are free from bacteria, the
natural passages as the respiratory and digestive tracts, being in more
direct contact with the exterior, become more readily infected. This is
particularly true with reference to the intestinal tract, for in the
undigested residue, bacterial activity is at a maximum. The result is
that fecal matter contains enormous numbers of organisms so that the
possibility of pollution of any food medium such as milk with such
material is sure to introduce elements that seriously affect the quality
of the product.