UC-NRLF

DAIRY BAC

RUSSELL

REESE LIBRARY

OF THK

UNIVERSITY OF CALIFORNIA

, 190 .
Accession No. 92011 . Class No.

\

OUTLINES

OF

DAIRY BACTERIOLOGY

A CONCISE MANUAL FOR THE USE OF
STUDENTS IN DAIRYING

BY

H. L, RUSSELL
* /

PROFESSOR OF BACTERIOLOGY, UNIVERSITY OF WISCONSIN

FIFTH EDITION

THOROUGHLY REVISED

UNIVERSITY

MADISON, WISCONSIN

PUBLISHED BY THE AUTHOR

1902

COPYRIGHTED 1903

BY
EL L. RUSSELL.

STATE JOURNAL PRINTING COMPANY,

PRINTERS AND STEREOTYPERS,

MADISON, wis.

PREFACE TO FIFTH EDITION.

Knowledge in claiming, like all other technical indus-
tries, 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 accu-
rate 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 r61e. Indeed, it may be safely predicted that
future progress in dairying will, to a large extent, depend
upon bacteriological research. As Fleischmanu, the emi-
nent German dairy scientist, says: "The gradual abolition
of uncertainty surrounding dairy manufacture is the pres-
ent important duty which lies before us, and its solution
can only be effected by bacteriology."

It is therefore natural that the subject of Dairy Bacteri-
ology has come to occupy an important place in the cur-
riculum 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 adopt-
ing the methods that have been developed as the result of
bacteriological study. The rapid development of the sub-
ject 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 prac-

92011

iv Preface to Fifth Edition.

tical dairy operator who wishes to understand the principles
and reasons underlying his art, numerous references to orig-
inal investigations have been added to aid the dairy inves-
tigator who wishes to work up the subject more thor-
oughly.

My acknowledgments are due to the following for the
loan of illustrations: Wisconsin Agricultural Experiment
Station; Creamery Package Mfg. Co., Chicago, 111.; and A. H.
Reid, Philadelphia, Pa,

H. L. RUSSELL.
UNrvEitsiTY OP WISCONSIN,
Madison, January, 1902.

CONTENTS.

CHAPTER 1 Structure of the bacteria and conditions gov-
erning 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 in-
fection of milk after withdrawal 94

CHAPTER VI. Preservation of milk for commercial purposes 102

CHAPTER VIL Bacteria and butter making 134

Bacterial defects in butter 156

CHAPTER VI1L 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 CON-
DITIONS GOVERNING THEIR DEVELOP-
MENT AND DISTRIBUTION*

BEFORE one can gain any intelligent conception of the
manner in which bacteria affect dairying, it is first neces-
sary to know something of the life history of these organ-
isms in general, how they live, move and react toward their
environment.

Nature of Bacteria. Toadstools, smuts, rusts and mil-
dews 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 ordi-
nary 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 con-
tinued to hold ground until after the middle of the present
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 chloro-
phyll, the green coloring matter of vegetable organisms.

2 Dairy Bacteriology.

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 mem-
brane 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

r * i »

Fio. 1. Different forms of bacteria, a, b, c, represent different types as to
form: a, coccus, *, bacillus, c, spirillum; </, diplococcus or twin coccus ;<?, staphy-
lococcus or cluster coccus; / anl #, different forms of bacilli, g shows internal
endospores within cell; h and /, bacilli with motile organs (cilia).

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 illus-
trate these different forms.

In size, the bacteria are the smallest organisms that are
known to exist. Relatively there is considerable difference in

Structure of the Bacteria. 3

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. Ig)
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

4: . Dairy Bacteriology.

spore forms that resist the action of heat in pasteurizing
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 tine thread-like processes
of protoplasm known as cilia (sing, cilium) that are de-
veloped 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 im motile, 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 wa}T 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. Cer-
tain conditions must prevail before development can occur.
Thus, the organism must be supplied with an adequate

Structure of the Bacteria. 5

and suitable food supply and with moisture. The tem-
perature must also range between certain limits, and finally,
the oxygen requirements of the. organism must be con-
sidered.

Food supply. Most bacteria are capable of living on
dead, inert, organic. matter, such as meats, milk and vege-
table material, in which case, they are known as sap-
rophytes. 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 organ-
isms which are concerned for the most part in the decom-
position 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 dis-
ease 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 avail-
able 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 im-

6 Dairy Bacteriology.

bibe 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 ap-
propriated. If nutritive liquids are too highly concen-
trated, 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 with-
in certain temperature limits, the extremes of which are
designated as the minimum and maximum. Below and
above these respective b'mits, 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 favor-
able temperature for growth, known as the optimum. The
temperature zone of most dairy bacteria in which growth
occurs ranges from 40°-45° F. to somewhat above blood-
heat, 105°-110° F., the optimum being from 80°-95° F.
Many parasitic species, because of their adaptation to the
bodies of warm-blooded animals, generally have a narrower
range, and a higher optimum, usually approxima'ing the
blood heat (9S°-99° F). The broader growth limits of bac-
teria in comparison with other kinds of life explain why
these organisms are so widely distributed in nature.

Air supply. No living organism, can thrive without
oxygen and most species require atmospheric oxygen; but
there exist certain bacteria (and yeasts as well), which
possess the peculiar property of not being able to utilize
elemental oxygen. These secure the necessary element for

Structure of the Bacteria. 7

respiratory purposes from oxygen in combination, and on
account of their ability to thrive in an atmosphere devoid
of free oxygen are called anaerobic, while those requiring
air are called aerobic. Some species grow strictly under
one condition or the other, and hence are obligate aerobes
or anaerobes; others known as facultative or optional forms
possess the ability of growing under either condition. The
majority of milk bacteria are either obligate or facultative
aerobes.

Rate Of Growth. Growth takes place very slowly at or
near the minimum temperatures, but as the temperature
is increased the rate of growth becomes more rapid, until
at the optimum point, a single bacterial cell, in an active
vegetative condition may divide into two cells in twenty
minutes. If the temperature is raised to the maximum the
rate of development is very rapidly lessened, until finally
cell-multiplication ceases altogether. Even under opti-
mum conditions 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, the sour milk germ grows rapidly at ordinary tem-
peratures until the accumulation of lactic acid checks its
own growth. If this is removed by neutralizing it with
chalk, the lactic bacteria renew their growth.

Detrimental effect of external conditions. Enviromental
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 knowl-
edge of the effect of these antagonistic forces is essential,
for it is often by their means that undesirable bacteria may
be killed out.

8 Dairy Bacteriology.

Effect Of cold. While it is true that chilling largely pre-
vents fermentative action, and actual freezing stops all
growth processes, still it does not follow that exposure to
low temperatures will effectually destroy the vitality of
bacteria, even in the vegetative condition. Numerous non-
spore-bearing species remain alive in ice for a prolonged
period, and recent experiments with liquid air show that
even a temperature of -310° F. for hours does not effect-
ually kill all exposed cells.

Effect of heat. High temperatures, on the other hand,
will destroy any form of life, whether in the vegetative or
latent stage. The temperature at which the vitality of the
cell is lost is known as the thermal death point. This limit
is not only dependent upon the nature of the organism, but
varies with the time of exposure and the condition in
which the heat is applied. In a moist atmosphere the
penetrating power of heat is great; consequently cell-death
occurs at a lower temperature than in a dry atmosphere.
An increase in time of exposure lowers the temperature
point at which death occurs.

For vegetating forms the thermal death point of most
bacteria ranges from 130°-140° F. where the exposure is
made for ten minutes which is the standard arbitrarily
selected. In the spore stage resistance is greatly increased,
some forms being able to withstand steam at 210°-212° F.
from one to three hours. If dry heat is employed, 260°-
300° F. for an hour is necessary to kill spores. Where steam
is confined under pressure, a temperature of 230°-240° F.
for 15-20 minutes suffices to kill all spores.

Drying. Spore-bearing bacteria like anthrax withstand
drying with impunity; even tuberculous material, although
not possessing spores retains its infectious properties for

Structure of the Bacteria. 9

many months. Most of the dairy bacteria do not produce
spores, and yet in a dry condition, they retain their vitalit}?-
unimpaired for considerable periods, if they are not sub-
jected to other detrimental influences.

Light. Bright sunlight exerts on many species a power-
ful disinfecting action, a few hours being sufficient to des-
troy 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 chem-
ical substances exert a more or less powerful toxic action
of various kinds of life. Many of these are of great ser-
vice 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 re-
tard 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 ex-
ceeded. 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
and so keep the air free from taints.

10 Dairy Bacteriology.

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 develop-
ment of a relatively small amount of cell-life. The souring
of milk, the formation of butyric acid, the making of vin-
egar 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, al-
kalies, 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 diges-
tion. Among the better known of these non-vital fer-
ments are rennet, the milk-curdling enzym; diastase or
ptyalin of the saliva, the starch-converting enzym; pepsin
and trypsiu, the digestive ferments of the animal body.

Enzyms of these types ar3 frequently found among the
bacteria and yeasts and it is by virtue of this characteristic

Structure of the Bacteria. 11

that these organisms are able to break clown such enor-
mous quantities of organic matter. Most of these en-
zyms 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 abund-
ance of nourishment. At the depth of several feet how-
ever, they. diminish rapidly in numbers, and in the deeper
soil layers, from six to ten feet or moi\), they are not
present, because of the unsuitable growth conditions.

The bacteria are found in the the air because of their de-
velopment 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 num-
bers than country air. Wherever dried fecal matter is
present, as in barns, the air contains many forms.

Water contains generally enough organic matter in so-
lution, 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 mi-
crobes. As the rain water percolates into the soil, it loses

12 Dairy Bacteriology.

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, ex-
cept as they become infected with soil organisms, as the
water issues from the soil. Water may serve to dissem-
inate 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 digest-
ive tracts, baing in more direct contact with the exterior, be-
come 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 organ-
isms so that the possibility of pollution of any food medium
such as milk with such material is sure to introduce ele-
ments that seriously affect the quality of the product.

CHAPTER II.
METHODS OF STUDYING BACTERIA.

Necessity of bacterial masses for study. The bacteria
are so extremely small that it is impossible to study indi-
vidual germs separately without the aid of first-class micro-
scopes. For this reason, but little advance was made in
the knowledge of these lower forms of plant life, until the
introduction of culture methods, whereby a single organ-
ism could be cultivated and the progeny of this cell in-
creased to such an extent in a short course of time, that
they would be visible to the unaided eye.

This is done by growing the bacteria in masses on vari-
ous kinds of food media that are prepared for the purpose,
but inasmuch as bacteria are so universally distributed, it
becomes an impossibility to cultivate any special form,
unless the medium in which they are grown is first freed
from, all pre-existing forms of germ life. So accomplish
this, it is necessary to subject the nutrient medium to
some method of sterilization, such as heat or filtration,
whereby all life is completely destroyed or eliminated.
Such material after it has been rendered germ-free is kept
in sterilized glass tubes and flasks, and is protected from
infection by cotton stoppers.

Culture media. For culture media, many different
substances are employed. In fact, bacteria will grow on
almost any organic substance whether it is solid or fluid,
provided the other essential conditions of growth are fur-
nished. The food substances that are used for culture
purposes are divided into two classes; solids and liquids.

14: Dairy Bacteriology.

Solid media may be either permanently solid like pota-
toes, or they may retain their solid properties only at cer-
tain temperatures like gelatin or agar. The latter two are
of utmost importance in bacteriological research, for their
use, which was introduced by Koch, permits the separation
of the different forms that may happen to be in any mix-
ture. Gelatin is used advantageously because the majority
of bacteria present wider differences due to growth upon
this medium than upon any other. It remains solid at
ordinary temperatures, bacoming liquid at about 70° F.
Agar, a gelatinous product derived from a Japanese sea-
weed, has a much higher melting point, and can be suc-
cessfully used, especially with those organisms whose
optimum growth point is above the melting point of gel-
atin.

Besides these solid media, different liquid substances are
extensively used, such as beef broth, milk, and infusions of
various vegetable and animal tissues. Skim-milk is of
especial value in studying the milk bacteria and may be
used in its natural condition, or a few drops of litmus solu-
tion may be added in order to detect any change in its
chemical reaction due to the bacteria.

Methods of isolation. Suppose for instance one wishes
to isolate the different varieties of bacteria found in milk.
The method of procedure is as follows : Sterile gelatin in
glass tubes is melted and cooled down so as to be barely
warm. To this gelatin which is germ-free a drop of milk
is added. The gelatin is then gently shaken so as to thor-
oughly distribute the milk particles, and poured out into a
sterile flat glass dish and quickly covered. This is allowed
to stand on a cool surface until the gelatin hardens. After
the culture plate has been left for twenty-four to thirty-

Methods of Studying Bacteria.

15

six hours at the proper temperature, tiny spots will begin
to appear on the surface, or in the depth of the culture
medium. These patches are called colonies and are com-
posed of an almost infinite number of individual germs,
the result of the continued growth of a single organism

Fia. 2. A gelatin plate culture showing appearance of different organisms
in a sample of milk. Each mass represents a bacterial growth (colony) derived
from a single cell. Different forms react differently toward the gelatin, some
liquefying the same, others growing in a restricted mass, a, represents a colony
of the ordinary bread mold; b, a liquefying bacterium; c, and dt solid forms.

that was in the drop of milk which was firmly held in
place when the gelatin solidified. The number of these
colonies represents approximately the number of germs
that were present in the milk drop. If the plate is not
too thickly sown with these germs, the colonies will con-
tinue 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

16

Dairy Bacteriology.

the manner in which it acts toward gelatin. In order to
make sure that the seeding in not too copious so as to in-
terfere with continued study, an attenuation is usually
made. This consists in taking a drop of the infected gel-

Fio. 8. Profile view of gelatin plate culture ; £, a liquefying form that dissolves
the gelatin; c and </, surface colonies that do not liquefy the gelatin.

atin in the first tube, and transferring it to another tube
of sterile media. Usually this operation is repeated again
so that these culture plates are made with different
amounts of seed with the expectation that in at least one
plate the seeding will not be so thick as to prevent further
stady. For transferring the culture a loop made of plat-
inum wire is used. By passing this through a gas flame, it
can be sufficiently sterilized.

To further study the peculiarities of different germs, the
separate colonies are transferred to other sterile tubes of
culture material and thus pure cultures of the various
germs are secured. These cultures then serve as a basis for
continued study and must be planted and grown upon all
the different kinds of media that are obtainable. In this
way the slight variations in the growth of different forms
are detected and the peculiar characteristics are determined,
so that the student is able to recognize this form when he
meets it again.

These culture methods are of essential importance in
bacteriology, as it is the only way in which it is possible
to secure a quantity of germs of the same kind.

Methods of Studying Bacteria.

17

The microscope in bacterial investigation. In order to
verify the purity of the cultures, the microscope is in con-
stant demand throughout all the different stages of the

cl

Fra 4. Pure cultures of different kinds of bacteria in gelatin tubes, a, growth
slight in this medium; b, growth copious at and near surface. Fine parallel fil-
aments growing out into medium liquefying at surface; c, a rapid liquefying
form; d, a gas-producing form that grows equally well in lower part of tube as
at surface (facultative anaerobe) ; *, an obligate anaerobe, that develops only in
absence of air.

isolating process. For this purpose, it is essential that the
instrument used shall be one of strong magnifying powers
(600-800 diameters), combined with sharp definition.
The microscopical examination of any germ is quite as
2

18 Dairy Bacteriology.

essential as the determination of culture characteristics;
in fact, the two must go hand in hand. The examination
reveals not only the form and si::e of the individual germs,
but the manner in which they are unite 1 with each other,
as well as any peculiarities of movement that they may
possess.

In carrying out the microscopical part of the work, not
only is the organism examined in a living condition, but
preparations are made by using solutions of auilin 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 micro-
scopical characters should be closely observed, but special
experiments must ba carried out along different lines, in
order to determine any special properties that the germ
may possess. Thus, the ability of any form to act as a
fermentative organism can be tested by fermentation ex-
periments; the property of causing disease, studied by the
inoculation of pure cultures into animals. A great many
different methods have been devised for the purpose of
studying special characteristics of different bacteria, but a
full description of these would necessarily be so lengthy
that in a work of this character they must be omitted.
For details of this nature consult standard reference books
on bacteriological technique.

CHAPTER III.
CONTAMINATION OF MILK*

Milk as food for bacteria. The fact that milk undergoes
decomposition changes so readily shows that it is well
suited to nourish bacterial life. Not only does it contain
all the substances necessary for nutrition, but they are
diluted in such proportions as to render most of them avail-
able for bacterial as well as mammalian lite.

The albumen which is in solution is readily assimilable.
The casein can not be appropriated directly, until it is first
rendered soluble, a process which occurs with those bac-
teria that secrete proteid-dissolving enzyms like pepsin
and trypsin. Of the carbon-containing compounds, the
fat is of little value for food as normally bacteria can not
act on this. The milk-sugar, however, is an admirable
food and the decomposition of this results in the produc-
tion of acids and gases. The mineral requirements of the
bacterial cell are so limited as to demand but little atten-
tion, as any organic substance contains sufficient inorganic
matter to satisfy the needs of the cell in the formation of
new cell substance.

Milk, germ-free in Udder. Under ordinary conditions,
when examined in the proper manner, milk always reveals
bacterial life. This germ contend, however, is due to in-
fection from without, for in the udder of a healthy animal,
as secreted, the milk like the other secretions and tissues
of the body is normally sterile.

Contamination Of milk. In withdrawing the milk from

the udder, it invariably comes in contact with germ life.

20

Dairy Bacteriology.

The same is true after it is milked. From the time of milk-
ing, until it is consumed in one form or another, it is con-
tinually subject to contamination from external sources.
In the main, germ life gains access while the milk is on the
farm, but even in the factory, the opportunities for infec-
tion are present in
greater or lesser de-
gree. Those forms
that become estab-
lished early generally
predominate in the
milk, as their intro-
duction enables them
to develop for a longer ^

period of time. It f
must be remembered
that by far the greater Sb&
part of these organisms
are relatively harmless.
While they are not
concerned in the production of disease, the larger majority
of them do affect, more or less seriously, the quality of
the milk. They are, therefore, undesirable from the milk
vendor's point of view, although in the processes of butter
and theese-making, the presence of some kinds is very de-
sirable.

Under the varying conditions in which milk is handled
it must of necessity be infected with bacteria of different
sorts, yet normally the type of fermentation that occurs is
quite the same, showing that certain species find milk such
a favorable nutrient medium that they soon gain the ascend-
ency over other forms.

Fio. 5. Microscopic appearance of milk show-
ing relative size of fat globules and bacteria.

Contamination of Milk. 21

Sources Of infection. The bacterial life that finds its
way into milk while it is yet on the farm may be traced to
several sources, which may be grouped under the following
heads : unclean dairy utensils, fore milk, coat of animal,
and general atmospheric surroundings. The relative im-
portance of these various factors fluctuates in each individ-
ual instance.

Dairy utensils. Of first importance are the vessels that
are used during milking, and also all storage cans and
other dairy utensils that come in contact with the milk
after it is drawn. By unclean utensils, actually visible
dirt need not always be considered, although its presence
in cracks and corners of pails and cans is often the
case. Unless cleansed with special care, these cracks and
joints are apt to be filled with foul and decomposing ma-
terial that suffices to abundantly seed the milk. Soxhlet1
found that the addition of 0.1 per cent of sour milk to
fresh milk decreased the keeping quality of the latter from
15 to 30 per cent ; the addition of 1.5 per cent diminished it
80 per cent. Where cans are not well cleansed the above
amount could easily be added to the milk from the ma-
terial that adhered to the walls of the can.

Through negligence, vessels are often used that are unfit
for handling milk. A rusty milk-can often spoils more
. milk than sufficient to purchase a new ves-

\ / sel. Wooden pails are no longer tolerated in

\ J^ a well-regulated dairy. Where possible, ves-
sels should be made of pressed tin. If join's
— £»— are necessary, they should be well flushed with
FIG. 6. solder so that they may be easily and thor-
oughly cleaned as shown in Fig. 6. In much of the

1 Soxhlet, Ber. d. Wandervers. bayer. Landwirthe, Oct., 1894.

22 Dairy Bacteriology.

cheaper class of tinware now found on the market, the
soldering of joints and seams is very imperfect, anJ. this
affords a place of refuge for bacteria and dirt, as shown in
c, Fig. 6.

Use of milk-cans for transporting factory by-products.
The general custom of using the milk-cans to carry back
to the farm the factory by-products (skim-milk or whey)
has much in it that is to be deprecated. These by-products
are generally rich in bacterial life, more especially where
the closest scrutiny is not given to the daily cleaning of
the vats and tanks. Too frequently the cans are not cleaned
immediately upon arrival at the farm, so that the condi-
tions are favorable for rapid fermentation. Many of the
taints that bother factories are directly traceable to such a
cause. A few dirty patrons will thus seriously infect the
whole supply. The responsibility for this defect shouldt
however, not be laid entirely upon the shoulders of the
producer. The factory operator should see that the refuse
material does not accumulate in the waste vats from day to
day and is not transformed into a more or less putrid mass.
A dirty whey tank is not an especially good object lesson
to the patron to keep his cans clean.

It is possible that abnormal fermentations or even con-
tagious diseases may thus be disseminated.

Suppose there appears in a dairy an infectious milk
trouble, such as bitter milk. This milk is taken to the
factory and passes unnoticed into the general milk-supply.
The skim-milk from the separator is of course infected
with the germ, and if conditions favor its growth, the
whole lot soon becomes tainted. If this waste product is
returned to the different patrons in the same cans that are
used for the fresh milk, the probabilities are strongly in

Contamination of Mi

favor of some of the cans being contaminated and thus in-
fecting the milk supply of the patrons. If the organism
is endowed with spores so that it can withstand unfavor-
able conditions, this taint may be spread from patron to
patron simply through the infection of the vessels that are
used in the transportation of the by-products. Connellhas
reported just such a case in a Canadian cheese factory where
an outbreak of slimy milk was traced to infected whey vats.
Quite a number of epidemics of typhoid fever have been
shown to have been disseminated in this way, and in Den-
mark and Germany with foot and mouth disease and tuber-
culosis, the danger is so great as to make it necessary to
heat all by-products taken back to the farm.1

Pollution of cans from whey vats. The danger is greater
in cheese factories than in creameries, for whey usually
represents a more advanced stage of fermentation than
skim-milk. The higher temperature at which it is drawn
facilitates more rapid bacterial growth, and the conditions
under which it is stored in many factories contribute to the
ease with which fermentative changes can go on in it.
Often this by-product is stored in wooden cisterns or tanks,
situated below ground, where it becomes impossible to
clean them out thoroughly. A custom that is almost uni-
versally followed in the Swiss cheese factories, here in this
country, as in Switzerland, is fully as reprehensible as any

1 Storch (40 Kept. Danish Expt. Stat., Copenhagen, 1898) has devised a test
whereby it can be determined whether this treatment has been carried out or
not: Milk contains a soluble enzym known as galactase which has the property
of decomposing hydrogen peroxid. If milk is heated to 176° F. (80° C.) or above,
this enzym is destroyed so that the above reaction w> longer takes place. If
potassium iodid and starch are added to unheated milk and the same treated
with hydrogen peroxid, the decomposition of the latter agent releases oxygen
which acts on the potassium salt, which in turn gives off free iodine that turns
the starch blue.

24 Dairy Bacteriology.

dairy custom could well be. In Fig. 7 the arrangement in
vogue for the disposal of the whey is shown. The hot
whey is run out through the trough from the factory into

FIG. 7. Swiss cheese factory (Wisconsin), showing careless way in which whey
is handled. Each patron's share is placed in a barrel, from which it is removed
by him. No attempt is made to cleanse these receptacles.

the large trough that is placed over the row of barrels, as
seen in the foreground. Each patron thus has allotted to
him in his individual barrel his portion of the whey, which
he is supposed to remove day by day. No attempt is made
to clean out these receptacles, and the inevitable result is
that they become filled with a foul, malodorous liquid, es-
pecially in summer. When such material is taken home
in the same set of cans that is used to bring the fresh
milk (twice a day as is the usual custom in Swiss factories),
it is no wonder that this industry is seriously handicapped
by " gassy " fermentations that injure materially the qual-
ity of the product. Not only is the above danger a very

Contamination of Milk. t 25

considerable one, but the quality of the factory by-product
for feeding purposes, whether it is skim-milk or whey, is
impaired through the development of fermentative changes.

Improved methods of disposal of by-products. The diffi-
culties which attend the distribution of these factory by-
products have led to different methods of solution. One is
to use another separate set of receptacles to carry back
these products to the farm. This method has been tried,
and while it is deemed impracticable by many to handle two
sets of vessels, yet some of the most progressive factories
report excellent results where this method is in use.

Large barrels could be used for this purpose to econo-
mize in wagon space.

Another method that has met with wider acceptance,
especially in creameries, is the custom of pasteurizing or
scalding the skim-milk immediately after it is separated,
so that it is returned to the farmer in a hot condition. In
factories where the whole milk is pasteurized, further
treatment of the by-product is not necessary. In most
factories steam, generally exhaust, is used directly in the
milk, and experience has shown that such milk, without
any cooling, will keep sweet for a considerable number of
hours longer than the untreated product. It is noteworthy
that the most advanced and progressive factories are the
ones that appreciate the value of this work, and although
it involves some time and expense, experience has shown
the utility of the process in that a better grade of milk is
furnished by the patrons of factories which follow this
practice.1 The exclusion of all danger of animal or human
disease is also possible in this way.

i McKay, N. Y. Prod. Rev.. Mch. 22, 1899.

2G Dairy Bacteriology.

Cleaning dairy utensils. The thorough cleaning of all
dairy apparatus that in any way conies in contact with the
milk is one of the most fundamental and important problems
in dairying. All such apparatus should be so constructed
as to permit of easy cleaning. Tinware, preferably of the
pressed variety, gives the best surface for this purpose and
is best suited for the handling of milk.

Milk vessels should never be allowed to become dry when
dirty, for dried particles of milk residue are extremely diffi-
cult to remove. In cleaning dairy utensils they should
first be rinsed in lukewarm instead of hot water, so as to
remove organic matter without coagulating the milk.
Then wash thoroughly in hot water, using soap or weak
alkali. A borax solution is sometimes recommended for
cleaning bottles. Strong alkalies should not be used.
After washing rinse thorough!}' in clean hot water. If
steam is available, as it always is in creameries, cans and
pails should be turned over jet for a few moments. While
a momentary exposure will not suffice to completely ster-
ilize such a vessel, yet many bacteria are killed in even a
short exposure, and the cans dry more thoroughly and
quickly when heated by steam.

Not only should the greatest care be paid to the condi-
tion of the cans and milk-pails, but all dippers, strainers,
and other utensils that come in contact with the milk
must be kept equally clean. Cloth strainers, unless attended
to, are objectionable, for the fine mesh of the cloth retains
so much moisture that they become a veritable hot-bed of
bacterial life, unless they are daily boiled or steamed.

Germ content of milk utensils. Naturally the number
of bacteria found in different milk utensils after they have
received their regular cleaning will be subject to great

Contamination of Milk. 27

fluctuations; but, nevertheless, such determinations are of
value as giving a scientific foundation for practical meth-
ods of improvement. The following studies may serve to
indicate the relative importance of the utensils as a factor
in milk contamination.

Two cans were taken, one of which had been cleaned in
the ordinary way, while the other was sterilized by steam-
ing. Before, milking, the udder was thoroughly cleaned
and special precautions taken to avoid raising of dust; the
fore milk was rejected. Milk drawn into these two cans
showed the following germ content:

No. bacteria Hours before
per cc. souring.

Steamed pail 165 28£

Ordinary pail 4265 23

Harrison1 showed the great variation in the bacterial
content in milk cans in the following way: Cans were
rinsed with 100 cc. of sterile water and numerical deter-
minations of this rinsing water made. The following data
are from cans poorly cleaned (Series A), cans washed in
tepid water and then scalded — the usual factory method —
(•Series B), and cans washed in tepid water and steamed for
five minutes (Series C).

Bacterial contents of cans cleaned in various ways.

Series A, dirty cans 238,525 342,875 215,400 618,200 806,320

510,270 230,100 610,510 418,810 317,250

Series B, ordinary method 89,320 84,750 26,800 24,000 38,400

76,800 15,200 13,080 44,160 93,400

Series C, approved method 1,170 1,792 890 355 416

A variation of this method was made by the writer as
follows: Three pails were thoroughly washed with 100 cc.

» Harrison, 22 Kept. Out. Agr'l Coll., 1896, p. 113.

I.

II.

III.

7,°00,000

1,450,000

49,000

283,000

43,400

35,000

1,685,000

105,000

61,400

28 Dairy Bacteriology.

of sterile water, using a sterile swab to remove all dirt.
This process was repeated with two otlrr rinse waters of
the same volume, and from each of these plates were made
with following results:

No. bacteria in different washings. Total No.

bacteria.
9,299,000
361,400
1,851,400

Infection of udder cavity. While it may be true that
milk as secreted in the glandular tissue of a healthy ani-
mal may be practically iree from bacteria, yet it does not
follow that it contains no microbes when first drawn. In-
deed, a bacterial examination of the first few streams taken
from each teat will invariably show a relatively high germ
content; much higher, in fact, than that which is subse-
quently withdrawn. The reason for this is evident when
the structure of the udder and its relation to the exte-
rior is noted (Fig. 8). The udder is composed of secreting
tissue (gland cells), held in place by fibrous connective tis-
sue. Ramifying throughout this glandular structure are
numerous channels (milk sinuses) that serve to convey the
milk from the cells where it is produced into the milk cis-
tern, a common receptacle just above the teat. This cavity
is connected with the exterior through the milk duct in the
teat, which is closed more or less tightly by the circular
sphincter muscles, thus preventing the milk from flowing
out.

According to the best authorities, the fat globules are
elaborated very largely during the actual manipulation of
the udder in milking, yet there is always a small residual
amount of milk that is not removed even by " clean" milk-
ers.

Contamination of Milk.

29

The ready infection1 of the external opening of the
milk duct gives an opportunity for bacteria to plant them-
selves on a moist mucous surface, and the result is, that

some organisms, at
least, find it possi-
ble to penetrate the
milk duct and so
enter the milk cis-
tern. When once
this reservoir is in-
fected, further
spread can be easily
made, even into the
glandular portion
of the udder.2 In
such a habitat, ideal
conditions would
seem to exist, at
least, for the faculta-
tive anaerobic type
of bacteria. Moist-
ture, warmth, suffi-
cient and nutritious
food, give optimum
conditions for de-
velopment, but
these may, in part

FIG. 8. Section of udder, showing teat, milk cis- , , , ,
tern and secreting tissue (Moore and Ward). at least> be

by the fact

that

1 According to Jos. Simon (Diss. Hyg. Inst. Erlangen, 1898) the udder is sterile
except at outer opening.
8 Moore and Ward, Bull. 153, Cornell Expt. Stat, Jan., 1899.

30 Dairy Bacteriology.

healthy mucous surfaces secrete more or less marked ger-
micidal fluids.1

Number of bacteria in fore milk. If the first few streams
of milk drawn from the udder are examined bacteriologic-
ally, it will invariably be found that the same contain a
relatively large number of organisms, as shown by the fol-
lowing data collected by Harrison,2 in which the bacterial
content of the fore milk is compared with the balance of
the milking.

Comparison in germ content of fore and whole milk.

Foremilk 26,070 25,630 38,420 18,110 54,800 32,700

43,520 27,830 18,500 29,400 45,630 48,700
Milk after removal

of foremilk..'.. 1,246 1,150 1,430 3,420 1,560 890

2,575 4,820 3,270 1,285 1,350

If successive bacterial determinations are made of milk
taken at different periods of the milking process, it is to be
noted, as in the following experiment, that a sudden dimi-
nution occurs after the first few streams are removed. In
this the bacterial distribution per cc. was as follows:

Bacterial content at different periods of milking.

Fore 200th 2000th 4300th 6500th Strip-
milk, cc. cc. cc. cc. pings.

Expt. 1 6,500 1,700 475 220 75 5

Expt. 2 3,100 1,650 400 240 50 10

In these cases contamination from all other sources was
excluded. The stoppings will sometimes be almost sterile,

^The germicidal properties of freshly drawn milk discovered by Fokker
(Zeit. f. Hyg., 1890, 9:41) is shown by the diminution in number of organisms that
may sometimes be noted (Park, N. Y. Univ. Bull., 1901, 1:85).

» Harrison, 22 Kept. Ont. Agr. Coll., 1896, p. 108; also Moore, 12 Kept. Bur. Animal
Ind. Washington, 1895-6, p. 261.

Contamination of Milk. 31

thus indicating that the abundance of bacteria found in
the fore milk is due to the flushing out of the lower por-
tion of the cistern and duct by the first few streams re-
moved.

Kinds Of bacteria in fore milk. The effect of these or-
ganisms on milk will depend upon the character of the
same. As a rule the number of the different species found
is usually small, a condition due in all probability to the
fact that the surroundings in the udder favor a rapid
growth of certain forms. According to Boiley1 the bac-
terial flora may vary considerably, although certain types
reappear with striking constancy if once found in the
udder or teat. What these forms are is a question of con-
siderable importance, for it would seem that the numerical
predominance of those present in the fore milk might in-
fluence the character of the fermentation of the whole
milk.

Harrison 2 claims to have found peptonizing bacteria in
the fore milk, while Marshall 3 reports organisms that re-
sist pasteurizing. Boiley, in thirty experiments, found
twelve out of sixteen species to belong to the lactic class.
Boiley and Hall failed to find gas-producing forms in the
milk of ten cows that were examined for a period of three
months; but the observations of Moore and Ward4 show
that gas-generating and taint-producing species are to be
found. This fact is important in the selection of milk
from a single animal for the cultivation of a starter. Hast-
ings has made the interesting observation in the writer's
laboratory, that the fore milk, although much richer in

i Boiley, Cent. f. Bakt., II Abt,, 1895, 1:795.

a Harrison, 1. c., p. 108.

•Marshall, Bull. 147, Mich. Expt. Stat., p. 42.

* Moore and Ward, Bull. 158, Cornell Expt. Stat., Jan., 1899.

32 Dairy Bacteriology.

bacteria than the whole milk, does not coagulate nearly as
soon. In a series of five trials the fore milk did not curdle
at room temperature on an average until after 84 hours,
while the average time of curdling of the whole milk was
38 hours.

Not all species of bacteria seem to be able to maintain
themselves in the udder even if they are introduced therein.
Dinwiddie1 injected into the milk cistern a facultative an-
aerobic lactic acid-producing form that grew at 99° F.
Several subsequent examinations failed to reveal the or-
ganism in any case. Ward 2 experimented with B. prodigi-
osus which he introduced through the milk duct. He was
able to determine its presence six days later. Experiments
have also been made with B. coli communis, B. cloacae and
B. lactis aerogenes? all of which are gas-generating species,
but in no case did these forms thrive.

In securing milk under conditions whereby the bacterial
content is reduced as much as possible, it is advisable to
throw out these first few streams. In doing so, the in-
trinsic loss is practically negligible, for the amount of
butter fat in even the first pint of a milking is only about
0.7,4 or one fifth of the normal.

Infection directly from animal. It is a popular belief
that much of the germ life that is found in milk is derived
from the food that is consumed by the animal, but such a
condition cannot prevail in the healthy animal for the
reason that bacteria in and on fodder are not absorbed into
the tissues proper, or if they are, they are quickly killed
by the germicidal properties of the body fluids. The

» Dinwiddie, Bull. 45, Ark. Expt. Stat., p. 57.
'Ward, Journ. of Appld. Mic., 1898, 1:305.
» Appel, Milch Zeit., 1900, No. 17.
« Snyder, Chemistry of Dairying, p. 10.

Contamination of Milk. 33

ger which does obtain directly from the animal, and which
to some extent is modified by the nature of the food par-
taken, is due to the fecal matter that is voided from the
intestine. Under careless conditions this is permitted to
soil the flanks and udder of the animal. In a dried state
it is readily dislodged by the movements of milking and so
falls into the open milk pail. The nature of the food con-
sumed modifies to some extent the character and consist-
ency of the manure, physically as well as bacteriologically.
The modern use of a more nitrogenous ration than for-
merly has resulted in the production of a softer, more fluid
manure, which is more likely to soil the coat of the ani-
mal. The same is true with animals on pasture in com-
parison with those fed dry fodder.

Wiithrich and Freudenreich * find a markedly higher
germ content in manure where animals are given dry
feed than where kept on grass. They found as many as
375,000,000 bacteria per gram in fresh manure, the major-
ity of which consisted of B. coli communis, the hay bacillus,
and other species able to peptonize casein.

Organisms of this type are more abundant in winter
milk than in summer, as the opportunity for infection is
greatly increased by closer housing.

The nature of the animal's coat favors greatly the re-
tention of dirt and dust. Cows wading in slime-covered
pools may cover the udder with material teeming with bac-
teria, which falls as an impalpable powder when dry. The
danger which may come from the introduction of such
matter is readily seen if hairs are removed from the coat
of the animal and laid on the surface of a sterile gelatin
plate as in Fig. 9. Almost invariably, a number of colo-

' Cent. f. Bakt., II. Abt. 1895, 1:8?3.

3

34: Dairy Bacteriology.

nies develop under these conditions, thus indicating the in-
fection that arises from this material. The introduction
of the dirt particles themselves, however, adds relatively
many more bac-
teria than come
from the hairs
themselves. The
amount of pol-
lution coming
from the coat of
the animal is
largely depend-
ent upon the
care taken in
bedding, and
even the nature
of the bedding
material has an

effect. Expen- 3^ 9 showing the bacterial contamination arising

ence has shown from hair. These hairs were allowed to fall on a sterile

1 j. • gelatin surface. The adherent bacteria developed

P6at 3 readily in jjjjg medium, and the number of bacteria

USed for this pur- thus introduced into the milk from these hairs can be

DOSe that the estimated by the number of developing colonies.

bacterial life is greatly reduced, due to the antiseptic ac-
tion which this strongly acid material possesses.

The amount of impurities that are often to be found in
milk, even after it is strained, attest beyond dispute the
careless methods of handling; it should, furthermore, be
noted that about one-half of fresh manure dissolves in
milk,1 and thus does not appear as sediment. It has been
determined by actual tests that the daily milk supply of

» Backhaus, Milch Zeit, 1897, 2G:357.

Contamination of Milk. 35

the city of Berlin, Germany, contains about three hundred
pounds of dirt and filth.

From a large number of determinations of the solid im-
purities found in market milk, Renk ! deduces the follow-
ing rule: If a sample of milk shows any evidence of im-
purity settling on a transparent bottom within two hours,
it is to be regarded as containing too much solid impuri-
ties. These solid particles, composed largely of manure
and dirt, are always teeming with bacteria, especially with
putrefactive and decomposition organisms.

While the number of bacteria that are hereby introduced
into milk is at times large, the character of the species is
even more significant. Derived primarily, as most of them
are, from fecal matter or from dirt, it is little wonder that,
their introduction should call forth abnormal fermenta-
tions. Undoubtedly bacteria of this class are intimately
concerned in the production of intestinal troubles in in-
fants. Eckles2 has shown that the digesting bacteria
that accompany fecal matter are closely connected with the
peculiar winter flavors that impair the quality of winter
butter.

Influence of the milker. The condition of the person
of the milker is not to be ignored in determining all pos-
sible factors of infection, for when clothed in dust-laden
garments, dislodgment of bacteria takes place readily.
Particular attention should be paid to the hands of the
milker. The filthy practice of moistening the hands with
a few drops of milk is to be deprecated from every point
of view. The milker should wash his hands in clean
water just before milking. If something is needed to en-

» Renk, Cent. f. Bakt., 1891, 10:19$.

2 Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.

36 Dairy Bacteriology.

able him to obtain a firmer grasp, a pinch of vaseline may
be used. Any scales or dirt rubbed from the teat would
be held by the vaseline and its effect on sore or chapped
teats is healing.

Freudenreich * reports some experiments in which the
germ content of milk was reduced from several thousand
to 200 where the hands were well rubbed with vaseline
before milking. Where a stringent control is exercised, it
is worth while to have the milker clothed in a suit kept
for this purpose, especially the upper portion of the body.
An outer garment could easily be slipped over the regular
working clothes. This garment should be white so as to
necessitate frequent washing.

Use Of milking machines. Numerous attempts have
been made to reduce the process of milking to a mechan-
ical basis by the introduction of a milking machine and
with some of these devices a bacteriological examination
has been made. Harrison 8 found that milk drawn from
the animal with the Thistle machine was much richer in
bacteria than hand-drawn milk. This was due to the
suction applied to the external surface of the teat and
udder, which caused the introduction of dust particles. In
the Murchland machine,3 the keeping quality of the milk
was fully equal to that drawn in the usual way.

Exclusion Of dirt. Scrupulous care will greatly mini-
mize the extent of infection from dirt and dust, carding
and brushing the udder and flanks will remove loose hairs
and considerable adherent dirt, but so long as the coat is
dry, dust particles and bacteria are readily dislodged. It
is generally thought that if these visible evidences of dirt

1 Freudenreich, Die Bakteriologie, p. 80.

a Harrison, Cent. f. Bakt., II Abt. 1809, 5:183.

• Dyrsdale, Trans. High. & Agr. Soc. Scotland, 5 Sen, 1898, 10:166.

Contamination of Milk. 37

are removed by straining or filtering the milk, the source
of trouble is eliminated. But in this operation only the
visible dirt is taken out. The invisible, and by far the
more dangerous material, is the bacterial life that thus be-
comes established, there to grow and develop. To remove
the dirt after it has once come in contact with the milk
only lessens the difficulty, but does not overcome it.

1. Moistening the udder. If alter brushing and remov-
ing the loose hairs and dirt that can be readily dislodged
in the milking, the udder and under parts are thoroughly
moistened with water, the fine dust-like particles will be
held in place. When moistened the surface does not want
to be dripping wet. The objection has been raised to
washing and cleaning the udder that the yield of milk is
reduced, but Eckles1 concludes from experiments that
when the animal becomes accustomed to the treatment, no
noticeable effect is produced either in amount of milk or
butter-fat.

The effect of this method on reducing the number of
bacteria dislodged is apparent from the following test
which was made on a cow kept on pasture and milked out
of doors. A sterile gelatin plate was exposed for sixty
seconds under the belly in close proximity to the milk
pail. Then, the udder, flank, and legs of the cow were thor-
oughly cleaned with water, and the milking resumed. A
second plate was then exposed in the same place for an
equal length of time; a control exposure being made at a
distance of ten feet from the animal and six feet from the
ground to ascertain the germ content of the surrounding
air.

From this experiment the following instructive data

» Eckles, Hoard's Dairyman, July 8, 1898.

38 Dairy Bacteriology.

were gathered. Where the animal was milked without
any special precautions being taken, there were 3,250
bacteria j00r minute deposited on an area equal to the ex-
posed top of a ten inch milk-pail. Where the cow received
the precautionary treatment as suggested above, there
were only 115 bacteria per minute deposited on the same
area. In the control plate sixty-five bacteria were found.
This indicates that a large number of organisms from the
dry coat of the animal can be kept out of milk if such
simple precautions as these are carried out. A consider-
able number of other observations have been collected in
the writer's laboratory, and it has frequently been found,
that in the case of well kept herds, the germ content of
the milk in the pail is increased from 20,000-40,030 bacteria
per minute during the milking period by the dislodg-
ment of organisms from the animal.

2. Diminishing exposed surface of pail. Another method
of excluding the dirt, in part at least, is to use a pail having
a less exposed surface. There are several different types of
these sanitary or hygienic pails that are ussd more or less
in the better type of dairies.

Eckles reports following data where a covered pail with
a small opening was used in comparison with a common
open pail. 43,200 bacteria per cc. were found in the
milk drawn in a common pail as against 3200 per cc. in
covered pail. The milk soured in 43 hours in the first
case; 64 hours in the latter instance. A series of experi-
ments made in writer's laboratory by Darrow with the
two sterilized pails shown in Fig. 10 were as follows:

No. bacteria per cc* in milk.

PailA 125 91 110 40 170 80 40

PailB.. 535 240 160 115 230 45 170

Contamination of Milk. 39

Pail A is provided with a small opening in a removable
top (C) to the pail. This top piece is arranged so that a layer
of muslin or flannel (a, a') can be stretched over opening.
On this is placed a layer of absorbent cotton (&), which in
turn is covered with another sheet of muslin.

Pail B is provided with a removable top in the lower
part of which is fitted another connection .5, that is also
removable. This part is provided with a cotton filter (a),
the milk also passing upward through a wire strainer (b).

FIG. 10. Sanitary milk pails designed to diminish introduction of hairs, etc.,
into milk.

The efficiency of either of these pails is apparent from
the data presented.

3. Cleaning milk by centrifugal force. Another method
that is in quite common use is to pass the milk through
separators. This not only removes practically all sus-
pended particles of foreign matter, as dirt, hairs, epithelial
cells or other debris,1 but eliminates a large part of the
bacteria with the centrifuge slime. By weighing the
skim milk, cream and slime carefully, and then determin-
ing the germ content of each, it is possible to compute the

1 Backhaus (Milch Ztg., 1897, 26:358) found that 95.6 per cent of impurities were
thus removed.

40 Dairy Bacteriology.

relative distribution of bacteria. Eckles and Barnes1 have
reported a series of studies of this sort in which they found
from 37 to 56 per cent of the organisms removed from
the milk by centrifugal force. An average of their re-
sults showed about 29 per cent of total bacterial life in the
skim milk, 24 per cent in the cream, and about 47 per
cent in the slime. Quantitatively the bacterial content of
the slime is always exceedingly high, ranging from tens of
millions to billions of bacteria per gram.

It is surprising that the elimination of snch a consider-
able proportion of organisms should not materially en-
hance the keeping quality of the purified milk. In the
work above reported, although the number of bacteria was
diminished 15 to 50 per cent, the diminution in develop-
ment of acidity in twenty-four hours in the purified sam-
ples was only a few hundredths of one per cent.

The effect of filtration through sand, gravel and other
substances is the same as when the milk is passed through
the separator, the aim in all cases being to purify or free
the milk from solid impurities that find their way into the
milk largely from the animal herself. Comparative experi-
ments made by Backhaus and Cronheim2 on various filter-
ing devices show that cellulose yields the best results.
Schuppan 8 found the supply of a Copenhagen company
that used a gravel filter reduced 38 to 48 per cent.

Influence of barn air. It is impossible to separate the
influence of the air entirely from that of the animal, as
the dust particles from the coat of the animal must of
necessity pass through the air.

Germ life cannot develop in the air, but in a dried con-

> Eckles and Barnes, Bull. 59, Iowa Expt. Stot., Aug. 1901.
•Journ. f. Landw., 1897,45:223.
» Cent. f. Bakt., 1893, 13:557.

Contamination of Milk. 41

dition, organisms retain their vitality for long periods of
time. The use of dry fodder, the bedding of animals with
straw adds greatly to the amount of dust particles, and
consequently to the germ life floating in the air, as seen in
Fig. 11. Taints in milk have frequently been traced to
infection arising from this source.

FIG. 11. Effect of contaminated air. The number of spots indicate the col-
onies that have developed from the bacteria which f ell in 30 seconds on the sur-
face of the gelatin plate (3 inches in diameter). This exposure was made at time
the cows were fed.

While the stable cannot be entirely freed from dust, yet
the effect of this factor can be greatly minimized by a little
forethought. Feeding before milking adds materially to
the germ content of the air, if feed is of dry character. If
moistened and given during milking, the same objection
does not inure. The following data collated by Harrison
shows number of bacteria per minute deposited in a 12-inch
pail. In Series A, the exposure was made during bedding;
in B, one hour after this operation.

42 Dairy Bacteriology.

Influence of dusty air on germ life.

Series A 16,000 13,536 12,216 12,890 15,340

19,200 23,400 27,342 42,750 18,730

Series B 483 610 820 715 1,880

2,112 1,650 990 1,342 2,370

These results indicate that the bacteria are in the main
attached to particles of considerable weight, as they settle
readily to the floor.

It has long been observed that the milk of stall-kept
animals does not keep as well as that milked out-of-doors.
In some of the better sanitary dairies, it is customary ta
have a milking room, the walls of which may be kept
moist, or at least free from dust, and in this way eliminate
the effect of air infection.

Relative importance of foregoing: factors. It is exceed-
ingly difficult to measure the relative values of these dif-
ferent methods of infection that have been cited, for they
are subject to so much fluctuation. Where the milk is
handled without any special care, unclean dairy utensils
and dirt from the animal are the most important sources
of pollution, not only with reference to the actual number
of bacteria introduced, but more particularly as to the effect
which bacteria of this class exert on the milk. If, how-
ever, careful supervision is given to the carrying out of ra-
tional methods of cleanliness, the most important factor
contributing to the germ content is often the fore milk.

Sanitary or hygienic milk. By putting into practice the
various suggestions that have been made with reference to-
diminishing the bacterial content of milk, it is possible to
greatly reduce the number of organisms found therein,
and at the same time materially improve the keeping qual-

Contamination of Milk. 43

ity of the milk. Backhaus1 estimates that the germ life
in milk can be easily reduced to one-two thousandth of its
original number by using care in milking. He reports a
series of experiments covering two years in which milk
was secured that averaged less than 10,000 bacteria per cc.,
while that secured under ordinary conditions averaged over
500,000.

Fig. 13 gives an illustration as to what care in milking
will do in the way of eliminating bacteria. Fig. 12 shows
a gelatin plate seeded with the same quantity of milk that
was used in making the culture indicated by Fig. 13. The
first plate was inoculated with milk drawn under ordi-
nary conditions, the germ content of which was found to
be 15,500 bacteria per cc., while the sample secured under
as nearly aseptic conditions as possible (Fig. 13) contained
only 330 organisms in the same volume.

Within recent years there has been more or less gener-
ally introduced into many cities, the custom of supplying
high grade milk that has been handled in a way so as to
diminish its germ content as much as possible. Milk of
this character is frequently known as usanitary,"uhygienic"
or "certified," the last term being used in connection with
a certification from veterinary authorities or boards of
health as to the freedom of animals from contagious dis-
ease. Frequently a numerical bacterial standard is exacted
as a pre-requisite to the recommendation of the board of
examining physicians. Thus, the Pediatric Society of
Philadelphia requires all children's milk that receives its
recommendation to have not more than 10,000 bacteria
per cc. Such a standard has its value in the scrupulous
cleanliness that must prevail in order to secure these re-

1 Backhaus, Ber. Landw. Inst. Univ. Konigsberg, 1897, 2:12.

44 Dairy Bacteriology.

suits. This in itself is practically a guarantee of the ab-
sence of those forms liable to produce trouble in children.
De Schweinitz1 has made a series of examinations ex-
tending throughout a year on such a sanitary dairy in

FIG. 19. Bacterial content of milk handled in ordinary way. Each spot rep-
resents a colony growing on gelatin plate. Compare with Fig. 13, where same
quantity of milk is used in making culture. Over 15,000 bacteria per cc. in this

Washington. The average of 113 samples was 6,485 or-
ganisms per cc. The following figures for a week in sum-
mer are taken from the regular analytical records of a
Philadelphia firm that makes a specialty of children's milk.

July ............... 1234 567

No. bacteria per cc.. 525 1,050 125 2,450 1,250 475 2,100

From a practical point of view, the improvement in
quality of sanitary milk, in comparison with the ordinary
product is seen in the enhanced keeping quality. During

» De Schweinitz, Nat. Med. Rev., Apr. 1899.

Contamination of Milk. 45

the Paris Exposition in 1900, milk and cream from several
such dairies in the United States were shipped to Paris,
arriving in good condition after 15-18 days transit. When
milk has been handled in such a way that by the time it

Fio. 18. Bacterial content of milk drawn with care. Diminished germ con-
tent is shown by smaller number of colonies (330 bacteria per cc.). Compare
this culture with that shown in Fig. 12.

reaches the consumer it contains no more germ life than
this, it is evident that it much more nearly approximates
the condition which exists in the udder of the animal than
the milk ordinarily sold by the milk dealer.

Considerable difference of opinion has existed in the
minds of the medical profession as to the relative merits of
such sanitary milk in comparison with pasteurized or
sterilized milk as food for children. While it can gener-
ally be shown that properly pasteurized milk will contain
less germ life than this which has been milked and handled
under careful sanitary conditions, yet the fact that the

4G Dairy Bacteriology.

low bacterial content is secured in the latter case by the
elimination rather than the destruction of bacteria is a
point in its favor. Unquestionably such sanitary milk is
less changed from the normal secretion of the cow than
that which has been subjected to heat sufficiently high to
•destroy the bacteria in the same.

It may not be practical under all conditions where milk
is produced to put into operation all of the precautions
tfcat have been previously referred to. But there can be
no doubt where milk is to be consumed as milk, that the
introduction of these or similar methods would greatly
improve the quality of the product. Even where milk is
utilized in the factory or creamery, it is quite essential
that it should be as nearly normal as possible, for in but-
ter and cheese making, the quality of the product is di-
rectly dependent upon the character of the raw material.

Dairymen have learned many lessons in the severe school
of experience, but it is earnestly to be hoped that future
conditions will not be summed up in the words of the
eminent German dairy scientist, Prof. Fleischmann, when
he says that "all the results of scientific investigation
which have found such great practical application in the
treatment of disease, in disinfection, and in the preserva-
tion of various products, are almost entirely ignored in
milking."

Effect of temperature on bacterial growth. After milk
is once seeded with bacterial life, no one factor exerts so
potent an effect as temperature upon the rate of growth.

Although different species vary in their rate of develop-
ment, yet moderately warm temperatures from 75° to 90°
F., encourage rapid growth. Unless the milk is quickly
deprived of its original heat, the rate of the fermentative

Contamination of Milk. 47

changes will be much increased as is shown in following
results obtained by Freudenreich.1

No. of bacteria per cc. in milk kept at different temperatures.

77° F. 95° F.

5 hrs. after milking < 10.000 30,000

8 " " " 25,000 12,000,000

12 u " " 46,000 35,283,000

26 " " " 5,700,000 50,000,000

PPOQEAY OF A SI/iQLE GERM
I/I 12 HOURS, I/I MILR ALLOWED
TO COOL /iATURALLY.

-f,** I/S MILfT COOLED WITH

''HUCA

COLD WATER.

Fro. 14. Effect of cooling milk on the growth of bacteria.

If a can of milk is allowed to cool naturally, it will take
several hours before it reaches the temperature of the sur-
rounding air. During this time the organisms in the fore
milk are continuing their rapid growth, while those forms
which come from dust, and are presumably in a latent
state, awake from their lethargy under the influence of
these favorable surroundings. If bacteria once gain an
entrance and begin to germinate, a considerably lower
temperature is required to successfully check development
than to hold latent organisms, like spores, in a condition
where germination will not occur.

To hasten this lowering of temperature artificial cooling
is a necessity. With good well water having a tempera-

1 Freudenreich, Ann. de Microg., 1890, 2:115.

48 Dairy Bacteriology.

ture of 48°-50° F., it is possible to chill milk sufficiently to
keep it. Where cold water is not available, ice water
should be used. In the production of the best quality of
milk for the factory, this factor of early and thorough
cooling is entitled to more weight than even the matter of
extreme care in milking.

Mixing: night and morning milk. Common experience
has often shown when old milk is mixed with new, that
the fermentative changes are more rapid than would have
been the case if the two milks had been kept apart. This
is most frequently observed when the night milk is cooled
down and mixed with the warm morning milk. This
often imperfectly understood phenomenon rests upon the
relation of bacterial growth to temperature. The night
milk may be cooled down to 50° F., but by the next morn-
ing it has considerably more bacteria than the freshly
drawn sample, the temperature of which may be 90° F.
Now, if these two milkings are mixed, the temperature of
the whole mass will be raised to a point that is more fa-
vorable for the growth of all of the contained bacteria
than it would be if the older milk was kept chilled.

Number of bacteria in milk. The germ content of
milk varies so greatly that unless the conditions are all
known, it is impossible to foretell what may be found
therein. An examination of milk will often reveal a dif-
ference in numbers, ranging from a few score of germs to
hundreds of millions per cc. The presence of such a vary-
ing number is dependent upon certain factors, as the age
of the milk, the care taken during the milking, and also
the way in which it has been handled since that time.
Disregarding milk of different ages, the number of germs
present in any sample bears a general relation to the

Contamination of Milk. 49

amount of dirt and filth with which it has come in contact
since it was drawn from the cow. Bacteria and filth of all
kinds are so intimately associated with each other that the
presence of one rightly presupposes that of the other.

As to the numerical content of any milk, there is such
a wide variation under different conditions that figures are
not of much worth unless surrounding conditions are
considered. No exact relation can be maintained between
the number of bacteria in milk and the development of
fermentative products.

Under American conditions data are gradually being
accumulated, but the subject has not been exhaustively
studied. Milk in this country as it reaches the consumer
usually contains fewer bacteria than are to be found in
European supplies, although as Conn has pointed out, it is
often older. As he intimates this is explained by the rela-
tively free use of ice in this country. A few determina-
tions of the bacterial content of European milks that
have been analyzed biologically will illustrate this point.

Renk1 found in Halle milk supply 6 to 30,000,000 germs
per cc.; Cnopf 2 in Munich milk supply 200,000 to 6,000,000
per cc.; Uhl8 in Giessen milk 83,000 to 170,000,000 per cc.;
Clauss 4 in Wurzburg 222,000 to 23,000,000 per cc.; Bujwid
in Warsaw an average of 4,000,000 per cc., and Knochen-
steirn 5 in Dorpat 25,000,000 per cc.

Sedgwick and Batchelder 6 report fifty-seven samples of
Boston milk as containing from 30,000 to 4,220,000 per cc.
In the country, they found in the milk fresh from the cow

J Renk, Cent. f. Bakt. 10:193.
8 Cnopf, Ibid. 6: 553.
» Uhl, Zeit. f. Hyg., 1892, 12:475.
4 Clauss, Diss. Wurzburg, 1889.

* Knochensteirn, Chem. Cent., 11:62.

• Sedgwick and Batchelder, Boston Med. Surg. Journ., Jan. 14, 1892.

4

50 Dairy Bacteriology.

30,000, and in the milk as used on the table about 70,000
organisms per cc. Loveland and Watson ' found in the
supply of Middletown, Conn., from 11,000 to 85,500,000
per cc.

Leighton8 studied seventeen dairies at Montclair, N. J.,
for a three-year period with the following results :

Class I, dairies averaging below 15,000 bacteria per cc.;

Class II, those averaging from 40,000 to 70,000 per cc.;
• Class III, those above 180,000.

In Class I, the dairies were found to be clean and well
improved; in Class II the conditions were as satisfactory
as possible with the crude appliances used; Class III was
as a whole careless. McDonnell3 sampled 352 lots from
eleven American cities. The worst samples were found in
restaurants and with small retail dealers. 28 per cent
of all samples contained less than 100,000 bacteria per cc.
while 34 per cent had less than 500,000. Park 4 finds in
New York City that the milk in the shops where it is
generally sold, averages during the coldest weather about
250,000 organisms per cc., during cool weather about
1,000,000, and in hot weather about 5,000,000 per cc. Eckles 6
has studied the flora of milk under factory conditions.
He finds from one to five million organisms per cc. in
winter, but in summer there may be from fifteen to
thirty millions.

Bacterial vs. other standards. As the germ content
of milk is subject to such wide variations, it is practi-
cally impossible to establish a numerical standard, al-

1 Loveland and Watson, Ch. 7 Rep. Storrs Stat. (Conn.), 1894, p. 72.

» Leighton, Science, Mar. 23, 1900.

1 McDonnell, Penn. Dept. Agr. Kept., 1897, p. 561.

«Park, N. Y. Univ. Bull., 1901, 1:85.

• Eckles, Bull. 59, Iowa Expt. Stat., Aug., 1901.

Contamination of Milk. 51

though Bitter states that 50,000 organisms per cc. should
be a maximum limit in milk intended as human food.
As a result of a study of the supply of New York city,
Park believes it is possible for a milk producer, without
adding materially to his expense, to secure milk which
when first drawn will not contain on an average more
than 30,000 bacteria in hot weather and 25,000 in cold
weather. If such milk is chilled immediately to 50° F., it
will not contain more than 100,000 per cc. in twenty-four
hours. Rochester, New York, has already tried the en-
forcement of a standard (100,000 per cc.) with good re-
sults it is claimed. The practical difficulties to contend
with in establishing a milk standard based upon a quanti-
tative bacterial determination are such as to render its gen-
eral adoption extremely problematical.

Acid test. It would seem that some other test which
can be more easily employed, even though it might not be
susceptible to an equal degree of accuracy, would be pref-
erable. Such a test is to be found in the acid test which
measures the acidity of the milk in question. There are
of course organisms to be found in milk that do not pro-
duce acid, and therefore it might be thought that the
presence of such would militate against the accuracy of
.such a test; but under normal conditions, the lactic acid-
producing organisms find in milk so congenial a sub-
stratum for their development, that in the great majority
of cases, the determination of the acid indicates the man-
ner in which milk has been handled. High acidity is either
due to old milk (long period of incubation) or insufficient
cooling (rapid incubation). Either of these conditions
permits of the accumulation of bacterial life, and therefore
impairs the quality of the milk. The determination of the

52

Dairy Bacteriology.

acid in milk can be made accurately by titration with
standard solutions of alkali, or more readily, in general
work, by employing the Farrington alkaline tablet, which
is a combination of a definite amount of standard alkali
and an indicator (phenolphthalein). It is possible by the
use of the tablet solution to make as accurate determina-
tion of acidity as with the usual standard solutions em-
ployed in the laboratory, but the method may also be
used rapidly in such a way as to give an approximate deter-
mination of acid. This modification is very serviceable at
the weigh-can or intake where it is necessary to pass judg-
ment very quickly on the quality of the milk.

Fia. 15. Apparatus used in making rapid acid test.

Making the acid test Fig. 15 gives the apparatus nec-
essary for this rapid testing of the acidity of the milk. A
solution of the alkaline tablets is first prepared by dissolv-

Contamination of Milk. 53

ing them in clean soft water, one tablet for each ounce of
water; thus eight tablets to an eight-ounce bottle of water.
In determining the acidity, a number of common white cups
are used, one for each patron. Two measures full ' of the
alkali solution are placed in each cup, and then as the milk
is received at the weigh-can, one measure of milk is added
to the alkali solution in the cup,2 and the whole gently, but
thoroughly shaken. If the pink color of the alkali solution
persists even fainthr, it shows that there is not enough acid
in the milk to neutralize the the same; if it disappears alto-
gether, leaving the milk white in color, it indicates that
there is more acid in the milk than can be neutralized by
the alkali of the tablet solution. When these proportions
of milk and alkali solution (2:1) are employed, it indicates
an acidity of 0.2 per cent, figured as lactic acid. Milk
should not contain more than this amount of acid, and
under good methods of handling, the acidity should be
brought down to 0.15 per cent if possible.

Circumstances may arise that might lead to an error if
this method is blindly followed. It has been pointed out *
that if milk is allowed to stand in rusty cans for some
time its acid content is diminished materially. The aver-
age acidity of nine samples of milk brought to a factory in
clean cans was .228 per cent acidity, while that of nine
other patrons brought in rusty cans was only .134 per cent.
Milk kept in rusty cans is sure to contain large quantities
of bacteria, even though its acid content may be low.

Kinds Of bacteria in milk. The number of bacteria in
milk is not of so much consequence as the kind present.

1 A brass cartridge shell provided with a handle serves admirably for a meas-
ure.

2 Farrington, Bull. 52, Wis. Expt. Stat.
8 Biddick, Hoard's Dairyman, July 30, 1897.

54 Dairy Bacteriology.

With reference to the number of kinds present, the more dirt
and foreign matter the milk contains, the larger the num-
ber of varieties found in the same. While milk may con-
tain forms that are injurious to man, still the great majority
of them have no apparent effect on human health. In
their effect on milk, the case is much different. Depend-
ing upon their action in milk, they may be grouped into
three classes:

1. Bacteria that exert no appreciable effect in milk.

2. Bacteria that are beneficial by reason of the products
which they form.

3. Bacteria that are injurious on account of the effect
which they produce in milk.

A surprisingly large number of bacteria that are found
in milk belong to the first class. Undoubtedly they af-
fect the chemical characteristics of the milk somewhat,
but not to the extent that it becomes physically percep-
tible. Eckles l reports in a creamery supply from 20 to 55
per cent of entire flora as included in this class.

Those species that are concerned in the production of
proper flavor and aroma in butter, and which are also
cdncerned in the development of acid and possibly asso-
ciated with formation of cheese flavor represent the sec-
ond type. Many of these organisms are lactic acid-pro-
ducing, but in addition to these, some of the casein
ferments are also associated with aroma production in but-
ter. Under normal conditions b}r far the larger proportion
of bacteria present in milk belong to the lactic acid type.
There are always present, though, digesting species that
are able to grow if the lactic acid forms are killed, as in
pasteurizing.

i Eckles, Bull. 59, Iowa Expt. Stat., Aug., 1901.

Contamination of Milk. 55

The third class includes those species that are able to
produce deleterious and undesirable flavors in milk and
milk products. The abnormal fermentations of milk re-
ferred to in the next chapter come under this head. Most
of these gain access to the milk through slovenly arid care-
less methods of handling. Those species associated with
ani mal excreta are particularly dangerous. The number of
different kinds that have been found in milk is quite con-
siderable, something over 200 species having been described
more or less thoroughly. In all probability, however, many
of these forms will be found to be identical when they are
subjected to a more critical study.

Direct absorption Of taints. A tainted condition in milk
may result from the development of bacteria, acting upon
various constituents of the milk, and transforming these in
such away 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 bac-
terial action, and is largely attributable to the physical
property which milk possesses of being able to absorb vola-
tile odors, the fat in particular, having a great affinity for
many of these substances. This direct absorption may
occur before the milk is withdrawn from the animal, or
afterwards if exposed to strong odors.

It is not uncommon for the milk of animals advanced in
lactation to have a more or less strongly marked odor and
taste; sometimes this is apt to be bitter, at other times
salty to the taste. It is a defect that is peculiar to indi-
vidual animals and is liable to recur at approximately the
same period in lactation.

56 Dairy Bacteriology.

The peculiar "cowy " or "animal odor" of fresh milk is
an inherent peculiarity that is due to the direct absorption
of volatile elements from the animal herself. This condi-
tion is very much exaggerated when the animal consumes
strong-flavored substances as garlic, leeks, turnips and cab-
bage. The volatile substances that give to these vegetables
their characteristic odor are quickly diffused through the
system, and if such foods are consumed some few hours be-
fore milking, the odor in the milk will be most pronounced.
The intensity of such taints is diminished greatly and
often wholly disappears, if the milking is not done for some
hours (8-12) after such foods are consumed.

This same principle applies in lesser degree to mary
green fodders that are more suitable as feed for animals, as
silage, green rye, rape, etc. Not infrequently, such fodders
as these produce so strong a taint in milk as to render it
useless for human use. Troubles from such sources could
be entirely obviated by feeding limited quantities of such
material immediately after milking. Under such condi-
tions the taint produced is usually eliminated before the
next milking. The milk of swill-fed cows is said to possess
a peculiar taste, and the urine of animals fed on this food
is said to be abnormally acid. Brewers' grains and distil-
lery slops when fed in excess also induce a similar condi-
tion in the milk.

Milk may also acquire other than volatile substances di-
rectly from the animal, as in cases where drugs, as bella-
donna, castor oil, sulfur, turpentine, jalap, croton oil,
and many others have been used as medicine. Such min-
eral poisons as arsenic have been known to appear eight
hours after ingestion, and persist for a period of three
weeks before being eliminated.

Contamination of Milk, 57

Absorption of odors after milking:. If milk is brought in
contact with strong odors after being drawn from the ani-
mal, it will absorb them readily, as in the barn, where fre-
quently it is exposed to the odor of manure and other
fermenting organic matter.

It has long been a popular belief that milk evolves
odors and cannot absorb them so long as it is warmer than
the surrounding air, but from experimental evidence, the
writer1 has definitely shown that the direct absorption of
odors takes place much more rapidly when the milk is
warm than when cold, although under either condition,
it absorbs volatile substances with considerable avidity.
In this test fresh milk was exposed to an atmosphere im-
pregnated with odors of various essential oils and other
odor-bearing substances. Under these conditions, the cooler
milk was tainted very much less than the milk at body
temperature even where the period of exposure was brief.
It is therefore evident that an exposure in the cow barn
where the volatile emanations from the animals themselves
and their excreta taint the air will often result in the ab-
sorption of these odors by the milk to such an extent as
to seriously affect the flavor.

The custom of straining the milk in the barn has long
been deprecated as inconsistent with proper dairy prac-
tice, and in the light of the above experiments, an additional
reason is evident why this should not be done.

Even after milk is thoroughly cooled, it may absorb
odors as seen where the same is stored in a refrigerator
with certain fruits, meats, fish, etc.

Distinguishing bacterial from non-bacterial taints. In

perfectly fresh milk, it is relatively easy to distinguish be-

* Russell, 15 Kept. Wis. Expt. Stat. 1898, p. 104.

58 Dairy Bacteriology.

tween taints caused by the growth of bacteria and those
attributable to direct absorption.

If the taint is evident at time of milking, it is in all
probability due to character of feed consumed, or possibly
to medicines. If, however, the intensity of the taint grows
more pronounced as the milk becomes older, then it is
probably due to living organisms, which require a certain
period of incubation before their fermentative properties
are most evident.

Moreover, if the difficulty is of bacterial origin, it can be
frequently transferred to another lot of milk (heated or
sterilized is preferable) by inoculating same with some of
the original milk. Not all abnormal fermentations are
able though to compete with the lactic acid bacteria, and
hence outbreaks of this sort soon die out by the re-estab-
lishment of more normal conditions.

Treatment of directly absorbed taints. Much can be
done to overcome taints of this nature by exercising greater
care in regard to the feed of animals, and especially as to
the time of feeding and milking. But with milk already
tainted, it is often possible to materially improve its con-
dition. Thorough aeration has been frequently recom-
mended, but most satisfactory results have been obtained
where a combined process of aeration and pasteurization
was resorted to. Where the milk is used in making but-
ter,, the difficulty has been successfully met by washing the
cream with twice its volume of hot water in which a little
saltpeter has been dissolved (one teaspoonful per gallon),
and then separating it again.1

The treatment of abnormal conditions due to bacteria
has been given already under the respective sources of in-

i Alvord, Circ. No. 9, U. S. Dept. Agric. (Div. of Sot.).

Contamination of Milk. 59

fection, and is also still further amplified in following
chapter.

Aeration. Practical experience has long demonstrated
the advantage of aerating the milk as soon after milking
as possible. This is accomplished in a variety of 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 and fall some distance so that it is brought inti-
mately in contact with the air. The benefit claimed for
aeration is that foul odors and gases which may be present
in the milk are thus allowed to escape by bringing the
finely divided milk into contact with the air. As ordi-
narily practiced, aeration is usually combined with cooling,
and it is noteworthy that the most effective aerators are
those that cool simultaneously. Under these conditions,
the keeping quality of the milk is increased, but where
milk is simply aerated without cooling, no material benefit
in keeping quality is observed. A satisfactory scientific
explanation of the advantages of aeration has not yet been
made. It is difficult to see how the process can have any
effect on the bacterial life in the milk. Its influence, un-
doubtedly, is on the odors directly absorbed by the milk.

Infection of milk in the factory. The problem of proper
handling of milk is not entirely solved when the milk is
delivered to the factory or creamery, although it might be
said that the danger of infection is much greater Avhile the
milk is on the" form. Then, too, contamination of milk at
time of withdrawal gives a longer period of incubation for
the bacteria in the milk, and consequently intensifies the
effect which they produce.

In the factory, infection can be minimized because ef-
fective measures of cleanliness can be more easity applied.

60 Dairy Bacteriology.

Steam is available in most cases, so that all vats, cans,
churns and pails can be thoroughly scalded. Special em-
phasis should be given to the matter of cleaning pumps
and pipes. The difficulty of keeping these utensils clean
often leads to neglect and subsequent infection. Care
must be taken relative to the use of worn apparatus. All
cans with rusty seams should be discarded. Permit no
vat to be repaired by putting in a false covering over the
old one. If a minute leak is established, such places be-
come a harbor of refuge for all kinds of putrefactive or-
ganisms. In a number of cases ill-smelling factory odors
have been traced to such a cause.

The influence of the air on the germ content of the milk
is, as a rule, overestimated. If the air is quiet, and free
from dust, the amount of germ life in the same is not rela-
tively large. In a creamery or factory, infection from this
source ought to be much reduced, for the reason that the
floors and wall are, as a rule, quite damp, and hence germ
life cannot easily be dislodged. The majority of organisms
found under such conditions come from the person of the
operators and attendants. Any infection can easily be
prevented by having the ripening cream-vats covered with
a canvas cloth. The clothing of the operator should be
different from the ordinary wearing-apparel. If made of
white duck, the presence of dirt is more quickly recog-
nized, and greater care will therefore be taken than if or-
dinary clothes are worn.

The surroundings of the factory have much to do with
the danger of germ infection. Many factories are poorly
constructed and the drainage is poor, so that filth and slime
collect about and especially under the factory. The ema-
nations from these give the peculiar " factory odor " that
indicates fermenting matter. Not only are these odors

Contamination of Milk. 61

absorbed directly, but germ life from the same is apt to
find its way into the milk. Connell1 has recently re-
ported a serious defect in cheese that was traced to germ
infection from defective factory drains.

The water supply of a factory is also a question of prime
importance. When taken from a shallow well, especially
if surface drainage from the factory is possible, the water
may be contaminated to such an extent as to introduce
undesirable bacteria in such numbers that the normal
course of fermentation may be changed. The quality of
the water, aside from flavor, can be best determined by
making a curd test (p. 76) which is done by adding some
of the water to boiled milk and incubating the same. If
" gassy " fermentations occur, it signifies an abnormal con-
dition. In deep wells, pumped as thoroughly as is gener-
erally the case with factory wells, the germ content should
be very low, ranging from a few score to a few hundred
bacteria per cc. at most.

Harrison 2 has recently traced an off-flavor in cheese in
a Canadian factory to an infection arising from the water-
supply. He found the same germ in both water and cheese
and by inoculating a culture into pasteurized milk suc-
ceeded in producing the undesirable flavor. The danger
from ice is much less, for the reason that good dairy prac-
tice does not sanction using ice directly in contact with
milk or cream. Then, too, ice is largely purified in the
process of freezing, although if secured from a polluted
source, reliance should not be placed in the method of
purification; for even freezing does not destroy all vege-
tating bacteria.

i Connell, Kept, of Commissioner of Agr ., Canada, 1897, part XVI, p. 15.
9 Harrison, Hoard's Dairyman, March 4, 1898.

CHAPTER IV.

FERMENTATIONS IN MILK AND THEIR
TREATMENT.

IT has been shown in the preceding chapter that the
contamination of milk with bacteria occurs so constantly
that under normal conditions it always contains a varying
amount of germ life. The result of this infection is to
cause in the milk, in due course of time, subsequent changes
of a fermentative nature. As a rule milk sours, due to the
production of acid from the decomposition of the milk
sugar; but, not infrequently, this more common change is
supplanted by other types of fermentative activity that re-
sult in the production of other kinds of by-products. These
fermentations are sometimes designated as abnormal, be-
cause of their less frequent occurrence.

It is impossible in the present state of knowledge to sat-
isfactorily classify these changes; but provisionally, they
may be grouped according to the substances on which they
act, or on the basis of the most prominent by-product
formed.

Milk is such a complex substance that the changes pro-
duced by a single germ are often so numerous that the
processes cannot be separated in their reactions. It must
be remembered then, in referring to the different types of
fermentations, that perhaps a distinct by-product is being
formed, but it is more than probable that there are a series
of changes, in which the most marked decomposition by-
product is alone taken into consideration. For example,
there is a fermentation classed under the head of the bu-

Fermentations in Milk. 63

tyric changes, a decomposition process in which butyric
acid is the chief product formed, but this may be associ-
ated with an alkaline condition of the milk and the pro-
duction of a bitter substance in the same. Thus, the sub-
division followed here will , of necessity be imperfect, and
occasional instances will be noted where some changes in
milk might well be described under several heads. It is
possible that milk may acquire an abnormal odor or taint,
such as is due to direct absorption, without having under-
gone any fermentative change, but the introduction of
various forms of bacteria is so common that fermentative
changes due to living ferments are constantly at work.

Souring Of milk. Milk naturally undergoes a change
known as souring, if allowed to stand for several days at
ordinary temperature. This is due to the formation of
lactic acid, which is produced by the decomposition of the
milk-sugar. While this change is wellnigh universal, it
does not occur without a pre-existing cause, and that is the
presence of certain living bacterial forms. These organ-
isms develop in milk with great rapidity, and the decom-
position changes that are noted in souring are due to the
by-products of their development.

The milk-sugar undergoes fermentation, the chief pro-
duct being lactic acid, although various other by-products,
.as other organic acids (acetic, formic and succinic), dif-
ferent alcohols and gaseous products, as C02, H, N and
methane (CH4) are produced in small amounts.

In this fermentation, the acidity begins to be evident to
the taste when it reaches about 0.3 per cent, calculated as
lactic acid. As the formation of acid goes on, the casein
is precipitated and incipient curdling or lobbering of the
milk occurs. This begins to be apparent when the acidity

64 Dairy Bacteriology.

is about 0.4 per cent, but the curd becomes more solid
with increasing acidity. The action of the bacteria is con-
tinued until about 0.8 to 1.0 per cent acid is formed, although
the maximum amount fluctuates considerably with differ-
ent organisms.1 Further formation then ceases, by reason
of the inability of the lactic acid organisms to continue
their development in such acid solutions. There is always
left in the milk a considerable amount of unfermented
milk-sugar which can be further acted upon by the con-
tinued growth of the bacteria, if a carbonate is added to
the milk to neutralize the developing acid that inhibits
their growth.

Cream never develops as much acid as milk, because a
larger proportion of its volume is made up of butter-fat
which is not subject to this change. In the ripening of
cream in butter-making, it is necessary to take this fact
into consideration where the cream varies widely in per
cent of fat.

The formation of lactic acid is a characteristic that is
possessed by a large number of bacteria, micrococci as well
as bacilli being numerously represented. Still the pre-
ponderance of evidence is in favor of the view that one
main type is responsible for most of this fermentation.
The most prominent organism associated with this change
is Bacillus acidi lactici, first described by Hiippe.9 Giin-
therand Thierfelder3 working on the spontaneous souring
of milk in the neighborhood of Berlin found what they
think is the same germ. Esten,4 in this country, studied
milks from thirty different localities in New England and

> Warrington, Jour. Chem. Soc., 1888, 53:727.
8 HUppe, Mitt. a. cL k. Gesundheitsamte, 1884, 2:309.
* Gtinther and Thierfelder, Arch. f. Hyg., 25:164.
« Esten, 9 Kept. Storrs Expt Stat., 1896, p. 44.

Fermentations in Milk. 65

the Middle States. He found a germ in all but two cases
that agreed in general with Gunther's description. Din-
widdie,1 studying the same question in Arkansas, arrives
at the same conclusion. This preponderance of evidence
makes it quite probable that there is a widely distributed
germ that is concerned in this change although undoubt-
edly different varieties exist. Besides this widely dissemi-
nated type, there are numerous other forms 2 that are
associated with this type of decomposition the most promi-
nent of which is known as B. lactis aerogenes. Conn and
Aikman refer to the fact that over one hundred species
are already known. It is fair to presume, however, that
a careful comparative study of these would show that
simply racial differences exist in many cases, and therefore,
that they are not distinct species.

This class of bacteria is characterized by their inability
to liquefy gelatin or develop spores. On account of this
latter characteristic they are easily destroyed when milk is
pasteurized. They live under aerobic or anaerobic condi-
tions, many of them being able to grow in either environ-
ment, although, according to McDonnell,3 they are more
virulent when air is not excluded.

The- temperature conditions as to growth vary somewhat
with different species. With most species this occurs at
50° F., but appreciable amounts of acid are not produced
until a higher temperature is reached.4 The optimum
temperatures for growth range from 90°-95° F.

While the souring of milk is a very wide-spread phenom-
enon, still lactic acid organisms are not universal^ dis-

1 Bull. 45, Ark. Expt. Stat., May 1897; Leichmann, Hyg. Rund., 1899, p. 1267.

2 Kayser, Ann. de Tlnst Past., 10:737.

3 McDonnell, Inaug. Diss., Kiel, 1899, p. 39.
* Kayser, Cent. f. Bakt., II Abt. 1:436.

5

66 Dairy Bacteriology.

tributed. According to Conn ' they are not very abun-
dant in perfectly fresh milk, but because of their ability to
thrive so luxuriantly in this liquid, they grow with great
rapidity, and therefore after a few hours milk always con-
tains them in abundance.

It is a wide-spread belief that thunder storms cause milk
to sour prematurely, but this idea has no scientific founda-
tion. Experiments9 with the electric spark, ozone and
loud detonations show no effect on acid development, but
the atmospheric conditions usually incident to a thunder
storm are such as permit of a more rapid growth of organ-
isms. There is no reason to believe but that the phenom-
enon of souring is wholly related to the development of
bacteria. Sterile milks are never affected by the action of
electric storms.

" Gassy " milks. A large number of bacteria possess
the property of fermenting sugars and producing gases of
various kinds as well as acids of a volatile or fixed char-
acter. The amount of acid formed is generally consider-
ably less than that produced by the normal lactic species.
Among the gases formed, H and C02 are most common,
although N and CH4 (methane) are sometimes produced.
In connection with these gases, there are also other de-
composition products of a more or less volatile nature that
frequently impart to the milk taints of an undesirable
character.

While these " gassy " defects can often be recognized in
the milk itself, they are much more apt to cause trouble
in the manufacture of cheese (see Fig. 16), where, in severe
cases, curds may " float" or be "pin holey."* There are

1 Conn, Agricultural Bacteriology, p. 191.
» Treadwell, Science, 1804, 17: 178.

» Freudenreich, Landw. Jahr. cL Schweiz, 1890, p. 17; Russell, 12 Eept Wte.
Ezpt. Stat., 1895, p. 189.

Fermentations in Milk. 67

a large number of organisms of this class found in surface
waters, soils and in decomposing organic matter. The
colon bacillus of the intestinal tract is a germ of this type

FIG. 16. Cheese made from " gassy " milk.

that finds its way into milk with manure particles. B. lactis
aerogenes, a common inhabitant of milk is also a gas-pro-
ducer. Abnormal fermentations of this class occur most
frequently in the hot summer months, but are not neces-
sarily confined to this season. Wherever carelessness pre-
vails in the matter of cleaning utensils, troubles from gassy
milks are very apt to occur.

"Sweet curdling" and digesting: fermentations. Not

infrequently milk, instead of undergoing spontaneous sour-
ing, curdles in a weakly acid or neutral condition, in which
state it is said to have undergone " sweet curdling." The
coagulation of the milk is caused by the action of enzyms
of a rennet type that are formed by the growth of various
species of bacteria. Later the whey separates more or less
perfectly from the curd, producing a "wheyed off" condi-
tion. Generally the coagulum in these cases is soft and

68 Dairy Bacteriology.

somewhat slimy. The curd usually diminishes in bulk,
due to the gradual digestion or peptonization of the casein
by proteid-dissolving enzyms (tryptic type) that are also
produced by the bacteria causing the change.

A large number of bacteria possess the property of af-
fecting milk in the above way. Generally they are able
to liquefy gelatin (also a peptonizing process) and form
spores. The Tyrothrix type of bacteria (so named by Du-
claux on account of the supposed relation to cheese ripen-
ing) belongs to this class. The hay and potato forms are
also digesters. Organisms of this type are generally as-
sociated with filth and manure, and find their way into
the milk from the accumulations on the coat of the aniinal.

Conn ' has separated the rennet enzym from bacterial
cultures in a relatively pure condition, while Fermi'2 has
isolated the digestive principle from several species.

Duclaux3 has given to this digesting enzym the name
casease or cheese ferment. These isolated ferments when
added to fresh milk possess the power of causing the char-
acteristic curdling and subsequent digestion quite inde-
pendent of cell development. The quantity of ferment
produced by different species differs materially in some
cases. In these digestive fermentations, the chemical trans-
formations are profound, the complex proteid molecule
being broken down into albumoses, peptones, amido-acids
(ty rosin and leucin) and ammonia as well as fatty acids.

Not infrequently these fermentations gain the ascend-
ency over the normal souring change, but under ordinary
conditions they are held in abeyance, although this type of
bacteria is always present to some extent in milk. When

' Conn, 5 Kept, Storrs Expt. Stat.. 1892. p. 396.
» Fermi, Arch. f. Hyg., 1892, 14:1.
* Duclaux, Le Lait, p. 121.

Fermentations in Milk. 69

the lactic acid bacteria are destroyed, as in boiled, sterilized
or pasteurized milk, these rennet-producing, digesting
species develop.

Butyric acid fermentations. The formation of butyric
acid in milk which may be recognized by the "rancid but-
ter" odor is not infrequently seen in old, sour milk, and
for a long time was thought to be a continuation of the
lactic fermentation, but it is now believed that these or-
ganisms find more favorable conditions for growth, not so
much on account of the lactic acid formed as in the ab-
sence of dissolved oxygen in the milk which is consumed
by the sour-milk organisms.

Most of the butyric class of bacteria are spore-bearing,
and hence they are frequently present in boiled or steril-
ized milk. The by-products formed in this series of changes
are quite numerous. In most cases, butyric acid is promi-
nent, but in addition to this, other organic acids, as lactic,
succinic, and acetic, are produced, likewise different alco-
hols. Concerning the chemical origin of butyric acid
there is yet some doubt. Duclaux l affirms that the fat,
sugar and casein are all decomposed by various forms.
In some cases, the reaction of the milk is alkaline, with
other species it may be neutral or acid. This type of fer-
mentation has not received th'e study it deserves.

In milk these organisms are not of great importance, as
this fermentation does not readily gain the ascendency
over the lactic bacteria.

Ropy or slimy milk. The viscosity of milk is often
markedly increased over that which it normally possesses.
The intensity of this abnormal condition may vary much;
in some cases the milk becoming viscous or slimy; in others

1 Duclaux, Principes de Laiterie, p. 67.

70

Dairy Bacteriology.

stringing out into long threads, several feet in length, as
in Fig. 17. Two sets of conditions are responsible for these
ropy or slimy milks. The most com-
mon is where the milk is clotted or
stringy when drawn, as in some forms
of garget. This is generally due to the
presence of viscid pus, and is often ac-
companied by a bloody discharge, such
a condition representing an inflamed
state of the udder. Ropiness of this
character is not usually communicable
from one lot of milk to another.

The communicable form of ropy milk
only appears after the milk has been
drawn from the udder for a day or so,
and is caused by the development of
various species of bacteria which find
their way into the milk after it is drawn.
These defects are liable to occur at any
season of the year. Their presence in
a dairy is a source of much trouble, as
the unsightly appearance of the milk
precludes its use as food, although there
is no evidence that these ropy fermen- Fl°- 17- R°PV milk-
tations are dangerous to health.

There are undoubtedly a number of different species of
bacteria that are capable of producing these viscid changes,1
but it is quite probable that they are not of equal im-
portance in infecting milk under natural conditions.

In the majority of cases studied in this country,* the

1 Guillebeau (Milch Zeit., 1892, p. 808) has studied over a dozen different forms
that possess this property.
* Ward, Bull. 165, Cornell Expt. Stat., Mch., 1899; also Bull. 195, Ibid., Nov., 1901.

Fermentations in Milk. 71

causal organism seems to be B. lactis viscosus. a form first
found by Adametz in surface waters.1 This organism pos-
sesses the property of developing at low temperatures
(45°-50° F.), and consequently it is often able in winter
to supplant the lactic-acid forms. Ward has found this
germ repeatedly in water tanks where milk cans are cooled;
and under these conditions it is easy to see how infection
of the milk might occur. Marshall 2 reports an outbreak
which he traced to an external infection of the udder; in
another case, the slime-forming organism was abundant
in the barn dust. A defect of this character is often per-
petuated in a dairy for some time, and may therefore be-
come exceedingly troublesome. In one instance in the
writer's experience, a milk dealer lost over $150 a month
for several months from ropy cream. Failure to properly
sterilize cans, and particularly strainer cloths, is frequently
responsible for a continuance of trouble of this sort.

The slimy substance formed in milk comes from vari-
ous constituents of the milk, and the chemical character
of the slime produced also varies with different germs. In
some cases the slimy material is merely the swollen outer
cell membrane of the bacteria themselves as in the case of
B. lactis viscosus; in others it is due to the decomposition
of the proteids, but often the chief decomposition product
appears to come from a viscous fermentation of the milk-
sugar.

An interesting case of a fermentation of this class being
utilized in dairying is seen in the use of ulangewei"
(long or stringy whey) which is employed as a starter in
Holland to control the gassy fermentations in Edam cheese.

1 Adametz, Landw. Jahr., 1891, p. 185.

2 Marshall, Mich. Expt. Stat., Bull. 140.

72 Dairy Bacteriology.

This slimy change is due to the growth of Streptococcus
Hollandicus.1

Alcoholic fermentations. Although glucose or cane-
sugar solutions are extremely prone to undergo alcoholic
fermentation, milk sugar does not readily decompose. The
more important alcoholic ferments are the yeasts, which
do not thrive readily in the milk, although Duclaux2 re-
ports a serious case in a dairy due to this cause.

Koumiss, a beverage originally made in the Orient from
mare's milk, is an example of an alcoholic fermentation
which is produced hy the addition of cane sugar and yeast
to ordinary cow's milk. It is used with success in gastric
troubles. In addition to the C02 developed which gives it
its effervescent qualities, alcohol, lactic acid, and casein-
dissolving ferments are also formed.

Kephir is another alcoholic drink made from milk that
is in common use among the people of Caucasus. It is
made by adding to milk kephir grains, which are merely a
mass of fermented cells (yeasts and bacteria) that start the
fermentation. This milk is then mixed with fresh milk
and kept in leather flasks until a mixed fermentation sets
in. The nature of the change is not yet thoroughly under-
stood,8 although it is quite probable that the alcoholic
change is produced by a yeast, while bacteria change the
casein more or less.

Bitter milk. The presence of bitter substances in milk
may be ascribed to a variety of causes. A number of plants,
such as lupines, wormwood and chicory, possess the prop-
erty of affecting milk when the same are consumed by ani-

' Milch Zeit., 1889, p. 982.

'Duclaux, Principes de Laiterie. p. 60.

' Freudenreich, Landw. Jahr. d. Schweiz, 1896, 10:.l.

Fermentations in Milk. 73

mals. At certain stages in lactation, a bitter salty taste is
occasionally to be noted that is peculiar to individual ani-
mals.

A considerable number of cases of bitter milk have, how-
ever, been traced to bacterial origin. For a number of
years the bitter fermentation of milk was thought to be
associated with the butyric fermentation, but Weigmann1
showed that the two conditions were not dependent upon
each other. He found that the organism which produced
the bitter taste acted upon the casein.

Conn2 observed a coccus form in bitter cream that was
able to impart a bitter flavor to milk. Sometimes a bitter
condition does not develop in the milk, but may appear
later in the milk products, as in the case of a micrococcus
which Freudenreich 3 found in cheese.

Cream ripened at low temperatures not infrequently de-
velops a bitter flavor, showing that the optimum tempera-
ture for this type of fermentation is below the typical
lactic acid change.

It has long been a question how to account chemically
for the bitter taste in milk. Various ideas have been ad-
vanced, but Freudenreich has demonstrated in one case
that a bitter substance is formed in the milk that can be
isolated by adding alcohol.

Milk that has been cooked is likely to develop a bitter
condition. The explanation of this is that the bacteria
producing the bitter substances usually possess endospores,
and that while the boiling or sterilizing of milk easily
kills the lactic acid germs, these forms on account of their
greater resisting powers are not destroyed by the heat.

i Weigmann. Milch Zeit., 1890, p. 881.

8 Conn, 3 Kept. Storrs Expt. Stat., 1890, p. 153.

a Freudenreich, Fiihl. Landw. Ztg., 43: 361.

74 Dairy Bacteriology.

Soapy milk: A soapy flavor in milk was traced by Weig-
mann and Ziru * to a specific bacillus, B. lactis saponacei,
that they found gained access to the milk in one case from
the bedding and in another instance from ha}r. A similar
outbreak has been reported in this country,2 due to a germ
acting on the casein and albumen.

Red milk. The most common trouble of this nature
in milk is due to presence of blood, which is most fre-
quently caused by some wound in the udder. The inges-
tion of certain plants as sedges and scouring rushes is also
said to cause a bloody condition; madders impart a red-
dish tinge due to coloring matter absorbed. Defects of this
class can be readily distinguished from those due to germ
growth because they are apparent at time of milking.
Where blood is actually present, the corpuscles settle out
in a short time if left undisturbed.

There are a number of chromogenic or color-producing
bacteria that are able to grow in milk, but their action is
so slow that generally they are not of much consequence.
Moreover their development is usually confined to the sur-
face of the milk as it stands in a vessel. The most import-
ant is the well-known B. prodigiosus. Another form found
at times in milk possessing low acidity3 is B. lactis erythro-
genes. This species only develops the red color in the dark.
In the light, it forms a yellow pigment. Various other
organisms have been reported at different times.4

Blue milk. Blue milk has been known for many years,
its communicable nature being established as long ago as
1838. It appears on the surface of milk first as isolated

» Miloh Zeit. 22:5(59.

'Marshall, Bull. 146, Mich. Expt. Stat., p. 16.

« Grotenfelt, Milch Zeit., 1833, p. 283.

«Menge, Cent. f. Bakt., 6:590; Keferstein, Cent. f. B?.kt., 21:177.

Fermentations in Milk. 75

particles of bluish or grey color, which later become con-
fluent, the blue color increasing in intensity as the acidity
increases. The causal organism, B. cyanogenes, is very re-
sistant toward drying,1 thus accounting for its persistence.
In Mecklenberg an outbreak of this sort once continued
for several years. It has frequently been observed in Eu-
rope in the past, but is not now so often reported. Occa-
sional outbreaks have been reported in this country.

Other kinds of colored milk. Two or three chromo-
genic forms producing still other colors have occasionally
been found in milk. Adametz 9 discovered in a sample of
cooked milk a peculiar form (Bacillus synxanthus) that
produced a citron-yellow appearance which precipitated
and finally rendered soluble the casein. Adanietz, Conn,
and List have described other species that confer tints of
yellow on milk. Some of these are bright lemon, others
orange, and some amber in color.

Still other color-producin 8 bacteria, such as those that
produce violet or green changes in the milk, have been ob-
served. In fact, almost any of the chromogenic bacteria
are able to produce their color changes in milk as it is such
an excellent food medium. Under ordinary conditions,
these do not gain access to milk in sufficient numbers so
that they modify the appearance ot it except in occa.jional
instances.

Treatment of abnormal fermentations. If the taint is
recognized as of bacterial origin (see p. 57) and is found in
the mixed milk of the herd, it is necessary to ascertain,
first, whether it is a general trouble, or restricted to one or
more animals. This can sometimes be done by separating

J Helm, Arb. a. d. Kais. Gesundheitsamte, 5:578.
a Adametz, Milch Zeit., 1890, p. 225.

76 Dairy Bacteriology.

the milk of the different cows and noting whether any ab-
normal condition develops in the respective samples.

Fermentation tests. The moot satisfactory way to de-
tect the presence of the taints more often present is to
make a fermentation test of one kind or another. These
tests are most frequently used at the factory, to enable the
maker to detect the presence of milk that is likely to prove
unfit for use, especially in cheese making. They are based
upon the principle that if milk is held at a moderately high
temperature, the bacteria will develop rapidly. A number
of different methods have been devised for this purpose. In
Walther^s lacto-fermentator samples of milk are simply
allowed to stand in bottles or glass jars until they sour.
They are examined at intervals of several hours. If the
curdled milk is homogeneous and has a pure acid smell, the
milk is regarded as all right. If it floats in a turbid serum,
is full of gas or ragged holes, it is abnormal. As generally
carried out, no attempt is made to have these vessels ster-
ile. Gerber's test is a similar test that has been extensively
employed in Switzerland. Sometimes a few drops of rennet
are added to the milk so as to curdle the same, and thus
permit of the more ready detection of the gas that is evolved.

Wisconsin curd test. The method of testing milk de-
scribed below was devised at the Wisconsin Experiment
Station in 1895 by Babcock, Russell and Decker.1 It was
used first in connection with experimental work on the
influence of gas-generating bacteria in cheese making, but
its applicability to the detection of all taints in milk pro-
duced by bacteria makes it a valuable test for abnormal
fermentations in general.

In the curd test a small pat of curd is made in a glass

» 12 Kept. Wis. Expt. Stat., 1895, p. 148; also Bull. 67, Ibid., June, 1898.

Fermentations in Milk.

77

jar from each sample of milk. These tests may be made
in any receptacle that has been cleaned in boiling water,
and to keep the temperature more nearly uniform these
jars should be immersed in warm water, as in a wash tub
or some other receptacle. When the milk is about 95° F.,
about ten drops of rennet extract are added to each sample
and mixed thoroughly with the milk. The jars should
then remain undisturbed until the milk is completely
curdled; then the curd is cut into small pieces with a case
knife and stirred to expel the whey. The whey should be
poured off at frequent intervals until the curd mats. If
the sample be kept at blood heat (98° F.) for six to eight
hours, it will be ready to examine.

FIG. 18. Improved bottles for making curd test. A, test bottle complete; B,
bottle showing construction of cover; S, sieve to hold back the curd when bottle
is inverted; C, outer cover with (D IT) drain holes to permit of removal of whey.

More convenient types of this test than the improvised
apparatus just alluded to have been devised by different
dairy manufacturers. Generally, they consist of a special
bottle having a full-sized top, thus permitting the easy

78 Dairy Bacteriology.

with a sieve of such construction that the bottles will drain
thoroughly if inclined in an inverted position.

Interpretation of results of test. The curd from a good
milk has a firm, solid texture, and should contain at most
only a few small pin holes. It may have some large, irreg-
ular, ''mechanical1' holes where the curd particles have
failed to cement, as is seen in Fig. 19. If gas-producing bac-

Fto. 19. Curd from a good milk. The large Irregular holes are Hiechanical.

teria are very prevalent in the milk, the conditions under
which the test is made cause such a rapid growth of the
same that the evidence of the abnormal fermentation may
bs readily seen in the spongy texture of the curd (Fig. 20).
If the undesirable organisms are not very abundant and the
conditions not especially suited to their growth, the " pin
holes " will be less frequent.

Sometimes the curds show no evidence of gas, but their
abnormal condition can be recognized by the "mushy"
texture and the presence of " off" flavors that are rendered
more apparent by keeping them in closed bottles. This
condition is abnormal and is apt to produce quite as serious
results as if gas was formed.

Fermentations in Milk. 79

Overcoming taints by use of starters. Another method
of combatting abnormal fermentations that is often fruit-
ful, is that which rests upon the inability of one kind of
bacteria to grow in the same medium in competition with
certain other species.

Some of the undesirable taints in factories can be con-
trolled in large part by the introduction of starters made

FIG. 20. Curd from a badly tainted milk. Large ragged holes are mechanical;
numerous small holes due to gas. This curd was a " floater."

from certain organisms that are able to obtain the ascend-
ency over the taint-producing germ. Such a method is
commonly followed when a lactic ferment, either a com-
mercial pure culture, or a home-made starter, is added to
milk to overcome the effect of gas-generating bacteria.

A similar illustration is seen in the case of the " lange
wei" (slimy whey), that is used in the manufacture of
Edam cheese to control the character of the fermentation
of the milk.

This same method is sometimes applied in dealing with
certain abnormal fermentations that are apt to occur on
the farm. It is particularly useful with those tainted milks
known as " sweet curdling." The ferment organisms con-

80 Dairy Bacteriology.

cerned in this change are unable to develop in the presence
of lactic acid bacteria, so the addition of a clean sour milk
as a starter restores the normal conditions by giving the
ordinary milk bacteria the ascendency.

Chemical disinfection. In exceptional instances it may
be necessary to employ chemical disinfectants to restore
the normal conditions. Of course with such diseases as
tuberculosis, very stringent measures are required, as they
are such a direct menace to human life, but with these
abnormal or taint-producing fermentations, care and clean-
liness, well directed, will usually overcome the trouble.

If it becomes necessary to employ chemical substances
as disinfecting agents, their use should always be preceded
by a thorough cleansing with hot water so that the germi-
cide may come in direct contact with the surface to be
disinfected.

It must be borne in mind that many chemicals act as
deodorants, i. e., destroy the offensive odor, without destroy-
ing the cause of the trouble.

Sulfur is often recommended as a disinfecting agent, but
its use should be carefully controlled, otherwise the vapors
have but little germicidal power. The common practice of
burning a small quantity in a room or any closed space for
a few moments has little or no effect upon germ life. The
effect of sulfur vapor (S02) alone upon germ life is relatively
slight, but if this gas is produced in the presence of mois-
ture, sulfurous acid (H2S03) is formed, which is much
more efficient. To use this agent effectively, it must be
burned in large quantities in a moist atmosphere (three Ibs.
to every 1,000 cubic feet of space), for at least twelve hours.
After this operation, the space should be thoroughly aired.

Formalin, a watery solution of a gas known as form-
aldehyde, is a new disinfectant that recent experience has

Fermentations in Milk. 81

demonstrated to be very useful. It may be used as a gas
where rooms are to be disinfected, or applied as a liquid
where desired. It is much more powerful in its action than
sulfur, and it has a great advantage over mercury and other
strong disinfectants, as it is not so poisonous to man as it
is to the lower forms of life.

Bleaching powder or chloride of lime is often recommended
where a chemical can be advantageously used. This sub-
stance is a good disinfectant as well as a deodorant, and if
applied as a wash, in the proportion of four to six ounces
of the powder to one gallon of v/ater, it will destroy most
forms of life. In many cases this a:;ent is inapplicable on
account of its odor.

Corrosive sublimate (Hg 01 2) for most purposes is a good
disinfectant, but it is such an intense poison that its use is
dangerous in places that are at all accessible to stock.

For the disinfection of walls in stables and barns, com-
mon thin white wash (Ca OH) is admirably adapted if made
from freshly-burned quick lime. It possesses strong germi-
cidal powers, increases the amount of light in the barn, is
a good absorbent of odors, and is exceedingly cheap.

Carbolic acid, creosote, and such products, while excellent
disinfectants, cannot well be used on account of their odor,
especially in factories.

For gutters, drains, and waste pipes in factories, vitriol
salts (sulfates of copper, iron and zinc) are sometimes used.
These are deodorants as well as disinfectants, and are not
so objectionable to use on account of their odor.

These suggestions as to the use of chemicals, however,
only apply to extreme cases and should not be brought into
requisition until a thorough application of hot water, soap,
a little soda, and the scrubbing brush have failed to do their

CHAPTER V.
RELATION OF DISEASE-BACTERIA TO MILK.

PRACTICAL experience with epidemic disease has abun-
dantly demonstrated the fact that milk not infrequently
serves as a vehicle for the dissemination o** contagion. At-
tention has been prominently called to this relation by
Ernest Hart,1 who in 1880 compiled statistical evidence
showing the numerous outbreaks of various contagious dis-
eases that had been associated with milk infection up to
that time. Since then, further compilations have been
made by Freeman,9 and also by Busey and Kober,3 who
have collected the data with reference to outbreaks from
1880 to 1899.

These statistics indicate the relative importance of milk
as a factor in the dissemination of disease.

The danger from this source is much intensified for the
reason that milk, generally speaking, is consumed in a raw
state; and also because a considerable number of disease-
producing bacteria are able, not merely to exist, but actu-
ally thrive and grow in milk, even though the normal
milk bacteria are also present. Moreover the recognition
of the presence of such pathogenic forms is complicated
by the fact that often they do not alter the appearance of

' Hart, Trans. Int. Med. Cong., London, 1881, 4:491-544.

» Freeman, Med. Rec., March 28, 1896.

* Busey and Kober, Kept. Health Off. of Dist. of CpL, Washington, D. C., 1895,
p. 299. These authors present in this report an elaborate article on morbific and
infectious milk, giving a very complete bibliography of 180 numbers. They ap-
pend to Hart's list (which is published in full) additional outbreaks which have
occurred since, together with full data as to extent of epidemic, circumstances
governing the outbreak, as well as name of original reporter and reference.

Relation of Disease-Bacteria to Milk. 83

the milk sufficiently so that their presence can be detected
by a physical examination. These facts which have been
experimentally determined, coupled with the numerous
-clinical cases on record, make a strong case against milk
serving as an agent in the dissemination of disease.

Origin of pathogenic bacteria in milk. Disease-produc-
ing 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, develop-
ing in the udder, and so infect the milk supply. It may,
of course, happen that diseases toward which domestic ani-
mals alone are susceptible may be spread from one animal
to another in this way without affecting human beings.

In the second group, the bacterial species lives a sapro-
phytic existence, growing in milk, if it happens to find its
way therein. In such cases milk indirectly serves as an
.agent in the dissemination of disease, by giving conditions
favorable to the growth of the disease germ.

By far the most important of diseases that may be trans-
mitted directly from animal to man through a diseased
milk supply is tuberculosis, but in addition to this, foot
and mouth disease (aphthous fever in children), anthrax
and acute enteric troubles have also been traced to a sim-
ilar source of infection.

84 Dairy Bacteriology.

The most important specific diseases that have been dis-
seminated through subsequent pollution of the milk are
typhoid fever, diphtheria, scarlet fever and cholera, but, of
course, the possibility exists that any disease germ capable
of living and thriving in milk may be spread in this way.
.In addition to these diseases that are caused by the intro-
duction of specific organisms (the causal organism of scar-
let 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 microorganisms that gain access to milk during
and subsequent to the milking, and which produce changes
in milk before or after its ingestion that result in the
formation of toxic products.

DISEASES TRANSMISSIBLE FROM ANIMAL TO MAN THROUGH
DISEASED MILK.

Tuberculosis. In view of the wide-spread distribution
of this disease in both the human and the bovine race, the
relation of the same to milk supplies is a question of great
importance. It is now generally admitted that the differ-
"ent types of tubercular disease found in different kinds of
animals and man are attributable to the development of
the same organism, Bacillus tuberculosis, although there
are varieties of this organism found in different species of
animals that are sufficiently distinct to permit of recogni-
tion.

The question of prime importance is, whether the bovine
type is transmissible to the human or not. Artificial in-
oculation of cattle with tuberculous human sputum as well
as pure cultures of this variety show that the human type

Relation of Disease-Bacteria to Milk. 85

is able to make but slight headway in cattle. This would
indicate that the danger of cattle acquiring the infection
from man would in all probability be very slight, but these
experiments offer no answer as to the possibility of trans-
mission from the bovine to the human. Manifestly it is
impossible to solve this problem by direct experiment upon
man except by artificial inoculation, but comparative ex-
periments upon animals throw some light on the question.

Theo. Smith ' and others2 have made parallel experiments
with animals such 'as guinea pigs, rabbits and pigeons, in-
oculated with both bovine and human cultures of this or-
ganism. The results obtained in the case of all animals
tested show that the virulence of the two types was much
different, but that the bovine cultures were much more se-
vere. While of course this does not prove that transmis-
sion from bovine to human is possible, still the importance
of the fact must not be overlooked.

In a number of cases record of accidental infection from
cattle to man has been noted.8 These have occurred with
persons engaged in making post-mortem examinations on
tuberculous animals, and the tubercular nature of the wound
was proven in some cases by excision and inoculation.

In addition to data of this sort that is practically experi-
mental in character, there are also strong clinical reasons
for considering that infection of human beings may occur
through the medium of milk. Naturally such infection
should produce intestinal tuberculosis, and it is noteworthy
that this phase of the disease is quite common in children

1 Smith, Theo., Journ. of Expt. Med., 1898, 3: 451.

2 Dmwiddie, Bull. 57, Ark. Expt. Stat., June, 1899; Ravenel, Univ. of Penn.
Med. Bull , Sept. 1901.

3 Ravenel, Journ. of Comp. Med. & Vet. Arch., Dec. 1897; Hartzell, Journ.
Amer. Med. Ass'n, April 16, 1893.

86 Dairy Bacteriology.

especially between the ages of two and five.1 It is difficult
to determine, though, whether primary infection occurred
through the intestine, for, usually, other organs also be-
come involved. In a considerable number of cases in which
tubercular infection by the most common channel, inhala-
tion, seems to be excluded, the evidence is strong that the
disease was contracted through the medium of the milk, but
it is always very difficult to exclude the possibility of pul-
monary infection.

Tuberculosis as a bovine disease has increased rapidly
during recent decades throughout many portions of the
world. This has been most marked in dairy regions. Its
extremely insidious nature does not permit of an early rec-
ognition by physical means, and it was not until the intro-
duction of the tuberculin test * in 1892, as a diagnostic aid
that accurate knowledge of its distribution was possible.
The quite general introduction of this test in many regions
has revealed an alarmingly large percentage of animals as
affected. In Denmark in 1894 over forty per cent were
diagnosed as tubercular. In some parts of Germany almost
as bad a condition has been revealed. Slaughter-house sta-
tistics also show that the disease has increased rapidly since
1890. In this country the disease on the average is much
less than in Europe and is also very irregularly distributed.
In herds where it gained a foothold some years a£O, often
the majority of animals are frequently infected; many

' Stille, Brit. Med. Journ., Aug. 19, 1899.

2 This test is made by injecting into the animal a small quantity of tuberculin,
which is a sterilized glycerin extract of cultures of the tubercle bacillus. In a
tuberculous animal, even in the very earliest phases of the disease, tuberculin
causes a temporary fever that lasts for a few hours. By taking the temperature
a number of times before and after injection it is possible to readily recognize
any febrile condition. A positive diagnosis is made where the temperature after
inoculation is at least 2.0° F. above the average normal, and where the reaction
fever is continued for a period of some hours.

Relation of Disease-Bacteria to Milk. 87

herds, in fact the great majority, are wholly free from all
taint. The disease has undoubtedly been most frequently
introduced through the purchase of apparently healthy but
incipiently affected animals. Consequently in the older
dairy regions where stock has been improved the most by
breeding, more of the disease exists than among the western
and southern cattle.

Infectiousness of milk of reacting animals. Where the
disease appears in the udder the milk almost invariably con-
tains the tubercle organism. Under such conditions the
appearance of the milk is not materially altered at first,

FIG. 21. Side view of a tuberculous udder, showing extent of swelling in
single quarter.

but as the disease progresses the percentage of fat generally
diminishes, and at times in the more advanced stages where
the physical condition of the udder is changed (Fig. 21),

88 Dairy Bacteriology.

the milk may become "watery"; but the percentage of
animals showing such udder lesions is not large, usually
not more than a few per cent.

On the other hand, in the earlier phases of the disease,
where its presence has been recognized solely by the aid of
the tuberculin test, before there are any recognizable phys-
ical symptoms in any part of the animal, the milk is gener-
all}r unaffected. Between these extremes, however, is found
a large proportion of cases, concerning which so definite
data are not available. The results of investigators on this
.point are conflicting and further information is much de-
sired. Some have asserted so long as the udder itself shows
no lesions that no tubercle bacilli would be present,1 but
the findings of a considerable number of investigators* in-
dicate that even when the udder is apparently not diseased
the milk may contain the specific organism as revealed by
inoculation experiments upon animals. In some cases,
however, it has been demonstrated by post-mortem exam-
ination that discoverable udder lesions existed that were
not recognizable before autopsy was made. In the experi-
mental evidence collected, a varying percentage of reacting
animals were found that gave positive results; and this
number wa? generally sufficient to indicate that the danger
of using milk from reacting animals was considerable, even
though apparently no disease could be found in the udder.

The infectiousness of milk can also be proven by the
frequent contraction of the disease in other animals, such
as calves and pigs which may be fed on the skim milk. The
very rapid increase of the disease among the swine of Ger-

1 Martin, Brit. Med. Journ. 1895, 1:937; Nocard, Les Tuberculoses animates, 1895.

a Bang. Schmidt's Jahrb., 235:22; Hirschberger, Arch. f. klin. Medicin, 1889;

Ernst, Infectiousness of Milk, 1895; Ravenel, Bull. 75, Penn. Dept. Agr, 1901.

Relation of Disease-Bacteria to Milk. 89

many and Denmark,1 and the frequently reported cases of
intestinal infection of young stock also attest the presence
of the organism in milk.

The tubercle bacillus is so markedly parasitic in its hab-
its, that, under ordinary conditions, it is incapable of grow-
ing at normal air temperatures. There is, therefore, no
danger of the germ developing in milk after it is drawn
from the animal, unless the same is kept at practically blood
heat.

Even though the milk of some reacting animals may not
contain the dangerous organism at the time of making the
test, it is quite impossible to foretell how long it will re-
main free. As the disease becomes more generalized, or if
tuberculous lesions should develop in the udder, the milk
may pass from a healthy to an infectious state.

This fact makes it advisable to exclude from milk sup-
plies intended for human use, all milk of animals that re-
spond to the tuberculin test; or at least to treat it in a
manner so as to render it safe. Whether it is necessary to
do this or not if the milk is made into butter or cheese is a
somewhat different question. Exclusion or treatment is
rendered more imperative in milk supplies, because the
danger is greater* with children with whom milk is often a
prominent constituent of their diet, and also for the reason
that the child is more susceptible to intestinal infection
than the adult.

The danger of infection is much lessened in butter or
cheese, because the processes of manufacture tend to dimin-
ish the number of organisms originally present in the milk,
and inasmuch as no growth can ordinarily take place in these
products the danger is minimized. Moreover, the fact that

i Ostertag, Milch Zeit., 22:672.

90 Dairy Bacteriology.

these foods are consumed by the individual in smaller
amounts than is generally the case where milk is used, and
also to a greater extent by adults, lessens still further the
danger of infection.

Notwithstanding this, numerous observers ' especially in
Germany have succeeded in finding the tubercle bacillus in
market butter, but this fact is not so surprising when it is
remembered that a very large fraction of their cattle show
the presence of the disease as indicated by the tuberculin
test, a condition that does not obtain in any large section
in this country.

These observations on the presence of the tubercle bacil-
lus in butter have been questioned somewhat of late2 by
the determination of the fact that butter may contain an
organism that possesses the property of being stained in
the same way as the tubercle organism. Differentiation
between the two forms is rendered more difficult by the
fact that this tubercle-like organism is also capable of pro-
ducing in animals lesions that simulate those of tubercu-
losis, although a careful examination reveals definite differ-
ences. Petri1 has recently determined that both the true
tubercle and the acid-resisting butter organism may be
readily found in market butter.

In the various milk products it has been experimentally
determined that the true tubercle bacillus is able to retain
its vitality in butter for a number of months and in cheese
for nearly a year.

Treatment of milk from tuberculous cows. While it has
been shown that it is practically impossible to foretell
whether the milk of any reacting animal actually contains

» Obermiiller, Hyg. Rund , 1897, p. 712; Petri, Arb. a. d. Kais. Ges. Amte, 1898,
14: 1; Hermann und Morgenrotb, Hyg. Rund., 1898, p. 217.
•Rabinowitsch. Zelt. f. Hyg., 1897, 26: 90.

Delation of Disease-Bacteria to Milk. 91

tubercle bacilli or not, still the interests of public health
demand that no milk from such stock be used for human
food until it has toen rendered safe by some satisfactory
treatment.

1. Heating. By far the best treatment that can be given
such milk is to heat it. The temperature at which this
should be done depends upon the thermal death point of
the tubercle bacillus, a question concerning which there
has been conisderable difference of opinion until very re-
cently. According to the work of some of the earlier in-
vestigators, the tubercle bacillus in its vegetative stage is
endowed with powers of resistance greater than those pos-
sessed by any other pathogenic organism. This work has
not been substantiated loy the most recent investigations
on this subject. In determining the thermal death point
of this organism, as of any other, not only must the tern-
perature be considered, but the period of exposure as well,
and where that exposure is made in milk, another factor
must be considered, viz., the presence of conditions per-
mitting of the formation of a " scalded layer," for as Smith l
first pointed out, the resistance of the tubercle organism
toward heat is greatly increased under these conditions.
If tuberculous milk is heated in a closed receptacle where
this scalded membrane cannot be produced, the tubercle
bacillus is killed at 140° F. in 15 to 20 minutes. These
results which were first determined by Smith, under labora-
tory conditions, and confirmed by Russell and Hastings,9
where tuberculous milk was heated in commercial pasteur-
izers, have also been verified by Hesse.8 A great practical
advantage which accrues from the treatment of milk at

1 Th. Smith. Journ. of Expt. Med., 1899, 4: 217.

2 Russell and Hastings, 18 Rept. Wis. Expt. Stat., 1901.
8 Hesse, Zeit. f. Hyg., 1900, 34:346.

92 Dairy Bacteriology.

140° F. is that the natural creaming is practically unaf-
fected. Of course, where a higher ternperatu re is employed,
the period of exposure may be materially lessened. If
milk is momentarily heated to 185° F., it is sufficient to
destroy the vitality of the tubercle bacillus. This is the
plan practiced in Denmark where all skim milk and whey
must be heated to this temperature before it can be taken
back to the farm, a plan which is designed to prevent the
dissemination of tuberculosis and foot and mouth disease
by means of the mixed creamery by-products. This course
renders it possible to utilize with perfect safety, for milk
supplies, the milk of herds reacting to the tuberculin test,
and as butter of the best quality can be made from cream
or milk heated to even high temperatures,1 it thus becomes
possible to prevent with slight expense what would other-
wise entail a large loss.

2. Dilution. Another method that has been suggested
for the treatment of this suspected milk is dilution with a
relatively large volume ot perfectly healthy milk. It is a
well known fact that to produce infection, it requires the
simultaneous introduction of a number of organisms, and
in the case of tuberculosis, especially that produced by in-
gestion, this number is thought to be considerable. Geb-
hardt9 found that the milk of tuberculous cows, which was
virulent when injected by itself into animals, was innocuous
when diluted with 40 to 100 times its volume of healthy
milk. This fact is hardly to be relied upon in practice, un-
less the proportion of reacting to healthy cows is positively
known.

It has also been claimed in the centrifugal separation of

1 Practically all of the finest butter made in Denmark is made from cream that
has been pasteurized at temperatures varying from 160°-185e F.
» Gebhardt, Virch. Arch., 1890, 119: 12.

Relation of Disease- Bacteria to Milk. 93

cream from milk * that by far the larger number of tubercle
bacilli were, thrown out with the separator slime. Moore*
has shown that the tubercle bacilli in an artificially in-
fected milk might be reduced in this way, so as to be no
longer microscopically demonstrable, yet the purification
was not complete enough to prevent the infection of ani-
mals inoculated with the milk.

Another way to exclude all possibility of tubercular in-
fection in milk supplies is to reject all milk from reacting
animals. This method is often followed where pasteuriza-
tion or sterilization is not desired. In dairies where the
keeping quality is dependent upon the exclusion of bacteria
by stringent conditions as to milking and handling ("sani-
tary " or " hygienic " milk), the tuberculin test is frequently
used as a basis to insure healthy milk.

Foot and mouth disease. The widespread extension of
this disease throughout Europe in recent years has given
abundant opportunity to show that while it is distinctively
an animal malady, it is also transmissible to man, although
the disease is rarely fatal. The causal organism has not
been determined with certainty, but it has been thoroughly
proven that the milk of affected animals possesses infec-
tious properties.8

Hertwig showed the direct transmissibiiity of the disease
to man by experiments made on himself and others. By
ingesting milk from an affected animal, he was able to
produce the symptoms of the disease, the mucous mem-
brane of the mouth being covered with the small vesicles
that characterize the malady. It has also been shown that

J Scheurlen, Arb. a. d. k. Ges. Amte, 1891, 7: 269; Bang, MUch Zeit., 1893, p. 672
2 Moore, Year Book of TJ. S. Dept. Agr., 1895, p. 432.

3Weigeland Noack, Jahres. d. Ges. Med., 1890, p. 642; Weissenberg, Allg. med.
Cent. Zeit., 1890, p. 1; Baum, Arch. f. Thierheilkunde, 1892, 18:16.

94 Dairy Bacteriology.

the virus of the disease may he conveyed in hutter.1 This
disease is practically unknown in this country, although
widely spread in Europe.

There are a number of other hovine diseases such as
anthrax,2 lockjaw,3 and hydrophobia4 in which it has been
shown that the virus of the disease is at times to be found
in the milk supply, but in most cases the secretion of milk
becomes visibly affected, so that the danger of using such
is greatly minimized.

There are also a number of inflammatory udder troubles
known as garget or mammitis which are produced by bac-
terial action. In most of these, the physical appearance
of the milk is so changed, and often pus is present to such
a degree as to give a very disagreeable appearance to the
milk. Pus-forming bacteria (staphylococci and strepto-
cocci) are to be found associated with such troubles.

DISEASES TRANSMISSIBLE TO MAN THROUGH INFECTION OF
MILK AFTER WITHDRAWAL.

Milk is so well adapted to the development of bacteria
in general, that it is not surprising to find it a suitable
medium for the growth of many pathogenic species. While
this statement applies primarily to milk in a sterile condi-
tion, yet in some cases, disease-producing bacteria are even
able to grow in raw milk in competition with the normal
milk bacteria, so that even a slight contamination may
suffice to produce infection.

The diseases that are most frequently disseminated in

i Schneider, Munch, med. Wochenschr., 1893, No. 27; Frohner, Ziet. L Fleisch
u. Milchhygiene, 1891, p. 66.
"Feser, Deutsche Zeit. f. Thiermed., 1880, 6:166.
•Nocard, Bull. Gen., 1885, p. 54.
« Deutsche Viertelsjahr. f. offentL Gesundheitspflege, 1890, 20:444.

Relation of Disease- Bacteria to Milk. 95

this way are typhoid fever, diphtheria, scarlet fever and
cholera, together with the various illy-defined intestinal
troubles of a toxic character that occur in children, es-
pecially under the name of cholera infantum, summer
complaint, etc.

Diseases of this class are not derived directly from ani-
mals because cattle are not susceptible to the same.

Modes of infection. In a variety of ways, however, the
milk may be subject to contaminating influences after it
is drawn from the animal, and so give opportunity for the
deyelopment of disease-producing bacteria. The more im-
portant methods of infection are as follows:

1. Infection directly from a pre-existing case of disease on
premises. Quite frequently a person in the early stage of
a diseased condition may continue at his usual vocation as
helper in the barn or dairy, and so give opportunity for
direct infection to occur. In the so-called cases of " walk-
ing typhoid," this danger is emphasized. Again during
the period of convalescence, a similar opportunity exists
for direct infection. This method functions more fre-
quently in scarlet fever than in typhoid. In some cases
infection has been traced to storage of the milk in rooms
in the house where it became polluted directly by the
•emanations of the patient.1 Among the dwellings of the
lower classes where a single room has to be used in com-
mon this source of infection has been most frequently ob-
served.

2. Infection through the medium of another person. Not
infrequently another individual may serve in the capacity
of nurse or attendant to a sick person, and also assist in
the handling of the milk, either in milking the animals or

1 E. Roth, Deutsche Vierteljahresschr. f. oflentl. Gesundheitspfl., 1890, 22:238.

96 Dairy Bacteriology.

caring for the milk after it has been drawn. Busey and
Kober report twenty-one outbreaks of typhoid fever in
which dairy employees also acted in the capacity of nurses.

3. Pollution of milk utensils. The most frequent method
of infection of cans, pails, etc., is in cleaning them with
water that may be polluted with disease organisms. Often
wells may be contaminated with diseased matter of intes-
tinal origin, as in typhoid fever, and the use of water at
normal temperatures, or even in a luke-warm condition,
give conditions permitting of infection. Intentional adul-
teration of milk with water inadvertently taken from pol-
luted sources has caused quite a number of typhoid out-
breaks.1 Sedgwick and Chapin * found in the Springfield,
Mass., epidemic of typhoid that the milk cans were placed
in a well to cool the milk, and it was subsequently shown
that the well was polluted with typhoid fecal matte*.

4. Pollution of udder of animal by leading in infected
water, or by washing same with contaminated water.
This method of infection would only be likely to occur in
case of typhoid. An outbreak at the University of Vir-
ginia in 18933 was ascribed to the latter cause.

5. Pollution of creamery by-products, skim-milk, etc.
Where the milk supply of one patron becomes infected
with pathogenic bacteria, it is possible that disease may be
disseminated through the medium of the creamery, the in-
fective agent remaining in the skim milk after separation
and so polluting the mixed supply. This condition is more
likely to prevail with typhoid because of the greater toler-
ance of this organism for acids such as would be found in raw

1 S. W. North, London Practitioner, 1889, 43:393.

•Sedgwick and Chapin, Boston Med. & Surg. Journ., 1893, 129:485.

» Dabney, Phila. Med. News, 1893, 68:630.

Relation of Disease- Bacteria to Milk. 97

milk. The outbreaks at Brandon,1 England, in 1893, Cas-
tle Island,2 Ireland, and Marlboro,3 Mass., in 1894, were
traced to such an origin.

While most outbreaks of disease associated with a pol-
luted milk supply originate in the use of the milk itself,
yet infected milk may serve to cause disease even when
used in other ways. Several outbreaks of typhoid fever
have been traced to the use of ice cream where there were
strong reasons for believing that the milk used in the man-
ufacture of the product was polluted.4 Hankin5 details a
case of an Indian confection made largely from milk that
caused a typhoid outbreak in a British regiment.

Although the evidence that milk may not infrequently
serve as an agent in spreading disease is conclusive enough
to satisfactorily prove the proposition, yet it should be borne
in mind that the organism of any specific disease in ques-
tion has rarely ever been found. The reasons for this are
quite the same as those that govern the situation in the
case of polluted waters, except that the difficulties of the
problem are much greater in the case of milk than with
water. The inability to readily separate the typhoid germ,
for instance, from thetiolon bacillus, an organism frequently
found in milk, presents technical difficulties not easily over-
come. The most potent reason of failure to find disease
bacteria is the fact that infection in any case must occur
sometime previous to the appearance of the outbreak. Not
only is there the usual period of incubation, but it rarely
happens that an outbreak is investigated until a number of

1 Welphy, London Lancet, 1804, 2:1085.

2 Brit. Med. Journ., 1894, 1:815.

3 Mass. Bd. Health Kept., 1894, p. 765.

4 Turner, London Practitioner, 1892, 49:141; Munro, Brit. Med. Journ., 1894, 2:829..
» Hankin, Brit. Med. Journ., 1894, 2:613.

7

98 Dairy Bacteriology.

cases have occurred. In this interim the original cause of
infection may have ceased to be operative.

Typhoid fever. With reference to the diseases likely to
to be disseminated through the medium of milk, infected
after being drawn from the animal, typhoid fever is the
most important. The reason for this is due (1) to the wide
spread distribution of the disease; (2) to the fact that the
typhoid bacillus is one that is capable of withstanding
considerable amounts of acid, and consequently finds even
in raw milk containing the normal lactic acid ba'cteria con-
ditions favorable for its growth.1 Ability to grow under
these conditions can be shown not only experimentally, but
there is abundant clinical evidence that even a slight infec-
tion often causes extensive outbreaks, as in the Stamford,
Conn., outbreak in 1895 where 386 cases developed in a few
weeks, 97 per cent of which occurred on the route of one
milkman. In this case the- milk cans were thoroughly and
properly cleaned, but were rinsed out with cold water from
a shallow well that was found to be polluted.

The most common mode of pollution of milk with typhoid
organisms is where the milk utensils are infected in one
way or another. Generally, this arises from the use of pol-
luted water in cleansing the vessels or in intentional water-
ing of the milk. Second in importance is the carrying of
infection by persons serving in the dual capacity of nurse
and dairy attendant.

Cholera. This germ does not find milk so favorable a
nutrient medium as the typhoid organism, because it is
much more sensitive toward the action of acids. Kitasato*

1 Heim (Arb. a. d. Kais. Gesundheitsamte, 1889, 5:303) finds it capable of living
from 20-30 days in milk.
'Kitasato, Arb. a. d. Kais. Gesundheitsamte, 1:470.

Relation of Disease- Bacteria to Milk. 99

found, however, that it could live in raw milk from one to
four days, depending upon the amount of acid present. In
boiled or sterilized milk it grows more freely, as the acid-
producing forms are thereby eliminated. In butter it dies
out in a few days (4 to 5).

On account of the above relation not a large number of
cholera outbreaks have been traced to milk, but Simpson *
records a very striking case in India where a number of
sailors, upon reaching port, secured a quantity of milk. Of
the crew which consumed this, every one was taken ill, and
four out of ten died, while those who did not partake es-
caped without any disease. It was later shown that the
milk was adulterated with water taken from an open pool
in a cholera infected district.

Diphtheria. According to Klein 2 the diphtheria organ-
ism is capable of developing in animals, attacking among
other organs the udder, and so infecting the milk; but
Abbott8 or Vladimirow4 failed to confirm these experi-
ments. There is abundant evidence that the diphtheria
organism is able to grow luxuriantly in milk, even more
so in raw than in sterilized.5

Infection in this disease is more frequently attributable
to direct infection from patient, or indirectly through
attendant.

Scarlet fever. Although it is more difficult to study the
relation of this disease to contaminated milk supplies, be-
cause the causal germ of scarlet fever is not yet known,
yet the origin of a .consider able number of epidemics has

1 Simpson, London Practitioner, 1887, 39:144.

» Klein, 19 Loc. Gov't. Bd. (Gt. Brit.) 1889, 167.

•Abbott, Vet. Mag. 1: 17.

* Vladimirow, Arch. Sci. biol. Inst. Med., St. Petersburg, 1892, p. 84.

*Schottelius and Ellerhorst, Milch Zeit., 1897, pp. 40 and 73.

100 Dairy Bacteriology.

been traced to polluted milk supplies. An outbreak oc-
curred at East Orange,1 New Jersey, a few years ago in
wbich from one to four cases developed in each of sixteen
families. The cause of the outbreak was traced to the son
of the milkman who, soon after convalescence from an at-
tack of the disease, resumed his work as milker.

Diarrhoeal diseases. Milk not infrequently acquires the
property of producing diseases of the digestive tract by
reason of the development of various bacteria that form
more or less poisonous by-products. These troubles occur
most frequently during the summer months, especially with
infants and children, as in cholera infantum and summer
complaint. The higher mortality of bottle-fed infants2 in
comparison with those that are nursed directly is explicable
alone on the theory that cows' milk is the carrier of the
infection, because in many cases it is not consumed until
there has been ample time for the development of organisms
in it. As a cause of sickness and death these diseases ex-
ceed in importance all other specific diseases previously
referred to.

The cause of these troubles is not to be ascribed to any
specific kind of bacteria, but there are undoubtedly a large
number of organisms which are able to develop toxic sub-
stances in food products, especially in milk. In some cases
it appears that the development of the poisonous products
takes place in the intestines after the food is ingested. The
origin of these bacteria is in all probability due to the in-
troduction of dirt and filth that find their way into the
milk at the time of milking. Fliigge3 has pointed out the

i Boston Med. Journ., 1897, 136: 44.
a Baginsky, Hyg. Rund., 1895, p. 176.
•Flttgge, Zeit. f. Hyg., 17: 272.

Relation of Disease-Bacteria to Milk. 101

fact that certain peptonizing species which are frequently
found in milk possess a toxic property for lower animals.
Ptomaine poisoning. Many cases of poisoning from food
products are also reported with adults. These are due to
the formation of various toxic products, generally pto-
maines, that are produced as a result of infection of foods
by different bacteria. One of these substances, tyrotoxicon,
was isolated by Vaughan J from cheese and various other
products of milk, and found to possess the propert}' of pro-
ducing ^symptoms of poisoning similar to those that are
noted in such cases. He attributes the production of this
toxic effect to the decomposition of the elements in the
milk induced by putrefactive forms of bacteria that develop
where milk is improperly kept.2 Often outbreaks of this
character3 assume the proportions of an epidemic, where a
large number of persons use the tainted food.

>Zeit. f. physiol. Chemie, 10:146; 9 Intern. Hyg. Cong. (London), 1891, p. 118.
a Vaughan and Perkins, Arch. f. Hyg., 27:308.

3 Newton and Wallace (Phila. Med. News, 1887, 50:570) report three outbreaks at
Long Branch, N. J., two of which occurred in summer hotels.

CHAPTER VI.

PRESERVATION OF MILK FOR COMMERCIAL
PURPOSES.

IN Chapter III it has been shown how milk becomes
contaminated with various kinds of bacteria which find in
this liquid most favorable conditions for development. The
result of this contamination is that the period during which
milk has a commercial value for food purposes, either in
the form of milk, or milk products, such as butter and
cheese, is greatly lessened, thereby causing losses of consid-
erable economic importance.

Moreover, it has been further shown (Chapter Y) that
this food product which is so admirably adapted to serve
as food may become infected with disease-producing organ-
isms, and so be the means of disseminating contagion.

From these two points of view, therefore,

1. the economic, as shown by the keeping quality of the
milk, and

2. the hygienic, as shown by its possibility to spread dis-
ease, it is highly important that means should be adopted,
if possible, that will result in improving the keeping qual-
ity so as to diminish these losses, and at the same time in-
sure freedom from bacteria capable of developing disease.
Inasmuch as the fermentative changes which ordinarily
occur, and which lessen the commercial life of the milk as
a food, depend entirely on the development of living or-
ganisms that may find their way into the milk, an improve-
ment in the condition of the milk may be secured (1) by
excluding bacterial life so far as practicable, at the time

Preservation of Milk. 103

the milk is drawn, and subsequently holding the milk at
temperatures unfavorable to the multiplication of the rela-
tively few organisms that do gain access; or (2) by remov-
ing those organisms wholly or in part after they have once
gained access to the milk. If all are not eliminated, it
then becomes necessary to keep the milk under such con-
ditions as to check the growth of those which are not re-
moved.

Preservation by exclusion. The first method is followed
in many dairies that supply high-grade milk for city deliv-
ery. The so-called " sanitary " or u hygienic " milk is usu-
ally a milk that has been handled in such a way as to pre-
vent the introduction of most bacteria that under ordinary
conditions would find their way into the same. The merits
of a milk of this character in comparison with one pre-
served by means of other methods, as pasteurization, is
a question concerning which there has been much discus-
sion. When it is considered, as will be shown later, that,
methods of preservation can be successfully applied thafc
will not apparently change the chemical and physical prop-
erties of milk, it is an open question that must be decided
in each case whether exclusion of bacteria can be as eco-
nomically and efficiently performed as the destruction of
the living organisms by heat. Certain it is that the pro-
cess of exclusion must be confined to dairies that are under
individual control. The impossibility of exercising ade-
quate control with reference to the milking process and the
care of the milk immediately thereafter, when the same is
produced on different farms, is evident.

In enhancing the keeping quality of milk by removing
the bacteria it is necessary to do so without in any way
materially interfering with the nutritive qualities of the

lOi Dairy Bacteriology.

fluid. The different methods that have been proposed to
accomplish this result depend upon the removal of the con-
tained organisms by mechanical means, or their destruc-
tion in the milk by means of either chemical or physical
processes. In removing the bacteria two means have been
more or less extensively employed, as filtration and cen-
trifugal force.

Filtration Of milk. Straining milk through cloth or
wire strainers has always been used as a means of cleaning
milk from dirt and foreign matter; but it is quite evident
that the removal of such material can only diminish the
germ content of the milk to the extent that bacteria would
adhere to such coarse particles. The individual organisms
floating in milk are capable of passing the finest strainer,
and consequently, such processes are more accurately meth-
ods of cleaning and purifying the milk rather than meth-
ods that enhance the keeping quality.

Along somewhat similar lines are the various methods
of filtration that have been devised. The use of germ-
proof filters, such as the Pasteur or Berkefeld type, are
inadmissible with milk, because the pores of these filters
are so fine as to hold back practically all suspended matter,
fat and casein as well as the bacteria.

For a number of years, gravel, sand or quartz filters
have been employed for the double purpose of cleaning
milk and preserving^ it. Several different types of these
filters are or have been in use. The most satisfactory are
built in several sections so as to permit of read}r cleansing,
a process which must be most thoroughly carried out with
apparatus of this kind. Bolle of Berlin washes his filters
first with boiling water, then dilute hydrochloric acid, and
finally, with water until all trace of acid is removed. The

Preservation of Milk. 105

Copenhagen Dairy Co. sterilize their gravel filters by high
heat. In other cases lime water is used in cleansing. Fil-
ters of this type remove practically all dirt and a consider-
able proportion of the contained bacteria, but they are in-
tended more to clean the milk than enhance its keeping
quality. Woodfiber (cellulose) has also been tested as a
filtering substance with success.

Centrifugal cleaning: of milk. The familiar coating of
slime and dirt that collects on the inner face of the sepa-
rator bowl shows that centrifugal force can be successfully
used in cleaning or clarifying milk. While the ordinary
types of cream separators are able to remove this material
in a satisfactory way, special machines have been devised
for this particular purpose. A bacteriological examination
of separator slime shows it to be teeming with myriads of
organisms, and the rapid decomposition which it undergoes
is evidence of its high germ content, but there is practi-
cally little or no improvement in the keeping quality of
milk that has been treated in this way. This is in part
due to the fact that bacteria reproduce so rapidly that a
marked variation in numbers is soon obscured by relatively
more rapid growth. Eckles and Barnes1 find that from
37 to 56 per cent of the total number of bacteria present in
milk are thrown out lay passing milk through separators.
Where milk is cleaned in this way, the cream and skim
milk is generally mixed again immediately, but the pas-
sage of cream through the separator bowl tends to break
down- the size of the normal fat-globule clusters and so
lessen the consistency of the product. Such a diminution
in " body " diminishes materially the ease with which cream
can be whipped.

i Eckles and Barnes, Bull. No. 59 Iowa Expt, Stat., Aug. 1901.

106 Dairy Bacteriology.

Chemical preservatives. Numerous attempts have been
made to find some chemical substance that could be added
to milk which would preserve it without interfering with
its nutritive properties, but as a general rule a substance
that is toxic enough to destro}" or inhibit the growth of
bacterial life exerts a prejudicial effect on the tissues of the
body. The use of chemicals, such as carbolic acid, mercury
salts and mineral acids, that are able to entirely destroy all
life, is of course excluded, except when milk is preserved for
analytical purposes; but a number of milder substances
are more or less extensively employed, although the statutes
of practically all states forbid their use.

The substances so used may be grouped in two classes:

1. Those that unite chemically with certain b3'-products
of bacterial growth to form inert substances. Thus bi-
carbonate of soda neutralizes the acid in souring milkr
although it does not destroy the lactic acid bacteria.

2. Those that act directly upon the bacteria in milk, re-
straining or inhibiting their development. The substances-
most frequently utilized are salicylic acid, formaldehyde
and boracic acid. These are nearly always sold to the
milk handler, under some proprietary name, at prices
greatly in excess of what the crude chemicals could be
bought for in the open market. Formaldehyde bas been
widely advertised of late, but its use is fraught with the
greatest danger, for it practically renders insoluble all al-
buminous matter and its toxic effect is greatly increased in
larger doses.

These substances are generally used by milk handlers
who know nothing of their poisonous action, and although
it ma}7 be possible for adults to withstand their use in
dilute form, without serious results, yet their addition to

Preservation of Milk. 107

general milk supplies that may be used by children is lit-
tle short of criminal. The sale of these preparations for
use in milk finds its only outlet with those dairymen who
are anxious to escape the exactions that must be met by
all who attempt to handle milk in the best possible man-
ner. Farrington has suggested a simple means for the
detection of preservalin (boracic acid).1 When this sub-
stance is added to fresh milk, it increases the acidity of
milk without affecting its taste. As normal milk tastes
sour when it contains about 0.3 per cent lactic acid, a milk
that tests as much or more than this without tasting sour
has been probably treated with this antiseptic agent.

Physical methods of preservation. Methods based upon
the application of physical forces are less likely to injure
the nutritive value of milk, and are consequently more
effective, if of any value whatever. A number of methods
have* been tried more or less thoroughly in an experimental
way that have not yet been reduced to a practical basis, as
electricity, use of a vacuum, and increased pressure.2 Con-
densation has long been used with great success, but in this
process the nature of the milk is materially changed. The
keeping quality in condensed milk often depends upon the
action of another principle, viz., the inhibition of bacterial
growth by reason of the concentration of the medium.
This condition is reached either by adding sugar and so in-
creasing the soluble solids, or by driving off the water by
evaporation, preferably in a vacuum pan. Temperature
changes are, however, of the most value in preserving milk,
for by a variation in temperature all bacterial growth can
be brought to a standstill, and under proper conditions
thoroughly destroyed.

1 Farrington, Journ. Amer. Chem. Soc., Sept., 1896.
»Hite, Bull. 58, West Va. Expt. Stat, 1899.

108

Dairy Bacteriology.

Use of low temperatures. The effect of chilling or
rapid cooling on the keeping quality of milk is well known.
When the temperature of milk is lowered to the neighbor-
hood of 45° F., the development of bacterial life is so slow
as to materially increase the period that milk remains
sweet. Within recent years, attempts have been made to
preserve milk so that it c'ould be shipped long distances by
freezing the product, which in the form of milk-ice could
be held for an indefinite period without change.1 A modi-
fication of this process known as Casse's system has been
tried on an extensive scale in Copenhagen and in several
places in Germany. This consists of adding a small block
of milk-ice (frozen milk) to large cans of milk (one part to
about fifty of milk) which may or may not be pasteur-
ized.2 This reduces the
temperature so that
the milk remains sweet
considerably longer.
Such a process permits
of the shipment of milk
for long distances with
safety. It is reported
that London receives
milk from Denmark and
Sweden that is treated
in this waj7.

Use of high tempera-

tiif ae TJ™4- 1 1 FIG. 22. Microscopic appearance of nor-

tures. Heat has long ^ m|lk showingpthe fat.globules aggre.
been used as a preserv- gated in clusters.
ing agent. Milk has been scalded or cooked to keep
it from time immemorial. Heat may be used at differ-

i Milch Zeit,, 1895, No. 9.
a Ibid., 1897, No. 83.

Preservation of Milk.

109

ent temperatures, and when so applied exerts a varying
effect, depending upon temperature employed. All meth-
ods of preservation by heat rest, however, upon the applica-
tion of the heat under the following conditions:

1. A temperature above the maximum growing-point
(105°-115° F.) and below the thermal death-point (130°-
140° F.) will prevent further growth, and consequently
fermentative action.

2. A temperature above the thermal death-point destroys
bacteria, and thereby stops all changes. This temperature
varies, however, with the condition of the bacteria, and for
spores is much higher than for vegetative forms.

Attempts have been made to employ the first principle
in shipping milk by rail, viz., prolonged heating above

growing temperature,
but when milk is so
heated, its physical ap-
pearance is changed.1
The methods of heating
most satisfactorily used
are known as steriliza-
tion and pasteurization,
in which a degree of
temperature is used ap-
proximating the boiling
and scalding points re-
spectively.

Effect of heat on milk.

When milk is subjected
to the action of heat, a
number of changes in its
physical and chemical properties are to be noted.

FIG. 23. Microscopic appearance of milk
heated above 140° F., showing the homo-
geneous distribution of fat-globules. The
physical change noted in comparison with
Fig. 22 causes the diminished consistency of
pasteurized cream.

i Bernstein, Milch Zeit., 1894, pp. 184, 200.

110 Dairy Bacteriology.

1. Diminished "body" When milk, but more especially
cream, is heated to 140° F. or above, it becomes thinner in
consistency or "body,1' a condition which is. due to a
change in the grouping of the fat globules. In normal
milk, the butter fat for the most part is massed in micro-
scopic clots as shown in Fig. 22. When exposed to a tem-
perature of 1400 F. or above, these fat-globule clots break
down, and the globules become homogeneously distributed
as in Fig. 23. Under these conditions milk does not cream
readily. When cream itself is so heated, its consistency is
materially reduced, giving the impression that it contains
a less per cent of butter fat. These changes seriously af-
fect the general adoption of heat as a means of preserving
milk for ordinary market use, but fortunately this defect
can be overcome.

2. Cooked taste. If milk is heated to 160° F., it acquires
a cooked taste that becomes more pronounced as the tem-
perature is further raised. Milk so heated develops on its
surface a pellicle or " skin.1' The cause of this change in
taste is not well known. Usually it has been explained
as being produced by changes in the nitrogenous elements
in the milk, particularly in the albumen. Recently, Thoer-
ner ! has pointed out the coincidence that exists between
the appearance of a cooked taste and the loss of certain
gases that are expelled by heating. He finds that the
milk heated in closed vessels from which the gas cannot
escape has a much less pronounced cooked flavor than if
heated in an open vessel. The so-called " skin " on the
surface of heated milk is not formed when the milk is
heated in a tightly-closed receptacle. By some 8 it is as-
serted that this layer is composed of albumen, but there is

.. » Thoerner, Chem. Zeit., 18: &45.
8 Snyder, Chemistry of Dairying, p. 59.

Preservation of Milk. Ill

evidence to show that it is modified casein due to the rapid
evaporation of the milk serum at the surface of the milk.

3. Digestibility. Considerable difference of opinion
has existed in the minds of medical men as to the relative
digestibility of raw and heated milks. A considerable
amount of experimental work has been done by making
artificial digestion experiments with enzyms, also diges-
tion experiments with animals, and in a few cases with
children. The results obtained by different investigators
are quite contradictory, although the preponderance of evi-
dence seems to be in favor of the view that heating does
impair the digestibility of milk, especially if the tempera-
ture attains the sterilizing point.1 It has been observed
that there is a noteworthy increase in amount of rickets,9
scurvy and marasmus in children where highly-heated milks
are employed. These objections do not obtain with refer-
ence to milk heated to moderate temperatures, as in pas-
teurization, although even this lower temperature lessens
slightly its digestibility. The successful use of pasteurized
milks in children's hospitals is evidence of its usefulness.

4- Fermentative changes. The normal souring change in
milk is due to the predominance of the lactic acid bacteria,
but as these organisms as a class do not possess spores,
they are readily killed when heated above the thermal death-
point of the developing cell. The destruction of the lactic
forms leaves the spore-bearing types possessors of the field,
and consequently the fermentative changes in heated milk

1 Doane and Price (Bull. 77, M«d. Expt. Stat., Aug. 1901) give quite a full resumS
of the work on this subject in connection with rather extensive experiments
made by them on feeding animals with raw, pasteurized and sterilized milks.

a Kickets is a disease in which the bones lack sufficient mineral matter to give
them proper firmness. Marasmus is a condition in which the ingested food seems
to fail to nourish the body and gradual wasting away occurs.

112 Dairy Bacteriology.

are not those that usually occur, but are characterized by
the curdling of the milk from the action of rennet enzyms.
5. Action of rennet. Heating milk causes the soluble lime
salts to be precipitated, and as the curdling of milk by ren-
net (in cheese-making) is dependent upon the presence of
these salts, their absence in heated milks greatly retards
the action of rennet. This renders it difficult to utilize
heated milks in cheese-making unless the soluble lime salts
are restored, which can be done by adding solutions of cal-
cium chlorid.

Sterilization. As ordinarily used in dairying, steriliza-
tion means the application of heat at temperatures approxi-
mating, if not exceeding, 212° F. It does not necessarily
imply that milk so treated is sterile, i. e., germ-free; for,
on account of the resistance of spores, it is practically im-
possible to destroy entirely all these hardy forms. If milk
is heated at temperatures above the boiling point, as is
done where steam pressure is utilized, it can be ren-
dered practically germ-free. Such methods are employed
where it is designed to keep milk sweet for a long period
of time. The treatment of milk by sterilization has not
met with any general favor in this country, although it has
been more widely introduced abroad. In most cases the
process is carried out after the milk is bottled; and consid-
erable ingenuity has been exercised in the construction of
devices which will permit of the closure of the bottles after
the sterilizing process has been completed. Milks heated
to so high a temperature have a more or less pronounced
boiled or cooked taste, a condition that does not meet with
general favor in this country. The apparatus suitable for
this purpose must, of necessity, be so constructed as to with-
stand steam pressure, and consequently is considerably

Preservation of Milk. 113

.more expensive than that required for the simpler pasteur-
izing process.

Pasteurization. In this method the degree of heat used
ranges from 140° to 185° F. and the application is made for
only a limited length of time. The process was first exten-
sively used by Pasteur (from whom it derives its name) in
combating various maladies of beer and wine. Its import-
ance as a means of increasing the keeping quality of milk
was not generally recognized until a few years ago; but the
method is now growing rapidly in favor as a means of pre-
serving milk for commercial purposes. The method does
not destroy all germ-life in milk; it affects only those or-
ganisms that are in a growing, vegetative condition; but if
the same is quickly cooled, it enhances the keeping quality
very materially. It is unfortunate that this same term is
used in connection with the heating of cream as a prepara-
tory step to the use of pure cultures in cream-ripening in
butter-making. The objects to be accomplished vary mate-
rially and the details of the two processes are dso quite dif-
erent. The experiments of Bitter 1 indicate that when stored
at 86° F., properly pasteurized milk will remain sweet from
six to eight hours longer than raw milk; at 77° F., ten
hours; at 73° F., twenty hours; and at 58° F., from fifty to
seventy hours. This enhances the keeping quality enough
so that it serves all practical purposes.

While pasteurizing can be performed on a small scale by
the individual, the process can also be adapted to the com-
mercial treatment of large quantities of milk. The appa-
ratus necessary for this purpose is not nearly so expensive
as that used in sterilizing, a factor of importance when
other advantages are considered. In this country pasteur-

' Bitter, Zeit. f. Hyg., 1890, 17:272.
8

114: Dairy Bacteriology.

ization has made considerable headway, not only in sup-
plying a milk that is designed to serve as children's food,
but even for general purposes.

Conditions that determine pasteurizing limits. Consid-
erable latitude with reference to temperature limits is per-
missible in pasteurizing, but there are certain conditions
which should be met, and these, in a sense, fix the limits
employed. They are as follows:

1. Physical requirement. Inasmuch as it is undesirable
to have any material change in taste and appearance in
pasteurized milk from that normally found in the raw pro-
duct, the pasteurizing temperature should be limited to the
degree of heat that can safely be employed without any
danger of imparting a cooked or scalded flavor to milk. If
the exposure is made for any considerable period of time,
say fifteen to twenty minutes, this change in taste appears
to be quite permanent when the milk is heated to 158° F.
This condition, therefore, determines the maximum limit
that should be used in pasteurizing, if one is to avoid
the production of a cooked flavor. Even below this tem-
perature a slight change in flavor occurs, although it dis-
appears upon chilling the milk- Where access of air is
excluded during heating, this cooked taste does not develop
so markedly.

2. Biological requirement. To be of value in increasing
the keeping quality of milk and to insure freedom from
disease bacteria, it is necessary, in all cases, to exceed the
thermal death-point of at least the actively developing bac-
teria in the milk. For most bacteria this limit is constant
and quite sharply defined, ranging from 130° to 140° F.
where the exposure is made for ten minutes. Where ex-
posed for a briefer period of time, the temperature limit is

Preservation of Milk. 115

necessarily higher. The organism that is invested with most
interest in this connection is the tubercle bacillus. On ac-
count of its more or less frequent occurrence in milk (see
p. 87) and its reputed high powers of resistance, it may
well be taken as a standard in pasteurizing.

Thermal death limits of tubercle bacillus. Concerning
the exact temperature at which this germ is destroyed there
is considerable difference of opinion. Part of this arises
from the inherent difficulty in determining exactly when
the organism is killed (due to its failure to grow readily on
artificial media), and part from the lack of uniform condi-
tions of exposure. The standards that previously have
been most generally accepted are those of De Man,1 who
found that thirty minutes exposure at 149° F., fifteen min-
utes at 155° F., or ten minutes at 167° F., sufficed to de-
stroy this germ.

More recently it has been conclusively proven,2 and these
results confirmed by different investigators,3 that if tuber-
culous milk is heated in closed receptacles where the
scalded surface pellicle does not form, the vitality of this
disease germ is destroyed at 140° F. in a brief period (15 to 20
minutes). If the conditions of heating are such that the
surface of the milk is exposed to the air, the resistance of
bacteria is greatly increased. When heated in open vessels
Smith found that the tubercle organism was not killed in
some cases where the exposure was made for at least an
hour. Russell and Hastings 4 have shown an instance where
the thermal death-point of a micrococcus isolated from
pasteurized milk was increased 12.5° F., by heating it under

»De Man, Arch. f. Hyg., 1893, 18:133.

aTh. Smith, Journ. of Expt. Med., 1899, 4: 217.

•Russell and Hastings, 17 Kept. Wis. Expt. Stat., 1900, p. 147.

4 Russell and Hastings, 18 Kept. Ibid., 1901.

116 Dairy Bacteriology.

conditions that permitted of the formation of the scalded
layer. This is a point of great practical importance in the
treatment of milk and necessitates the use of machinery
that will prevent the formation of this surface film. It
follows, therefore, from these results that a temperature of
140° F. can be used if advisable, although higher temper-
atures are not inadmissible.

Sanitary advantages of pasteurized milk. Not only does
pasteurized milk keep longer and is also free from specific
disease bacteria, but its use has been of utmost importance
in checking infant mortality from diarrhoeal disturbances.
This is shown in the diminished death rate in children's
hospitals, and is again exemplified on a large scale in the
results that have been obtained in New York City through
the liberality of Nathan Strauss, who, for several years,
has furnished the poor children of that city with pasteurized
milk. Since the introduction of this milk, the death rate
of those under five years of age has dropped over ten per
thousand living persons, a condition which is explicable
in large measure to the use of a relatively germ-free milk.

Pasteurized milk should be consumed within twenty-four
hours if it is used by children. If left under conditions
favorable to germination, bacterial growth will go on, and
it has been shown that the type of fermentation produced
may sometimes be deleterious.1

High vs. low temperature pasteurization. The limit
which has been most generally employed has been the
maximum at which a cooked flavor did not appear, and in
practice this has been 155° F. for a period of exposure of
twenty minutes. Under these conditions, pasteurization
is efficiently and thoroughly performed, but the applica-
tion of this degree of heat to milk results in a diminution

» Fltlgge, Zeit. f. Hyg., 1894, 17: 272.

Preservation of Milk. 117

of creaming property, and especially in cream of a marked
decrease in thickness. Both of these conditions seriously
interfere with the general extension of pasteurization, be-
cause the consumer does not like a milk on which the
cream does not rise thoroughly. The reasons which under-
lie this physical change have already been noted (p. 109),
and it should be further observed that if milk is not
pasteurized at a temperature exceeding 140° F., this
change in condition does not obtain. Milk heated to this
temperature satisfactorily fills the biological requirement, in
that the vegetating forms of the acid-producing as well as
the disease bacteria are destroyed. The consequence is
that the keeping quality of such milk is practically as good
as if it was heated to a temperature of 155° F. The appli-
cation of this temperature results in the preparation of a
milk or cream that more closely approximates the condi-
tion of the normal product, while at the same time such
milk possesses practically all of the advantages that are
found in that heated to a higher temperature.

Bacteriological studies. The following bacteriological
studies as to the effect which a variation in temperature
exerts on bacterial life in milk are of importance as indi-
cating the proper temperature limits to be selected. In
the following table the exposures were made for a uniform
period (20 minutes):

The bacterial content of milk heated at different temperatures.

Number of bacteria per cc. in milk.

45°C. 50°C. 55°C. 60°C. 65°C. 70°C.

Unheated 113°F. 122°F. 131°F. 140°F. 149°F. 158°F.

Series I. 2,895,000 1,260,000 798,000 82,000 5,770 3,900

Series II. 750,000 665,000 262,400 201,000 950 700 705

Series III. 1,350,000 1,100,000 260.000 215,000 575 610 650

Series IV. 1,750,000 • 87,3ftO 4,000 3,500 3,600

118 Dairy Bacteriology.

It appears from these results that the most marked de-
crease in temperature occurs at 140° F. (60° C.). At 113° F.
(45° C.) no marked diminution was noted; at 122° F. (50° C.)
many cells were killed, but the larger part of them were
not killed until a temperature of 140° F. (60° C.) was
reached. It should also be observed that an increase in
heat above this temperature did not materially diminish the
number of organisms present, indicating that those forms
remaining were in a spore or resistant condition. It was
noted, however, that the developing colonies grew more
slowly in the plates made from the highly heated milk,
showing that their vitality was injured to a greater ex-
tent.

Applicability to general use. This method of low tem-
perature pasteurization has now been tried under practical
conditions for a sufficient period to determine its utility as a
means of preserving general market milk. The fact that
it does not modify in any essential particular the normal
characters of milk is a point much in its favor. Enhance-
ment in keeping quality and freedom from disease organ-
isms are factors of such value that they readily commend
such milk to the general consumer. With the improve-
ments that have been made in pasteurizing machinery, it
is now possible to handle considerable quantities and so
reduce very much the cost of treatment per gallon. This
method is especially applicable to the treatment of cream.
The extreme perishable nature of this milk product makes
it imperative that it should be handled in such a way as to
check as far as possible germ growth, and this can be
readily accomplished when the same is pasteurized and
kept at low temperatures. The higher intrinsic value of
this product lessens the relative cost of operation per unit
of volume.

Preservation of Milk. 119

Within the last few years this system has been quite
widely introduced into a number of cities, and the results
obtained are, on the whole, successful. As a system for
general use it does not meet with nearly as much opposition
as is offered to the use of the higher limits.

One marked advantage accruing to this system is its
general applicability. Milk can be pasteurized where it is
produced under a single management as in the individual
dairy, or the product of several patrons can be treated to-
gether, as in a factory. The fresher and better the milk is,
though, the more suitable it is for pasteurizing. Therefore,
while it is possible to somewhat improve milk that has
been collected for some hours (12 to 24) if it is properly
pasteurized, still better results will be obtained if the treat-
ment is given nearer the animal. Under practical condi-
tions, however, pasteurizing near the place of consumption
has some advantages and is preferable, if it is possible to
transport the raw material quickly from the place of pro-
duction.

For the preparation of high-grade milk supplies it may
be said that either the elimination of the bacteria by pas-
teurization, or preventing their gaining access to the milk,
as in sanitary dairies, is the most feasible and successful
way to deal with this question.

Restoration of "body" of pasteurized cream. The ac-
tion of heat causes the tiny groupings of -fat globules in
normal milk (Fig. 22) to break up, and with this change,
which occurs in the neighborhood of 140° F., the consist-
ency of the liquid is diminished, notwithstanding the fact
that the fat-content remains unchanged. Babcock and
the writer * devised the following " cure " for this apparent

1 Babcock and Russell, Bull. 54, Wis. Expt. Stat., also 13 Eept. Wis. Expt. Stat.
1896, p. 81.

120

Da iry Ba cier ioloyy.

defect. If a strong solution of cane sugar is added to
freshly slacked lime and the mixture allowed to stand, a
clear fluid can be decanted off. The addition of this alka-
line liquid, which is called u viscogeu," to pasteurized cream
in proportions of about one part of sugar-lime solution to
100 to 150 of cream, restores the consistencyof the cream, as
it causes the fat globules to cluster together in small groups.
The relative viscosity of creams can easily be determined
by the following method (Fig. 24):

A B

FIG. 24. Relative consistency of pasteurized cream before (A) and after (B)
treatment with viscogen as shown by rate of flow down inclined glass plate.

Take a perfectly clean piece of glass (plate or picture
glass is preferable, as it is less liable to be wavy). Drop on

Preservation of Milk. 121

one edge two or three drops, of cream at intervals of an
inch or so. Then incline piece of glass at such an angle
as to cause the cream to flow down surface of glass. The
cream, having the heavier body or viscosity, will move more
slowly. If several samples of each cream are taken, then
the aggregate lengths of the different cream paths may be
taken, thereby eliminating slight differences due to condi-
tion of glass.

Pasteurizing details. While the pasteurizing process is
exceedingly simple, yet, in order to secure the best results,
certain conditions must be rigidly observed in the treat-
ment before and after the heating process.

It is important to select the best possible milk for pas-
teurizing, for if the milk has not been milked under clean
conditions, it is likely to be rich in the spore-bearing bac-
teria. Old milk, or milk that has not been kept at a low
temperature, is much richer in germ-life than perfectl}r
fresh or thoroughly chilled milk.

The true standard for selecting milk for pasteurization
should be to determine the actual number of bacterial
spores that are able to resist the heating process, but this
method is impracticable under commercial conditions.

The following method, while only approximate in its re-
sults, will be found helpful: Assuming that the age or
treatment of the milk bears a certain relation to the pres-
ence of spores, and that the acid increases in a general way
with an increase in age or temperature, the amount of acid
present may be taken as an approximate index of the suit-
ability of the milk for pasteurizing purposes. Biological
tests were carried out in the author's laboratory * on milks
having a high and low acid content, and it was shown that

i Siiockley, Thesis, Univ. of Wis., 1896.

122

Dairy Bacteriology.

THERMAL
DEATH POINT

MAXIMUM
GROWTH POINT
BLOOJ) HEAT-

MINIMUM

'GROWTH POINT

FIG. 25. Diagram showing tem-
perature changes in pasteurizing,
and the relation of same to bac-
terial growth.

Shaded zone represents limits
of bacterial growth, 50°-109° F.
(10°-43° C.), the intensity of shad-
ing indicating rapidity of develop-
ment. The solid black line shows
temperature of milk during the
process. The necessity for rapid
cooling is evider.t as the milk fal's
in temperature to that of growing
zone.

Preservation of Milk. 123

the milk with the least acid was, as a rule, the freest from
spore-bearing bacteria.

This acid determination can be made at the weigh-can
by employing the Farrington alkaline tablet which is used
in cream-ripening. Where milk is pasteurized under gen-
eral creamery conditions, none should be used containing
more than 0.2 per cent acidity. If only perfectly fresh milk
is used, the amount of acid will generally be about 0.15 per
cent with phenolphthalein as indicator.

Emphasis has already been laid on the selection of a
proper limit of pasteurizing (p. 114). It should be kept
constantly in mind that the thermal death-point of any
organism depends not alone on the temperature used, but on
the period of exposure. With the limits given, 140° to 155°
F., it is necessary to expose the milk for not less than fif-
teen minutes. If a higher heat is employed (and the
cooked flavor disregarded) the period of exposure may be
curtailed.

Chilling the milk. It is very essential in pasteurizing
that the heated milk be immediately chilled in order to
prevent the germination of the resistant spores, for if ger-
mination once occurs, growth can go on at relatively low
temperatures.

The following experiments by Marshall ' are of interest
as showing the influence of refrigeration on germination
of spores :

Cultures of organisms that had been isolated from pas-
teurized milk were inoculated into bouillon. One set was
left to grow at room temperature, another was pasteurized
%and allowed to stand at same temperature, while another
heated set was kept in a refrigerator. The unheated cul-

» Marshall, Mich. Expt. Stat., Bull. 147, p. 47.

124 Dairy Bacteriology.

tares at room temperature showed evidence of growth in
thirty trials in an average of 26 hours; 29 heated cultures
at room temperature all developed in an average of 50
hours, while the heated cultures kept in refrigerator showed
no growth in 45 days with but four exceptions.

After the milk is pasteurized, it must of necessity be
stored and handled in germ-free receptacles. All utensils,
such as cans, dippers, bottles, etc., must be thoroughly
sterilized. For this purpose a sterilizing oven should be
had which is fitted with steam. Material of this sort, after
being thoroughly cleansed, should be steamed for one-half
to three-quarters of an hour. Sterilized bottles should be
kept protected from dust until they are used.

Bottling and handling: the product. In bottling the
product it is necessary to keep the milk protected from re-
infection. It may be bottled from a large can with a bottom
faucet, or, on a large scale, with commercial bottling ma-
chines that fill several bottles at once. If " viscogen " is
added to restore consistency of cream, it should be done
before bottling, but not before the cream is thoroughly
cooled. The best bottles for the purpose are those that
have a plain pulp cap. All metal fastenings or stoppers
are dirt-catchers and are likely to get out of order. It is
our practice to heat pulp caps in paraffin, thereby render-
ing them more pliable and at the same time sterilizing
them. Bottles sealed with hot caps in this way are tightly
closed.

In delivering pasteurized products, it is always neces-
sary to use care in handling to prevent the cream and milk
from being warmed up, and thus inciting into activity the
latent spores.

Preservation of Milk. 125

Pasteurizing: apparatus. The problems to be solved in
the pasteurization of milk and cream designed for direct
consumption are so materially different from those where
butter is to be made that the type of machinery best adapted
to each purpose is quite different. The equipment neces-
sary for the first purpose may be divided into two general
classes:

1. Apparatus of limited capacity designed for family use.

2. Apparatus of sufficient capacity to pasteurize on a
commercial scale.

Domestic pasteurizers. In pasteurizing milk for indi-
vidual use, it is not desirable to treat at one time more than
will be consumed in one day; hence an apparatus holding
a few bottles will suffice. In this case the treatment can
best be performed in the bottle itself, thereby lessening the
danger of infection. Several different types of pasteurizers
are on the market; but special apparatus is by no means
necessary for the purpose. The process can be efficiently
performed by any one with the addition of an ordinary dairy
thermometer to the common utensils found in the kitchen.
Fig. 26 indicates a simple contrivance that can be readily
arranged for this purpose.

The following suggestions indicate the different steps of
the process:

1. Use only fresh milk.

2. Place milk in clean bottles or fruit cans, filling to a
uniform level, closing bottles tightly with a cork or cover.
If pint and quart cans are used at the same time, an inverted
bowl will equalize the level. Set these in a flat-bottomed
tin pail and fill with warm water to same level as milk.
An inverted pie tin punched with holes will serve as a stand
on which to place the bottles during the heating process.

126

Dairy Bacteriology.

3. Heafc water in pail until the temperature of same
reaches 155° to 160° F. ; then remove from source of direct
heat, cover with a cloth or tin cover, and allow the whole

FIG. 26. A home-made pasteurizer.

to stand for half an hour. In the preparation of milk for
children, it is not advisable to use the low-temperature
treatment (140° F.) that is recommended for commercial
city delivery.

4. Remove bottles of milk and cool them as rapidly as
possible without danger to bottles and store in a refriger-
ator.

Commercial pasteurizers. As noted before, the object
in commercial pasteurization depends upon whether it is
desired to treat milk for general milk supply or to make
into butter. The ends to be attained are so widely different
that it naturally follows that the apparatus best suited for
the respective purposes must vary considerably. In pas-
teurizing milk in butter-making, capacity is one of the

Preservation of Milk. 127

most important desiderata, but in preparing milk for human
use, fulfillment of sanitary conditions is the first requisite.
It is to be regretted that milk dealers so frequently lose
sight of this requirement in their attempts to secure appa-
ratus that will handle large amounts so as to reduce the
cost of operation. Pasteurizing involves considerable time
and trouble, and it is better not to have the milk treated at
all than to have the process imperfectl}T performed.

The various types of machinery that have been suggested
for this use may be grouped as follows, depending upon
their method of operation:1

1. Continuous- flow machines.

2. Intermittent machines.

The continuous-flow pasteurizers were originally designed
for the treatment of milk and cream for butter-making, but
in many cases they have been applied to the preservation
of milk for direct consumption. The difficulty with them
is not that the milk cannot be readily heated in the same,
but as customarily arranged there is no provision for the
retention of the milk at a temperature that would be fatal
to the organisms in the same.

Continuous-flow pasteurizers. Apparatus of this class
vary much in detail, but all possess this common principle,
that the milk enters the machine in a continuous stream
and is generally discharged in the same way. The objec-
tion to this type of apparatus is that the time of heating
cannot be regulated with any certainty, although the tem-
perature can be controlled in part by varying the speed of
flow. Recent tests made at the Wisconsin Dairy School

1 For a more detailed description of pasteurizing machinery, reference should
be made to Monrad's Pasteurization of Milk, or Weigmann's Conservierung der
Milch.

128 Dairy Bacteriology.

on a machine of this type showed that it took only one
and one-half minutes for milk to pass through the ap-
paratus, although it was claimed that it was in the machine
for a period of ten minutes. Another objection is that in
the rapid heating, where steam is employed, the proteids of
the milk scald on to the walls of the pasteurizer.

In some of these machines (Thiel, Kuehne, Lawrence,
De Laval, and Hochmuth), a ribbed surface is employed
over which the milk flows, while the opposite surface is
heated with hot water or steam. Monrad, Lefeldt and
Lentsch employ a centrifugal apparatus in which a thin
layer of milk is heated in a revolving drum.

In the Hill and Miller pasteurizers (both American ma-
chines), the milk is forced in a thin sheet between heated
surfaces and overflows at the top.

One of the most economical types of apparatus is the
regenerator type (a German machine), in which the milk
passes over the heating surface in a thin stream and then
is carried back over the incoming cold milk so that the
heated liquid is partially cooled by the inflowing fresh milk.

A number of machines have been constructed on the
principle of a reservoir which is fed by a constantly flow-
ing stream. In some kinds of apparatus of this type no
attempt is made to prevent the mixing of the recently in-
troduced milk with that which has been partially heated.
The pattern for this reservoir type is Fjord's heater, in
which the milk is stirred by a stirrer. This apparatus was
originally designed as a heater for milk before separation.
Reid was the first to introduce this type of machine into
America. A recently devised machine of this type (Pas-
teur) has been tested by Lehmann, who found that it was
necessary to heat the milk as high as 176° to 185° F., in

Preservation of Milk. 129

order to secure satisfactory results on the bacterial content
of the cream.

Tests Of apparatus. But few of the continuous-flow
type of machines have been subjected to rigid bacterio-
logical control, and their efficiency is questionable. By
their use it may be possible to enhance the keeping qual-
ity of milk in a fairly satisfactory way, and yet not in-
sure complete freedom from disease-producing bacteria.
One grave defect in many of them is that all parts of the
milk are not heated uniformly. It is easily possible for one
part to be over-heated while the remainder is under-heated;
and while the outlet may show a suitable temperature, still
it does not follow that all parts of the milk have been
thoroughly treated.

The following simple method enables the factory operator
to test the period of exposure in the machine: Start the
machine full of water, and after the same has become heated
to the proper temperature, change the inflow to full-cream
milk, continuing at the same rate. Note the exact time of
change and also when first evidence .of milkiness begins to
appear at outflow. If samples are taken from first appear-
ance of milky condition and thereafter at definite intervals
for several minutes, it is possible, by determining the amount
of butter-fat in the same, to calculate with exactness how
long it takes for the milk to entirely replace the water.

Intermittent pasteurizers. Inasmuch as the biological
and physical requirements of successful pasteurizing neces-
sitate milk being heated between the temperatures of
140° and 160° F., it is desirable that the temperature should
also be under complete control. Moreover, the treatment
should also be in such a way as absolutely to insure all the
milk being treated for a given period of time. A fulfill-
9

130 Dairy Bacteriology.

ment of these conditions necessitates the use of the inter-
mittent type of apparatus, or continuous apparatus ar-
ranged so as practically to conform to the discontinuous
process.

The simplest way in which these conditions can he car-
ried out is to employ a number of shot-gun cans immersed
in a tank of hot water. By means of this crude device, milk
or cream can he pasteurized more effectually than in many
of the specially designed apparatus. Tanks surrounded
with water spaces can also be used quite successfully.

The use of the Boyd cream ripening vat has been sug-
gested, and this fulfills the necessary conditions as'to a com-
mercial pasteurizer. The cream in this is heated by means
of a swinging coil immersed in the same, through which
hot water circulates.

In some of the pasteurizers, steam is introduced directly
into the milk or cream, as in Bentley's apparatus. It is
obvious that while this may be a cheaper method by which
to heat the milk, still the proteids of the fluid must be
scalded in part, although the temperature of the whole
mass may not exceed the proper pasteurizing point. The
impurities in the condensed steam are also objectionable.

The first American pasteurizer to be built on the intermit-
tent plan that was made to conform to biological require-
ments was devised by the writer1 in 1894. It consists of a
long, narrow vat, surrounded by a water chamber which is
heated by steam. To facilitate the heating of the milk,
both the milk and water reservoirs are supplied with agita-
tors having a to-and-fro movement.

The Potts pasteurizer is another machine of the inter-
mittent type that has since been quite generally introduced

> Russell, Wis. Expt. Stat., Bull. 44.

Preservation of Milk.

131

FIG. 27. Potts pasteurizer.

and which conforms to the necessary biological conditions.
This apparatus has a central milk chamber that is sur-
rounded with an
outer shell con-
taining hot water.
The whole ma-
chine revolves on
a horizontal axis,
and the cream or
milk is thus thor-
ough^ agitated
during the heating
process.

Coolers, A speedy cooling of the heated product is
essential to success in pasteurizing. Some of the machines
have been devised for a combination purpose, being used
for the heating and subsequent cooling of the milk. This
is an evident advantage in some ways, as it lessens the
amount of apparatus necessary, also the work involved in
cleaning the same, but at the same time the problems of
quick heating and cooling involve somewhat different
principles, so that for the most economical manipulation of
the product, separate pieces of apparatus are advisable
where the business warrants such expense.

The simplest method of treatment in cooling is to draw
off the milk in shot-gun cans and place these first in water,
then in ice-water.

To cool milk most economically, two coolers should be
provided. With one of these, cold water can be used, and
with this the temperature can be reduced to nearly that
of the water in a short time. In order, however, to
lower the temperature below a point where spore germi-
nation will readily occur, milk should be chilled by the

132 Dairy Bacteriology.

aid of ice. This may be applied in the same cooler that is
used for running cold water, by supplying ice-water for the
latter part of the cooling process. To use ice economically,
the ice itself should be applied as closely as possible to the
milk to be cooled, for the larger part of the chilling value
of ice comes from the melting of the same. To convert a
pound of ice at 32° F. into a pound of water at the same
temperature, if we disregard radiation, would require as
much heat as would suffice to raise 142 pounds of water
one degree F., or one pound of water 142° F. The ab-
sorptive capacity of milk for heat (specific heat) is not quite
the same as it is with water, being .847 for milk in com-
parison with 1.0 for water.1 Hot milk would therefore
require somewhat less ice to cool it than would be required
by an equal volume of water at the same temperature.

In the mere melting of a pound of ice, if expended on
cooling heated milk, the temperature of pasteurized milk
would be reduced to a keeping temperature. To take ad-
vantage of this, the ice should be brought in close contact
with the milk, rather than to utilize the specific heat in
cooling water which is later applied to the milk. If broken
ice is used directly, it is utilized most economically if the
milk surrounds it, as in this way the ice does not absorb
heat from the outside.

Bacterial efficiency of pasteurizing; apparatus. The bac-
terial content of pasteurized milk and cream will depend
somewhat on the number of organisms originally present
in the same. Naturally, if mixed milk brought to a cream-
ery is pasteurized, the number of organisms remaining
after treatment would be greater than if the raw material
was fresh and produced on a single farm.

An examination of milk and cream pasteurized on a com-

1 Fleischmann, Landw. Versuchsstat., 17: 251.

Preservation of Milk.

133

mercial scale in the Russell vat at the Wisconsin Dairy
school showed that over 99.8 per cent of the bacterial life in

raw milk or cream
was destroyed by
the heat employed,
i. e., 155° F. for
twenty minutes du-
ration.1 In nearly
one-half of the sam-
ples of milk, the
germ content in the
pasteurized sample
fell below 1,000
bacteria per cc.,
and the average of
twenty-five samples
contained 6,140 bac-

FIG. 28. Effect of pasteurizing on germ content £erja per cc Jn
of milk. Black square represents bacteria of raw
milk; small white square, those remaining after Cream the germ COn-

pasteurization. tent was higher, av-

eraging about 25,000 bacteria per cc. This milk was taken
from the general creamery supply, which was high in organ-
isms, containing on an average 3,675,000 bacteria per cc.
De Schweinitz2 has reported the germ content of a supply
furnished in Washington which was treated at 158° to 160°
F. for fifteen minutes. This supply came from a single
source. Figures reported were from 48-hour-old agar plates.
Undoubtedly these would have been higher if a longer pe-
riod of incubation had been maintained. The average of
82 samples, taken for the period of one year, showed 325
bacteria per cc.

1 Russell, 12 Wis. Expt. Stat. Kept , 1895, p. 160.
2De Schweinitz, Nat. Med. Rev., 1899, No. 11.

CHAPTER VII.
BACTERIA AND BUTTER-MAKING.

IN making butter from the butter fat in milk, it is neces-
sary to concentrate the fat globules into cream, preliminary
to the churning process. The cream may be raised by the
gravity process or separated from the milk by centrifugal
action. In either case the bacteria that are normally pres-
ent in the milk differentiate themselves in varying numbers
in the cream and the skim-milk. The cream always con-
tains per cc. a great many more than the skim-milk, the
reason for this being that the bacteria are caught and held
in the masses of fat globules, which, on account of their
lighter specific gravity, move toward the surface of the
milk or toward the interior of the separator bowl. This
filtering action of the fat globules is similar to what happens
in muddy water upon standing. As the suspended particles
fall to the bottom they carry with them a large number of
the organisms that are in the liquid.

Various creaming methods. The creaming method has
an important bearing on the kind as well as the number
of the bacteria that are to be found in the cream. The
difference in species is largely determined by the difference
in ripening temperature, while the varying number is gov-
erned more by the age of the milk.

1. Primitive gravity methods. In the old shallow-pan
process, the temperature of the milk is relatively high, as
the milk is allowed to cool naturally. This comparatively
high temperature favors especially the development of
those forms whose optimum growing-point is near the air

Bacteria and Butter- Making. 135

temperature. By this method the cream layer is exposed
to the air for a longer time than with any other, and conse-
quently the contamination from this source is greater.
Usually cream obtained by the shallow-pan process will
contain a larger number of species and also have a higher
acid content.

2. Modern gravity methods. In the Cooley process, or
any of the modern gravity methods where cold water or ice
is used to lower the temperature, the conditions do not
favor the growth of a large variety of species. The number
of bacteria in the cream will depend largely upon the man-
ner in which the milk is handled previous to setting. If
care is used in milking, and the milk is kept so as to ex-
clude outside contamination, the cream will be freer from
bacteria than if carelessness prevails in handling the milk.
Only those forms will develop in abundance that are able
to grow at the low temperature at which the milk is set.
Cream raised by this method is less frequently infected with
undesirable forms than that which is creamed at a higher
temperature.

3. Centrifugal method. Separator cream should contain
less germ-life than that which is secured in the old way.
It should contain only those forms that have found their
way into the milk during and subsequent to the milking,
for the cream is ordinarily separated so soon that there is
but little opportunity of infection, if care is taken in the
handling. As a consequence, the number of species found
therein is smaller.

Where milk is separated, it is always prudent to cool the
cream so as to check growth, as the milk is generally heated
before separating in order to skim efficiently.

Although cream is numerically much richer in bacteria

138 Dairy Bacteriology.

than milk, yet the changes due to bacterial action are slower
so that it usually spoils sooner than cream. For this same
reason, cream will sour sooner when it remains on the milk
than it will if it is separated as soon as possible. This fact
indicates the necessity of early creaming, so as to increase
the keeping quality of the product, and is another argu-
ment in favor of the separator process.

Ripening Of cream. If cream is allowed to remain at
ordinary temperatures, it undergoes a series of fermenta-
tion changes that are exceedingly complex in character, the
result of which is to produce in butter made from the
same the characteristic flavor and aroma that are so well
known in this article. We are so accustomed to the de-
velopment of these flavors in butter that they are not gen-
erally recognized as being intimately associated with bac-
terial activity unless compared with butter made from per-
fectly fresh cream. Sweet-cream butter lacks the aromatic
principle that is prominent in the ripened product, and
while the flavor is delicate, it is relatively unpronounced.

In the primitive method of butter-making, where the but-
ter was made on the farm, the ripening of cream became a
necessity in order that sufficient material might be accumu-
lated to make a churning. The ripening change occurred
spontaneously without the exercise of any especial control.
With the development of the creamery system came the
necessity of exercising a control of this process, and there-
fore the modern butter-maker must understand the prin-
ciples which are involved in this series of complex changes
that largely give to his product its commercial value.

In these ripening changes three different factors are to
be taken into consideration: the development of acid, flavor
and aroma. Much confusion in the past has arisen from a

Bacteria and Butter- Making. 137

failure to discriminate between these qualities. While all
three are produced simultaneously in ordinary ripening, it
does not necessarily follow that they are produced by the
same cause. If the ripening changes are allowed to go too
far, undesirable rather than beneficial decomposition pro-
ducts are produced. These greatly impair the value of but-
ter, so that it becomes necessary to know just to what extent
this process should be carried.

In cream ripening there is a very marked bacterial
growth, the extent of which is determined mainly by the
temperature of the cream. Conn and Esten l find that the
number of organisms may vary widely in unripened cream,
but that the germ content of the ripened product is more
uniform. When cream is ready for the churn, it often
contains 500,000,000 organisms per cc., and frequently
even a higher number. This represents a germ content
that has no parallel in any natural material.

The larger proportion of bacteria in cream as it is found
in the creamery belong to the acid-producing class, but in
the process of ripening, these forms seem to thrive still
better, so that when it is ready for churning the germ con-
tent of the cream is practically made up of this type.

Effect on Churning. In fresh cream the fat globules which
are suspended in the milk serum are surrounded by a film of
albuminous material which prevents them from coalescing
readily. During the ripening changes, this enveloping
substance is modified, probably by partial solution, so that
the globules cohere when agitated, as in churning. The
result is that ripened cream churns more easily, and as it
is possible to cause a larger number of the smaller fat-
globules to cohere to the butter granules, the yield is

1 Conn and Esten, Cent. f. Bakt., II Abt., 1901, 7: 746.

138 Dairy Bacteriology.

slightly larger — a point of considerable economic impor-
tance where large quantities of butter are made.

Development Of acid. The result of this enormous bac-
terial multiplication is that acid is produced in cream, lac-
tic being the principal acid so formed.

Other organic acids are undoubtedly formed as well as
certain aromatic products. While the production of acid
as a result of fermentative activity is usually accompanied
with a development of flavor, the flavor is not directly pro-
duced by the formation of acid. If cream is treated in
proper proportions with a commercial acid, as hydrochloric,1
it assumes the same churning properties as found in nor-
mally ripened cream, but is devoid of the desire J aromatic
qualities. Lactic acid8 has also been used in a similar way
but with no better results.

The amount of acidity that should be developed under
natural conditions so as to secure the optimum quality a»
to flavor and aroma is the most important question in
cream ripening. Concerning this there have been two some-
what divergent views as to what is best in practice, some
holding that better results were obtained with cream rip-
ened to a high degree of acidity than where a less amount
was developed.3 The present tendency seems to be to de-
velop somewhat more than formerly, as it is thought that
this secures more of the "high, quick" flavor wanted in
the market. On the average, cream is ripened to about
0.5 to 0.65 per cent acidity, a higher percentage than this
giving a strong-flavored butter. In the determination of
acidity, the most convenient method is to employ the Far-

'Tiemann, Milch Zeit., 23:701.

• Milch Zeit, 1889, p. 7; 1894, p. 634: 1895, p. 383.

« Dean, Out. Agr. Coll., 1897, p. 60.

Bacteria and Butter- Making. 139

rington alkaline tablet, which permits of an accurate and
rapid estimation of the acidity in the ripening cream. The
amount of acidity to be produced must of necessity be gov-
erned by the amount of butter-fat present, for the forma-
tion of acid is confined to the serum of the cream; conse-
quently, a rich cream would show less acid by titration
than a thinner cream, and still contain really as much acid
as the other. The importance of this factor is evident in
gathered-cream factories.

The rate of ripening is dependent upon the conditions
that affect the rate of growth of bacterial life, such as time
and temperature, number of organisms in cream and also
the per cent of butter fat in the cream. Some years ago
it was customary to ripen cream at about 50° to 60° F.,
but more recently better results have been obtained, it is
claimed, where the ripening temperature is increased and
the period of ripening lessened. As high a temperature
as TO0 to 75° F. has been recommended. It should be said
that this variation in practice may have a valid scientific
foundation, for the temperature of the ripening cream is un-
doubtedly the most potent factor in determining what
kind of bacteria will develop most luxuriantly. It is well
known that those forms that are capable of producing bit-
ter flavors are able to thrive better at a lower temperature
than some of the desirable ripening species.

The importance of this factor would be lessened where
a pure culture was used in pasteurized cream, because here
practically the selected organism alone controls the field.

It is frequently asserted that better results are obtained
by stirring the cream and so exposing it to the air as much
as possible. Experiments made at the Ontario Agricul-
tural College, however, show practically no difference in

140 Dairy Bacteriology.

the quality of the butter made by these two methods.
The great majority of the bacteria in the cream belong to
the facultative class, and are able to grow under conditions
where they are not in direct contact with the air,

Flavor and aroma. The basis for the peculiar flavor or
taste which ripened cream-butter possesses is due, in large
part, to the formation of certain decomposition products
formed by various bacteria. Aroma is a quality often
confounded with flavor, but this is produced by volatile
products only, which appeal to the sense of smell rather
than taste. Generally a good flavor is accompanied by a
desirable aroma, but the origin of the two qualities is
not necessarily dependent on the same organisms. The
quality of flavor and aroma in butter is, of course, also af-
fected by other conditions, as, for instance, the presence or
absence of salt, as well as the inherent qualities of the
milk, that are controlled, to some extent at least, by the
character of the feed which is consumed by the animal.
The exact source of these desirable butevancescent qualities
in butter is not yet satisfactorily determined. According
to Storch,1 flavors are produced by the decomposition of
the milk sugar and the absorption of the volatile flavors
by the butter fat. Conn 2 holds that the nitrogenous ele-
ments in cream serve as food for bacteria, and in the de-
composition of which the desired aromatic substance is pro-
duced. The change is unquestionably a complex one, and
cannot be explained as a single fermentation.

There is no longer much doubt but that both acid-forming
and casein-digesting species can take part in the production
of proper flavors as well as desirable aromas. The researches

1 Storch, Nogle. Unders. over Floed. Syrning, 1890.

2 Conn, 6 Storrs Expt. Stat., 1893, p. 66.

Bacteria and Butter-Making. 141

of Conn,1 who lias studied this question most exhaustively,
indicate that both of these types of decomposition partici-
pate in the production of flavor and aroma. He has shown
that both flavor and aroma production are independent
of acid; that many good flavor-producing forms belong
to that class which renders milk alkaline, or does not
change the reaction at all. Some of these species liquefied
gelatin and would therefore belong to the casein-dissolv-
ing class. Those species that produced bad flavors are also
included in both fermentative types. Conn has found
a number of organisms that are favorable flavor-pro-
ducers; in fact they were much more numerous than de-
sirable aroma-yielding species. None of the favorable
aroma forms according to his investigations were lactic-
acid species, — a view which is also shared by Weigmann.9

McDonnell8 has found that the production of aroma in
certain cases varies at different temperatures, the most pro-
nounced being evolved near the optimum growing tem-
perature, which, as a general rule, is too high for cream
ripening.

The majority of bacteria in ripening cream do not seem
to exert any marked influence in butter. A considerable
number of species are positively beneficial, inasmuch as
they produce a good flavor or aroma. A more limited
number are concerned in the production of undesirable
ripening changes. This condition being true, it may seem
strange that butter is as good as it is, because so frequently
the requisite care is not given to the development of proper
ripening. In all probability the chief reason why this is
so is that those bacteria that find milk and cream pre-emi-

i Conn, 9 Storrs Expt. Stat., 1896, p. 17.

a Wiegmann, Milch Zeit., 1891, p. 793.

• McDonnell, u. Milchsaure Bakterien (Diss. Kiel, 1899), p. 43.

142 Dairy Bacteriology.

nently suited to their development, e. g. the lactic-acid
class, are either neutral or beneficial in their effect on
butter.

Use of Starters. Experience has amply demonstrated
that it is possible to control the nature of the fermentative
changes that occur in ripening cream to such an extent as
to materially improve the quality of the butter. This is
frequently done by the addition of a " starter." While
starters have been employed for man}' years for the purpose
mentioned, it is only recently that their nature has been
understood. A starter may be selected from widely diver-
gent sources, but in all cases it is sure to contain a large
number of bacteria, and the presumption is that they are
of such a nature as to produce desirable fermentative
changes in the cream.

In the selection of these so-called natural starters, it fol-
lows that they must be chosen under such conditions as
experience has shown to give favorable results. For this
purpose, whole milk from a single animal is often used
where the same is observed to sour with the production of
no gas or other undesirable taint. A skim-milk starter
from a mixed supply is recommended by many. Butter
milk is frequently employed, but in the opinion of butter
experts is not as suitable as the others mentioned.

It not infrequently happens that the practical operator
may be misled in selecting a starter that is not desirable,
or by continuing its use after it has become contaminated.

In 1890 1 a new system of cream ripening was intro-
duced in Denmark by Storch that possesses the merit of
being a truly scientific and at the same time practical
method. This consisted in the use of pure cultures of

» Storch, Milch Zeit., 1890, p. 304.

Bacteria and Butter-Making, 143

specific organisms that were selected on account of their
ability to produce a desirable ripening change in cream.
The introduction of these so-called culture starters has be-
come almost universal in Denmark and in parts of Ger-
many. Their use is also extending in this country, Aus-
tralia and New Zealand.

Principles of pure-culture cream-ripening:. In the proper
use of pure cultures for ripening cream, it is necessary first
to eliminate as far as possible the bacteria already present
in cream before the culture starter is added. This result is
accomplished by heating the cream to a temperature suf-
ficiently high to destroy the vegetating organisms. The
addition of a properly selected starter will then give the
chosen organism such an impetus as will generally enable
it to gain the ascendency over any other bacteria and so
control the character of the ripening. The principle em-
ployed is quite like that practiced in raising grain. The
farmer prepares his soil by plowing, in this way killing the
weeds. Then he sows his selected grain, which is merely a
pure culture, and by the rapid growth of this, other forms
are held in check.

The attempt has been made to use these culture starters
in raw sweet cream, but it can scarcely be expected that
the most beneficial results will be attained in this way.
This method has been justified on the basis of the following
experiments. Where cream is pasteurized and no starter
is added, the spore-bearing forms frequently produce unde-
sirable flavors. These can almost always be controlled if
a culture starter is added, the obnoxious form being re-
pressed by the presence of the added starter. This condi-
tion is interpreted as indicating that the addition of a starter
to cream which already contains developing bacteria will

Dairy Bacteriology.

prevent those originally present in the cream from grow-
ing.1 This repressive action of one species on another is a
well-known bacteriological fact, but it must be remembered
that such an explanation is only applicable in those cases
where the culture organism is better able to develop than
those forms that already exist in the cream.

If the culture organism is added to raw milk or cream
which already contains a flora that is well suited to develop
in this medium, it is quite doubtful whether it would gain
the supremacy in the ripening cream. The above method
of adding a culture to raw cream renders cream-ripening
details less burdensome, but at the same time Danish ex-
perience, which is entitled to most credence on this ques-
tion, is opposed to this method.

Reputed advantages of culture starters. 1. Flavor and
aroma. Naturally the flavor produced by pure-culture fer-
ments depends upon the character of the organism used.
Those which are most extensively used are able to produce
a perfectly clean but mild flavor, and a delicate but not
pronounced aroma. The uhigh, quick" flavor and aroma
that is so much desired in the American market is not
readily obtained by the use of cultures. It is quite problem-
atical whether the use of any single species will give any
more marked aroma than normally occurs in natural ripen-
ing.

2. Uniformity of product. Culture starters produce a
more uniform product because the type of fermentation is
under more complete control, and herein is the greatest
advantage to be derived from their use. Even the best
butter-maker at times will fail to secure uniform results
if his starter is not perfectly satisfactory.

1 Conn, 9 Storrs Expt. Stat, 1890, p. 25.

Bacteria and Butter-Making. 145

3. Keeping quality of product. Butter made from pas-
teurized cream to which a pure-culture starter has been
added will keep much better than the ordinary product, be-
cause the diversity of the bacterial flora is less and the milk
is therefore not so likely to contain those organisms that
produce an "off" condition.

4. Elimination of taints. Many defective conditions in
butter are attributable to the growth of undesirable bacteria
in the cream that result in the formation of "off" flavors
and taints. If cream is pasteurized, thereby destroying these
organisms, then ripened with pure ferments, it is generally
possible to eliminate the abnormal conditions.1 Taints
may also be present in cream due to direct absorption from
the cow or through exposure to foul odors.2 Troubles of
this sort may thus be carried over to the butter. This is
particularly true in regions where leeks and wild onions
abound, as in some of the Atlantic States. The heating of
the cream tends to expel these volatile taints, so that a
fairly good article of butter can be made from what would
otherwise be a relatively worthless product.

Characteristics desired in culture starters. Certain con-
ditions as the following are desirable in starters made from
pure cultures:

1. Vigorous growth in milk at ordinary ripening tem-
peratures.

2. Ability to form acid so as to facilitate churning and
increase the yield of butter.

3. Able to produce a clean flavor and desirable aroma.

4. Impart a good keeping quality to butter.

i Milch Zeit., 1891, p. 122; 1894, p. 284; 1895, p. 56; 1896, p. 163.
8 McKay, Bull. 32, Iowa Expt. Stat, p. 477.
10

146 Dairy Bacteriology.

5. Not easily modified in its flavor-producing qualities
by artificial cultivation.

These different conditions are difficult to attain, for the
reason that some of them seem to be in part incompatible.
Weigmann1 found that a good aroma was generally an
evanescent property, and therefore opposed to good keep-
ing quality. Conn has shown that the functions of acid-
formation, flavor and aroma production are not necessarily
related, and therefore the .chances of finding a single or-
ganism that possesses all the desirable attributes are not
very good.

In all probability no one germ possesses all of these de-
sirable qualities, but natural ripening is the resultant of
the action, of several forms.8 This idea has led to the at-
tempt at mixing selected organisms that have been chosen
on account of certain favorable characteristics which they
might possess. The difficulty of maintaining such a com-
posite culture in its correct proportions when it is propa-
gated in the creamery is seemingly well nigh insuperable,
as one organism is very apt to develop more or less rapidly
than the other.

A very satisfactory way in which these cultures are mar-
keted is to mix the bacterial growth with some sterile,
inert, dry substance. This is the method used in most of
the Danish cultures. In this country, some of the more
prominent cultures employed are marketed in a liquid form.

Culture vs. home-made starters. One great advantage
which has accrued from the use of culture or commercial
starters has been that in emphasizing the need of closer
control of the ripening process, greater attention has been

» Weigmann, Lamlw. Woch. f. Schl. Hoi., No. 2, 1890.
« Weigmann, Cent. f. Bakt., II Abt., 3:49 7, 1897.

Bacteria and Butter- Making. 147

paid to the carrying out of the details. In the hands of
the better operators, the differences in flavor of butter
made with a culture or a natural starter are not marked,1
but in the hands of those who fail to make a good product
under ordinary conditions, an improvement is often se-
cured where a commercial culture is used.

Pasteurization as applied to butter-making:. This pro-
cess, as applied to butter making, is often confounded with
the treatment of milk and cream for direct consumption.
It is unfortunate that the same term is used in connection
with the two methods, for they have but little in common
except in the use of heat to destroy the germ life of the
milk. In pasteurizing cream for butter-making, it is not
•necessary to observe the stringent precautions that are to
be noted in the preservation of milk; for the addition of a
rapidly developing starter controls at once the fermenta-
tive changes that subsequently occur. Then again, the
physical requirement as to the production of a cooked taste
is not so stringent in butter-making. While a cooked
taste is imparted to milk or even cream at about 158P F.r
it is possible to make butter that shows no permanent
cooked taste from cream that has been raised as high as
185° or even 195° F. This is due to the fact that the fat
does not readily take up those substances that give to
scalded milk its peculiar flavor.

Unless care is taken in the manipulation of the heated
cream, the grain or body of the butter may be injured.
This tendency can be overcome if the ripened cream is

i At the National Creamery Buttermakers1 Association lor 1901, 193 out of 240
exhibitors used starters. Of those that employed starters, nearly one-half used
commercial cultures. There was practically no difference in the average score
of the two classes of starters, but those using starters ranked nearly two points
higher in flavor than those that did not.

148 Dairy Bacteriology.

chilled to 48° F. for about two hours before churning.1 It
is also essential that the heated cream should be quickly
and thoroughly chilled after being pasteurized.

The Danes, who were the first to employ pasteurization
in butter-making, used, in the beginning, a temperature
ranging from 158° to 167° F., but owing to the prevalence
of such diseases as tuberculosis and foot-and-mouth disease,
it became necessary to treat the milk so as to thoroughly
destroy the virus of the disease. This can be done by mo-
mentarily heating the same to the temperature of 185° F.,
and this temperature is now generally employed. With
the use of this higher temperature the capacity of the pas-
teurizing apparatus is considerably reduced, as not more
than one-half to two-thirds as much milk can be handled
at 185° as at 158° F.

When the system was first introduced in Denmark, two
methods of procedure were followed: the whole milk was
either heated before separation, or the cream was pasteur-
ized afterwards, the skim milk being treated separately.
At the present time the latter system is gaining grou'nd.

The present law makes it compulsory to heat all skim
milk to 185° F. to avoid the dissemination of the diseases
previously mentioned.

Apparatus for pasteurizing. As it is not necessary to
heat the milk or cream for butter-making under such a
narrow range of conditions as when designed for direct con-
sumption, it is permissible to employ machinery that be-
longs to the continuous-flow type. These pasteurizers have
a large capacity and it is possible to handle in them several
thousand pounds per hour. The majority of apparatus for
this purpose has originated in Denmark and Germany.

* N. Y. Prod. Rev., October, 1899.

Bacteria and .Butter-

A quantitative determination of the bacteria found in milk
and cream when, treated in machinery of this class almost
always shows a degree of variation in results that is not to
be noted in the discontinuous apparatus.

FIG. 29. Reid's Continuous Pasteurizer.

Harding and Rogers l have tested the efficiency of one
of the Danish type of continuous pasteurizers. These ex-
periments were made at 158°, 176° and 185° F. They

* Harding and Rogers, Bull. 182, N. Y. (Geneva) Expt. Stat., Dec., 1899.

150 Dairy Bacteriology.

found the efficiency of the machine not wholly satisfactory
at the lower temperatures. At 158° F. the average of four-
teen tests gave 15,300 bacteria per cc., with a maximum to
minimum range from 62,790 to 120. Twenty-five examina-
tions at 176° F. showed an average of only 117, with a
range from 300 to 20. The results at 185° F. showed
practically the same results as noted at 176° F. Consider-
able trouble was experienced with the " scalding on " of
the milk to the walls of the machine when milk of high
acidity was used.

The writer1 tested Reid's pasteurizer at 155° to 165° F.
with the following results: in some cases as many as 40
per cent of the bacteria survived, which number in some
cases exceeded 2,000,000 bacteria per cc.

Development of pasteurization and pure-culture ripening.

Since the introduction of this system into Denmark in
1890, creamery methods have been completely revolution-
ized. At the present time practically all of the butter ex-
ported to England is prepared in this way and by far the
larger part of that which is consumed at home. There are
several different selected commercial cultures that are used.
In Sweden, in 1897, 67 per cent of creameries pasteurized
their product.

In Germany the system has been adopted most exten-
sively in the north, and may be said to be practically an ex-
tension of the Danish system. In southern Germany the
method is not employed to any extent.

In this country considerable agitation has been given
the matter, but the process has been but slowly adopted.
Under the auspices of the Department of Agriculture at
Washington, considerable effort has been put forth in the

'Russell, Bull. 69, Wis. Expt. Stat.

Bacteria and Butter- Making. 151

preparation of culture butter especially for export trade.
In some cases this work has borne fruitage,1 but in general
the flavor of butter made from pasteurized cream is not as
" high " and " quick " as that made in the other way, and
therefore it does not meet this desired requisite of the
American market. The mild, clean flavor which charac-
terizes culture butter is particularly desired by the English
market, to which the bulk of the Danish butter goes. Con-
siderable " pasteurized " or "culture" butter has been ex-
ported from Australia 2 and New Zealand to England and
it is said that the system is gaining ground slowly.

Where the market demands are satisfied with the quality
of butter that can best be made by this system, the method
is undoubtedly destined to be adopted more and more gen-
erally, as the uniformity of product obtained is a great ad-
vantage. The system entails considerable labor and some
expense, and the question as to its more general adoption
will be determined by the advantages gained.

Propagation of starters for cream-ripening:. The prepa-
ration-and propagation of a starter for cream-ripening is a
process involving considerable bacteriological knowledge,
whether the starter is of domestic origin or prepared from
a pure-culture ferment. In any event, it is necessary that
the starter should be handled in a way so as to prevent the
introduction of foreign bacteria as far as possible. The
following points should be kept in mind in manipulating
the starter:

1. If a pure-culture ferment is used, see that it is fresh
and that the seal has not been disturbed.

1 Some of the larger western creamery syndicates are pasteurizing a large
part of the cream they receive.

2 Cherry, Farm and Home, Aug., 1900.

152 Dairy Bacteriology.

2. If a home-made starter is employed, use the greatest
possible care in selecting the milk that is to be used as a
basis for the starter.

3. For the propagation and perpetuation of the starter
from day to day, it is necessary that the same should be
grown in milk that is as germ-free as it is possible to secure
it. For this purpose sterilize some fresh skim-milk in a
covered can that has previously been well steamed. This
can be done easily by setting cans containing skim-milk in
a vat filled with water and heating th* same to 180° F. or
above. The temperature should be maintained for a half
hour or more. This destroys all but a few of the most re-
sistant spore-bearing organisms. This will give a cooked
flavor to the milk, but will not affect the cream to which
the starter is added. Dairy supply houses are now intro-
ducing the use of starter cans that are specially made for
this purpose.

4. After the heated milk is cooled down to about 70° or
80° F., it can be inoculated with the desired culture. Some-
times it is desirable to " build up " the starter by propagat-
ing it first in a smaller volume of milk, and then after this
has developed, adding it to a larger amount.

This method is of particular value where a large amount
of starter is needed for the cream-ripening.

5. After the milk has been inoculated, it should be kept
at a temperature that is suitable for the rapid development
of the contained bacteria, 65°-75° F., which temperature
should be kept as uniform as possible.

6. This can best be done by setting can in vat filled with
warm water and covering the same with a wooden cover
or heavy cloth during the night to maintain proper tem-
perature.

Bacteria and Butter- Making. 153

7. The starter should not be thoroughly curdled and solid
when it is needed for use, but should be well soured and
partially curdled. This point is of importance for the fol-
lowing reasons:

a. It is difficult to thoroughly break up curd particles if
the starter is completely curdled. If these curd masses are
added to ripening cream, white specks may appear in the
butter.

b. The vigor of the starter is in all probability stronger
when the milk is on the point of curdling than it is after
the curd has been formed some time. The continued for-
mation of lactic acid kills many of the bacteria and thus
weakens the fermentative action. It is therefore highly
important that the acidity of the starter should be closely
watched.

8. The starter should be propagated from day to day by
adding a small quantity to a new lot of freshly prepared
milk. For this purpose two propagating cans should be
provided so that one starter may be in use while the other
is being prepared.

9. The butter-maker must exercise his judgment as to
the condition of his starter. If the same should appear
moldy or contain evidence of gas, the skim milk has been
imperfectly handled.

How long should a starter be propagated? No hard-and-
fast rule can be given for this, for it depends largely upon
how carefully the starter is handled during its propaga-
tion. If the starter is grown in sterilized milk kept in
steamed vessels and is handled with sterile dippers, it is
possible to maintain it in a state of relative purity for a
considerable period of time; if, however, no especial care is
given, it will soon become infected by the air, and the

154: Dairy Bacteriology.

retention of its purity will depend more upon the ability
of the contained organism to choke out foreign growths
than upon any other factor. Experience seems to indicate
that pure-culture starters " run out " sooner than domestic
starters. While it is possible, by bacteriological methods,
to determine with accuracy the actual condition of a starter
as to its germ content, still such methods are inapplicable
in creamery practice. Here the maker must rely largely
upon the general appearance of the starter as determined
by taste and smell. The supply houses that deal in cult-
ures of this class generally expect to supply a new culture
at least every month.

Bacteria in butter. As ripened cream is necessarily rich
in bacteria, it follows that butter will also contain germ
life in varying amounts, but as butter- fat is not well adapted
for bacterial food, the number of germs in butter is usually
less than in ripened cream.

Sweet-cream butter is naturally poorer in germ life than
that made from ripened cream. Grotenfelt1 reports in
sweet-cream butter, the so-called " Paris butter," only
120 to 300 bacteria per cc., while in butter from sour crearn
2,000 to 55,000 germs per cc. were found. Pammel8 found
from 125,000 to 730,000 per gram, while Lafar3 found in
butter sold in Munich from 10,000,000 to 20,000,000 organ-
isms per gram.

The germ content of butter on the outside of a package
is much greater than it is in the middle of a mass, this
doubtless being due to the freer access of air favoring the
growth of aerobic forms.

» Grotenfelt-Woll, Prin. Mod. Dairy Practice, p. 244.
« Pammel, Bull. 21, Iowa Expt. Stat., p. 801.
« Lafar, Arch. f. Hyg., 1891, 18:1.

Bacteria and Butter- Making. 155

Changes in germ content. The bacteria that are incor-
porated with the butter as it first "comes" undergo a
slight increase for the first few days. The duration of this
period of increase is dependent largely upon the condition
of the butter. If the buttermilk is well worked out of the
butter, the increase is slight and lasts for a few days only,
while the presence of so nutritious a medium as buttermilk
affords conditions much more favorable for the continued
growth of the organisms.

While there may be many varieties in butter when it is
fresh, they are very soon reduced in kind as well as num-
ber. The lactic acid group of orgariisms disappear quite
rapidly; the sppre-bearing species remaining for a some-
what longer time. Butter examined after it is several
months old is often found to be almost free from germs.

In the manufacture of butter there is much that is de-
pendent upon the mechanical processes of churning, wash-
ing, salting and working the product. These processes do
not involve any bacteriological principles other than those
that are incident to cleanliness. The cream, if ripened
properly, will contain such enormous numbers of favorable
forms that the access of the few organisms that are derived
from the churn, the air, or the water in washing will have
little effect, unless the conditions are abnormal.

Rancid change in butter. Fresh butter has a peculiar
aroma that is very desirable and one that enhances the
market price, if it can be retained; but this delicate flavor
is more or less evanescent, soon disappearing, even in the
best makes. While a good butter loses with age some of
the peculiar aroma that it possesses when first made, yet a
gilt-edged product should retain its good keeping qualities
for some length of time. All butters, however, sooner or

15G Dairy Bacteriology.

later undergo a change that renders them worthless for table
use. This change is usually a rancidity that is observed
in all stale products of this class. The cause of this rancid
condition in butter has been attributed to the action of
living organisms, particularly those that form butyric acid,
to the influence of light, air, etc. Undoubtedly under cer-
tain conditions, rancidity may be produced by the opera-
tion of all of the above agents. Although the subject has
been quite extensively studied, there is yet considerable
variation in opinion as to the exact nature of the causal
agents.1

BACTERIAL DEFECTS IN" BUTTER.

Lack of flavor. Often this may be due to improper
handling of the cream in not allowing it to ripen far
enough, but sometimes it is impossible to produce a high
flavor. The lack of flavor in this case is due to the ab-
sence of the proper flavor-producing organisms. This con-
dition can usually be overcome by the addition of a proper
starter. The relation between flavor and desirable bacteria
is very intimate, and troubles of this kind usually arise
because the proper forms commonly found in the cream
have been supplanted by other species that do not possess
the ability of forming these aromatic substances so neces-
sary in sour-cream butter.

Putrid butter. This specific butter trouble has been ob-
served in Denmark, where it has been studied by Jensen.8
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.

iReinmann. Cent. f. Bakt., 1900, 6:131; Jensen, Landw. Jahr. d. Schweiz, 1901.
•Jensen, Cent. f. Bakt., 1891, 11:409.

Bacteria and Butter- Making. 157

Jensen found the trouble to be due to several different
putrefactive bacteria. One form which he called Bacillus
fcetidus lactis, a close ally of the common feces bacillus,
produced this rotten odor and taste in milk in a very short
time. Fortunately, this organism was easily killed by a
comparatively low heat, so that pasteurization of the cream
and use of a culture starter quickly eliminated the trouble,
where it was tried.

Turnip-flavored butter. Butter sometimes acquires a
peculiar flavor recalling the order of turnips, rutabagas,
and other root crops. Often this trouble is due to feed-
ing, there being in several of these crops, aromatic sub-
stances that pass directly into the milk, but in some in-
stances the trouble arises from bacteria that are able to
produce decomposition products,1 the odor and taste of
which strongly recalls these vegetables.

"Cowy" butter. Frequently there is to be noted in
milk a peculiar odor that resmbles that of the cow stable.
Usually this defect in milk has been ascribed to the absorp-
tion of impure gases by the milk as it cools, although the
gases and odors naturally present in fresh mills: have this
peculiar property that is demonstrable by certain methods
of aeration. Occasionally it is transmitted to butter, and
recently Pammel" has isolated from butter a bacillus that
produced in milk the same peculiar odor so commonly pres-
ent in stables.

Lardy and tallowy butter. The presence of this un-
pleasant taste in butter may be due to a variety of causes.
In some instances, improper food seems to be the source of
the trouble; then again,. butter exposed to direct sunlight

» Jensen, Milch Zeit., 1892, 6, Nos. 5 and 6.

Bull. 21, Iowa Expt. Stat., p. 803.

158 Dairy Bacteriology.

bleaches in color and develops a lardy flavor.1 In addition
to these, cases have been found in which the defect has been
traced to the action of bacteria. Storch * has described a
lactic-acid form in a sample of tallowy butter that was able
to produce this disagreeable odor.

Oily butter. Jensen has isolated one of the causes of the
dreaded oily butter that is reported quite frequently in
Denmark. The specific organism that he found belongs
to the sour-milk bacteria. In twenty-four hours it curdle8
milk, the curd being solid like that of ordinary sour milk.
There is produced, however, in addition to this, an unpleas-
ant odor and taste resembling that of machine oil, a pe-
culiarity that is transmitted directly to butter made from
affected cream.

Bitter butter. Now and then butter develops a bitter
taste that may be due to a variety of different bacterial
forms. In most cases, the bitter flavor in the butter is
derived primarily from the bacteria present in the cream
or milk. Several of the fermentations of this character
in milk are also to be found in butter. In addition to
these defects produced by a biological cause, bitter flavors
in butter are sometimes produced by the milk being im-
pregnated with volatile, bitter substances derived from
weeds.

Moldy butter. This defect is perhaps the most serious
because most common. It is produced by the development
of a number of different varieties of molds. The trouble
appears most frequently in packed butter on the outside of
the mass of butter in contact with the tub. Mold spores
are so widely disseminated that if proper conditions are

1 Fischer, Hyg. Bund, 5:573.

2 Storch, 18 Kept. Danish Agric. Expt, Stat., 1890.

Bacteria and Butter-Making. 159

given for their germination, they are almost sure to develop.
In some cases the mold is due to the growth of the ordinary
bread mold, Penicillium glaucum; in other cases a black
mold develops, due often to Cladosporium butyri. Not in-
frequently trouble of this character is associated with the
use of parchment wrappers. The difficulty can easily be
held in check by soaking the parchment linings and the
tubs in a strong brine.

Fishy butter. Considerable trouble has been experienced
in Australian butter exported to Europe in which a fishy
flavor developed. It was noted that the production of this
defect seemed to be dependent upon the storage tempera-
ture at which the butter was kept. When the butter was
refrigerated at 15° F. no further difficulty was experienced.
It is claimed that the cause of this condition is due to the
formation of trimethylamine (herring brine odor) due to
the growth of the mold fungus Oidium lactis, developing
in combination with the lactic-acid bacteria.

CHAPTER VIII.
BACTERIA IN CHEESE.

THE art of cheese-making, like all other phases of dairy-
ing, has been developed mainly as a result of empirical
methods. Within the last decade or so, the subject has
received more attention from the scientific point of view
and the underlying causes determined to some extent.
Since the subject has been investigated from the bacterio-
logical point of view, much light has been thrown on the
cause of many changes that were heretofore inexplicable.
Our knowledge, as yet, is quite meager, but enough has
already been determined to indicate that the whole indus-
try is largely based on the phenomena of ferment action, and
that the application of bacteriological principles and ideas
is sure to yield more than ordinary results, in explaining,
in a rational way, the reasons underlying many of the pro-
cesses to be observed in this industry.

The problem of good milk is a vital one in any phase of
dairy activity, but it is pre-eminently so in cheese-making,
for the abilitjr to make a first-class product depends to a
large extent on the quality of the raw material. Cheese
contains so large a proportion of nitrogenous constituents
that it is admirably suited, as a food medium, to the devel-
opment of bacteria; much better, in fact, than butter.

INFLUENCE OF BACTERIA IN NORMAL CHEESE PROCESSES .

In the manufacture of cheddar cheese bacteria exert a
marked influence in the initial stages of the process. To
produce the proper texture that characterizes cheddar
cheese, it is necessary to develop a certain amount of acid

Bacteria in Cheese. 1611

which acts upon the casein. This acidity is measured by>
the development of the lactic-acid bacteria that normally
abound in the milk; or, as the cheese-maker expresses it,
the milk is "ripened " to the proper point. The action of
the rennet, which is added to precipitate the casein of the
milk, is markedly affected by the amount of acid present,
as well as the temperature. Hence it is desirable to have
a standard amount of acidity as well as a standard tem-
perature for coagulation, so as to unify conditions. It
frequently happens that the milk is abnormal with refer-
ence to its bacterial content, on account of the absence of
the proper lactic bacteria, or the presence of forms capable
of producing fermentative changes of an undesirable char-
acter. In such cases the maker attempts to overcome the
effect of the unwelcome bacteria by adding a "starter; " or.
he must vary his method of manufacture to some extent
to meet these new conditions.

Use of Starters. A starter maybe employed to hasten
the ripening of milk that is extremely sweet, so as to cur-
tail the time necessary to get the cheese to press; or it
may be used to overcome the effect of abnormal conditions.

The starter that is employed is generally one of domestic
origin, and is usually taken from skim milk that has been
allowed to ferment and sour under carefully controlled con-
ditions. Of course much depends upon the quality of the
starter, and in a natural starter there is always the possibility
that it may not be perfectly pure.

Within recent years the attempt has been made to con-
trol the effect of the starter more thoroughly by using pure
cultures of some desirable lactic-acid form.1 This has ren-

1 Russell, 13 Rept. Wis. Expt. Stat., 1896, p. 112; Campbell, Trans. High. &
Agr. Soc. Scotland, 5 ser., 1898, 10:181.
11

162 Dairy Bacteriology.

dered the making of cheese not only more uniform, but has
aided in repressing abnormal fermentations particularly
those that are characturized by the production of gas.

Recentl3T, pure cultures of Adametz's B. nobilis, a di-
gesting organism that is claimed to be the cause of the
breaking down of the casein and also of the peculiar aroma
of Emmenthaler cheese, has been placed on the market
under the name Tyrogen. It is claimed that the use of this
starter, which is added directly to the milk and also rubbed
on the surface of the cheese, results in the improvement of
the curds, assists in the development of the proper holes,
imparts a favorable aroma and hastens ripening.1

Campbell 2 states that the discoloration of cheese in Eng-
land, which is due to the formation of white spots that are
produced by the bleaching of the coloring matter in the
cheese, may be overcome by the use of lactic-acid starters.

The use of stringy or slimy whey has been advocated in
Holland for some years as a means of overcoming the
tendency toward gas formation in Edam cheese which is
made from practically sweet milk. This fermentation, the
essential feature of which is produced by a culture oi Strep-
tococcus Hollandicus? develops acid in a marked degree,
thereby inhibiting the production of gas.

The use of masses of moldy bread in directing the fer-
mentation of Roquefort cheese is another illustration of the
empirical development of starters, although in this in-
stance it is added after the curds have been prepared for
the press.

Pasteurizing milk for cheese-making;. If it were pos-
sible to use properly pasteurized milk in cheese-making,

* Winkler, Milch Zeit. (Hildesheim}, Nov. 24, 1900.
'Campbell, No. Brit., Agric., May 12, 1897. ,
• Weigmann, Milch Zeit., No. 50, 1889.

J3acteria in Cheese. 163

then practically all abnormal conditions could be controlled
by the use of properly selected starters. Numerous at-
tempts have been made to perfect this system with refer-
ence to cheddar cheese, but so far they have been attended
with imperfect success. The reason for this is that in pas-
teurizing milk, the soluble lime salts are precipitated by the
action of heat, and under these conditions rennet extract
does not curdle the casein in a normal manner. This con-
dition can be restored, in part at least, by the addition of
soluble lime salts, such as calcium chlorid; but in our ex-
perience, desirable results were not obtained where heated
milks to -which this calcium solution had been added were
made into cheddar cheese. Considerable experience has been
gained in the use of heated milks in the manufacture of cer-
tain types of foreign cheese. Klein1 finds that Brick cheese
•can be successfully made even where the milk is heated as
high as 185° F. An increased weight is secured by the addi-
tion of the coagulated albumin and also increased moisture.
Bacteria in rennet. In the use of natural rennets, such
as are frequently employed in tbe making of Swiss cheese,
considerable numbers of bacteria are added to the milk. Al-
though these rennets are preserved in salt, alcohol or boric
acid, they are never free from bacteria. Adametz 8 found
ten different species and from 640,000 to 900,000 bacteria
per cc. in natural rennets. Freudenreich has shown that
rennet extract solutions can be used in Swiss cheese-mak-
ing quite as well as natural rennets; but to secure the best
results, a small quantity of pure lactic ferment must be
added to simulate the conditions that prevail when natural
rennets are soaked in whey, which, it must be remembered,
is a fluid rich in bacterial life.

* Klein, Milch Zeit. (Hildesheim), No. 17, 1900.
8 Adametz, Landw. Jahr., 18:256.

164 Dairy Bacteriology.

Where rennet extract or tablets are used, as is gener-
erally the case in cheddar making, the number of bacteria
added is so infinitesimal as to be negligible.

Development Of acid. In the manufacture of cheddar
cheese, the development of acid exerts an important influ-
ence on the character of the product. This is brought
about by holding the curds at temperatures favorable to the
growth of the bacteria in the same. Under these conditions
the lactic-acid organisms, which usually predominate, de-
velop very rapidly, producing thereby considerable quan-
tities of acid which change materially the texture of the
curds. This acid unites to some extent with the casein,
thereby producing compounds of a character different from
those existing in the green curds. The acid also ex-
erts a slight solvent effect on the casein, as is seen in the
" strings " made on the " hot iron." This causes the curds
to mat, producing a close, solid body free from mechan-
ical holes. Still further, the development of this acid is
necessary for the digestive activity of the pepsin in the
rennet extract.

In some varieties of cheese, as the Swiss, acid is not de-
veloped and the character of the cheese is much different
from that of cheddar. In all such varieties, a great deal
more trouble is experienced from the production of " gassy "
curds, because the development of the gas-producing bac-
teria is held in check by the rapid growth of the lactic acid-
producing species.

Bacteria in green cheese. The conditions under which
cheese is made permit of the development of bacteria
throughout the entire process. The cooking or heating of
curds to expel the excessive moisture is never so high as to
be fatal to germ life; on the contrary, the acidity of the

Bacteria .in Cheese.

165

curd and whey is continually increased by the development
of bacteria in the same.

The body of green cheese fresh from the press is, to a
considerable extent, dependent upon the acid produced in
the curds. If the curds are put to press in a relatively
sweet condition the texture is open and porous. The curd
particles do not mat closely together and u mechanical
holes," rough and irregular in outline, occur. Very often,

FIG. 30. L, a sweet curd cheese direct from the press. " Mechanical " holes due
to lack of acid development; P, same cheese fpur days later, mechanical holes
distended by development of gas.

at relatively high temperatures, such cheese begin to
u huff," soon after being taken from the press, a condition
due to the development of gas, produced by gas-generating
bacteria acting on the sugar in the curd. This gas finds
its way readily into these ragged holes, greatly distending
them, as in Fig. 30.

Physical changes in ripening cheese. When a green
cheese is taken from the press, the curd is tough, firm, but
elastic. It has no value as a food product for immediate use,

166 Dairy Bacteriology.

because it lacks a desirable flavor and is not readily digesti-
ble. It is nothing but precipitated casein and fat. in a
short time, a deep-seated change occurs. Physically this
change is demonstrated in the modification that the
curd undergoes. Gradually it breaks down and becomes
plastic, the elastic, tough curd being changed into a soft-
ened mass. This change in texture of the cheese is also ac-
companied by a marked change in flavor. The green cheese
has no distinctively cheese flavor, but in course of time,
with the gradual change of texture, the peculiar flavor in-
cident to ripe cheese is developed.

The characteristic texture and flavor are susceptible of
considerable modification that is induced not only by varia-
tion in methods of manufacture, but by the conditions
under which the chaese are cured. The amount ot moist-
ure incorporated with the curd materially affects the phys-
ical appearance of the cheese, and the rate of change in the
same. The ripening temperature, likewise the moisture
content of the surrounding air, also exerts a marked in-
fluence on the physical properties of the cheese. To some
extent the action of these forces is purely physical, as in
the gradual loss by drying, but in other respects they are
associated with chemical transformations.

Chemical changes in ripening cheese. Coincident with
the physical breaking down of the curd comes a change in
the chemical nature of the casein. The hitherto insoluble
casein is gradually transformed into soluble nitrogenous
substances (caseone of Duclaux, or caseogluten of Weig-
mann). This chemical phenomenon is a breaking-down
process that is analogous to the peptonization of proteids,
although in addition to the peptones and albumoses char-
acteristic of peptic digestion, amido-acids and ammonia are

Bacteria in Cheese. 167

to be found. The quantity of these lower products in-
creases with the age of the cheese.

The chemical reaction of cheese is normally acid to
phenolphthalein, although there is generally no free acid,
as shown by Congo red, the lactic acid being converted
into salts as fast as formed. In very old cheese, undergo-
ing putrefactive changes, especially on the outside, an alka-
line reaction may be present, due to the formation of free
ammonia.

The changes that occur in a ripening cheese are for the
most part confined to the proteids. According to most in-
vestigators the fat remains practically unchanged, although
the researches of Weigrnann and Backe l show that fatty
acids are formed from the fat. In the green cheese con-
siderable milk-sugar is present, but, as a result of the fer-
mentation that occurs, this is rapidly converted into acid
products.

Bacterial flora Of Cheese. It might naturally be expected
that the green cheese, fresh from the press, would contain
practically the same kind of bacteria that are in the milk,
but a study of cheese shows a peculiar change in the char-
acter of the flora. In the first place, fresh cottage cheese,
made by the coagulation of the casein through the action
of acid, has a more diversified flora than cheese mada with
rennet, for the reason, as given by Lafar,2 that the fermen-
tative process is farther advanced.

When different varieties of cheese are made from milk in
the same locality, the germ content of even the ripened
product has a marked similarity, as is illustrated by
Adametz's work 3 on Emmenthaler or Swiss hard cheese,

1 Milch Zeit., 1898, No. 49,

2 Lafar, Technical Mycology, p. 21t>.
8 Adametz, Landw. Jahr., 18: 228.

168 Dairy Bacteriology.

and Schweitzer Hauskase, a soft variety. Of the nln3
species of bacilli and cocci found in mature Emmenthaler,
eight of them were also present in ripened Hauskase.

Different investigators have studied the bacterial flora of
various kinds of cheese, but as yet little comparative sys-
tematic work has been done. Freudenreich * has determined
the character and number of bacteria in Emmenhtaler
cheese, and Russell 2 the same for cheddar cheese. The same
general law has also been noted in Canadian 8 and Eng-
lish4 cheese. At first there is found a marked decrease in
numbers, lasting for a day or two. This is followed by an
enormous increase, caused by the rapid growth of the
lactic-acid type. The development may reach scores oi
millions and often over a hundred million organisms per
gram. Synchronous with this increase, the peptonizing
and gas-producing bacteria gradually disappear. This rapid
development, which lasts only for a few weeks, is followed
by a general decline.

In the ripening of cheese a question arises as to whether
the process goes on throughout the entire mass of cheese,
or whether it is more active at or near the surface. In the
case of many of the soft cheese, such as Brie and limburger,
bacterial and mold development is exceedingly active on the
exterior, and the enzyms secreted by these organisms dif-
fuse toward the interior. That such a condition occurs in
the hard type of cheese made with rennet is extremely im-
probable. Most observers agree that in this type of cheese
the ripening progresses throughout the entire mass, al-
though Adametz opposes this view and considers that in

1 Freudenreich, Landw. Jahr. d. Schweiz, 4: 17; 5: 16.

'Russell, 13 Kept. Wis. Expt. Stat., 18%, p. 95.

8 Harrison, Unpublished Data.

* Lloyd, Bath ajd West of Eng. Soc. Kept., 1892, 2: 180.

Bacteria in Cheese. 169

Emmenthaler cheese the development of the specific aroma-
producing organism occurs in the superficial layers. Jen-
sen has shown, however, that the greatest amount of
soluble nitrogenous products are to be found in the inner-
most part of the cheese, a condition that is not reconcil-
able with the view that the most active ripening is on the
exterior.1

The course of development of bacteria in cheddar cheese
is somewhat influenced by the ripening temperature. In
cheese ripened at relatively low temperatures (50°-55° F.),2
the bacterial flora retains for a considerable period the
same general aspect as in the milk. Under these condi-
tions the lactic-acid type does not gain the ascendancy so
readily. In cheese cured at high temperatures (80°-86° F.),
the number of organisms is greatly diminished, and they
fail to persist in appreciable numbers for as long a time as
in cheese cured at temperatures more frequently employed.

Influence of temperature on curing. Temperature exerts
a most potent influence on the quality of the cheese, as de-
termined not only by the rate of ripening but the nature
of the process itself. Much of the poor quality of cheese
is attributable to the effect of improper curing conditions.
Probably in the initial stage of this industry cheese were
allowed to ripen without any sort of control, with the
inevitable result that during the summer months the tem-
perature generally fluctuated so much as to impair seriously
the quality. The effect of high temperatures (70° F. and
above) is to produce a rapid curing, and, therefore, a short
lived cheese; also a sharp, strong flavor, and generally a

i Freudenreich, Landw. Jahr. d. Schweiz, 1900; Adametz, Oest. Molk. Zeit.,
1899, No. 7. -
» Russell, 14 Wis. Expt. Stat., 1897, p. 203.

170

Dairy Bacteriology.

more or less open texture. Unless the cheese is made
from the best quality of milk, it is very apt to undergo
abnormal fermentations, more especially those of a gassy
character.

Where cheese is ripened at low temperatures, ranging
from 50° F. down to nearly the freezing temperatures, it is

9ozs.

FIG. 31. Influence of curing temperature on texture of cheese. Upper row
ripened eight months at 60° F. ; lower row at 40° F.

found that the quality is greatly improved.1 Such cheese
are thoroughly broken down from a physical point of view
even though they majr not show such a high per cent of
soluble nitrogenous products. They have an excellent
texture, generally solid and firm, free from all tendency to
openness; and, moreover, their flavor is clean and entirely
devoid of the sharp, undesirable tang that so frequently
appears in old cheese. The keeping quality of such cheese
is much superior to the ordinary product. The introduc-

» Babcock and Russell, 18 Kept. Wis. Expt. Stat., 1901.

Bacteria in Cheese. 171

tion of this new system of cheese-curing promises much
from a practical point of view, and undoubtedly a more
complete study of the subject from a scientific point of view
will aid materially in unraveling some of the problems as
to flavor production.

Theories of cheese curing. Within the last few years
considerable study has been given the subject of cheese cur-
ing or ripening, in order to explain how this physical and
chemical transformation is brought about.

Much of the misconception that has arisen relative to
the cause of cheese ripening comes from a confusion of
terms. In the ordinary use of the word, ripening or cur-
ing of cheese is intended to signify the sum total of all
the changes that result in converting the green product
as it comes from the press into the edible substance that is
known as cured cheese. As previously shown, the most
marked chemical transformation that occurs is that which
has to do with the peptonization or breaking down of the
casein. It is true that under ordinary conditions this de-
composition process is also accompanied with the forma-
tion of certain flavor-producing substances, more or less
aromatic in character; but it by no means follows that these
two processes are necessarily related to each other. The
majority of investigators have failed to consider these two
questions of casein decomposition and flavor as independ-
ent, or at least as not necessarily related. They are un-
doubtedly closely bound together, but it will be shown later
that the problems are quite different and possibly suscep-
tible of more thorough understanding when considered
separately.

In the earlier theories of cheese ripening it was thought
to be purely a chemical change, but, with- the growth of

172 Da try Bacteriology.

bacteriological science, evidence was forthcoming that
seemed to indicate that the activity of organisms entered
into the problem. SchafFer1 showed that if milk was boiled
and made into cheese, the casein failed to break down.
Adametz 2 added to green cheese various disinfectants, as
creolin and thymol, and found that this practically stopped
the curing process. From these experiments he drew the
conclusion that bacteria must be the cause of the change,
because these organisms were killed; but when it is con-
sidered that such treatment would also destroy the activity
of enzyms as well as vital ferments, it is evident that these
experiments were quite indecisive.

A determination of the nature of the by-products found
in maturing cheese indicates that the general character of
the ripening change is a peptonization or digestion of the
casein.

Until recently the most widely accepted views relating
to the cause of this change have been those which ascribed
the transformation to the activity of micro-organisms,
although concerning the nature of these organisms there
has been no unanimity of opinion. The overwhelming
development of bacteria in all cheeses naturally gave sup-
port to this view; and such experiments as detailed above
strengthened the idea that the casein transformation could
not occur where these ferment organisms were destroj'ed.

The very nature of the changes produced in the casein
signified that to take part in this process any organism
must possess the property of dissolving the proteid mole-
cule, casein, and forming therefrom by-products that are
most generally found in other digestive or peptonizing
changes of this class.

. 'Schaffer, Milch Zeit, 1889, p. 146.
'Adametz, Land w. Jahr., 18:261.

Bacteria in Cheese. 173

Digestive bacterial theory. The first theory propounded
was that of Duclaux,1 who in 1887 advanced the idea that
this change was due to that type of bacteria which is able
to liquefy gelatin, peptonize milk, and cause a hydrolytic
change in proteids. To this widely-spread group that he
found in cheese, he gave the generic name Tyrothrix
(cheese hairs). According to him, these organisms do not
function directly as ripening agents, but they secrete an
enzym or unorganized ferment to which he applies the
name casease. This ferment acts upon the casein of milk,
converting it into a soluble product known as caseone.
These organisms are found in normal milk, and if they
function as casein transformers, one would naturally expect
them to be present, at least frequently, if not predominat-
ing in the ripening cheese; but such is not the case. In
typical cheddar or Swiss cheese, they rapidly disappear
(p. 168), although in the moister, softer varieties, they per-
sist for considerable periods of time. According to Freud-
enreich, even where these organisms are added in large
numbers to the curd, they soon perish, an observation that
is not regarded as correct by the later adherents to the di-
gestive bacterial theory, as Adametz and Winkler.

Duclaux's experiments were made with liquid media for
isolation purposes, and his work, therefore, cannot be re-
garded as satisfactory as that carried out with more modern
technical methods. Recently this theory has been revived
by Adametz,2 who claims to have found in Em men thaler
cheese a digesting species, one of the Tyrothrix type, which
is capable of peptonizing the casein and at the same time
producing the characteristic flavor of this class of cheese.

i Duclaux, Le Lait, p. 213.

« Adametz, Oest. Molk. Zeit., 1900, Nos. 16-18.

174: Dairy Bacteriology.

This organism, called by him Bacillus nolilis, the Edelpilz
of Emmenthaler cheese, has been subjected to comparative
experiments, and in the cheese made with pure cultures of
this germ better results are claimed to have been secured.
Sufficient experiments have not as yet been reported by
other investigators to warrant the acceptance of the claims
made relative to the effect of this organism.

Lactic-acid bacterial theory. It has already been shown
that the lactic-acid bacteria seems to find in the green
cheese the optimum conditions of development; that they
increase enormously in numbers for a short period, and
then finally decline. This marked development, coincident
with the breaking down of the casein, has led to the view
which has been so ably expounded by Freudenreich ' that
this type of bacterial action is concerned in the ripening of
cheese. This group of bacteria is, under ordinary condi-
tions, unable to liquefy gelatin, or digest milk, or, in fact,
to exert, under ordinary conditions, any proteolytic or pep-
tonizing properties. This has been the stumbling-block to
the acceptance of this hypothesis, as an explanation of
the breaking down of the casein. Freudenreich has re-
cently carried on experiments which he believes solve the
problem. By growing cultures of these organisms in milk,
to which sterile, freshly precipitated chalk had been added,
he was able to prolong the development of bacteria for a
considerable period of time, and as a result finds that an
appreciable part of the casein is digested; but this action is
so slow compared with what normally occurs in a cheese,
that exception may well be taken to this type of experi-
ment alone. Weigmann 2 inclines to the view that the

1 Freudenreich, Landw. Jahr. d. Schweiz, 1897, p. 85.

* Weigmann, Cent. f. Bakt., II Abt., 1898, 4:593; also 1899, 5:630.

Bacteria in Cheese. 175

lactic-acid bacteria are not the true cause of the peptoniz-
ing process, but that their development prepares the soil,
as it were, for those forms that are more directly concerned
in the peptonizing process. This they do by developing
an acid substratum that renders possible the more luxuriant
growth of the aroma-producing species. According to
Goriiii,1 certain of the Tyrothrix forms function at high
temperatures as lactic acid producing bacteria, while at
lower temperatures they act as peptonizers. On this basis
he seeks to reconcile the discrepancies that appear in the
experiments of other investigators.

Milk enzym (galactase) theory. In 1897 B ibcock and
the writer2 showed experimentally that milk is digested
spontaneously when treated with various anaesthetics like
ether, chloroform and benzol. Under these conditions it w.is
demonstrated that bacterial activity was entirely suppressed.
Furthermore the3r showed that the nature of the by-prod-
ucts produced in such cases was identical with those found
in a maturing cheese, albumoses, peptones, arnido-acids and
ammonia being present in varying amounts. When milk
or curd was heated to 175° F. or above, or treated with
strong chemicals, this digestive process was stopped.

Under these conditions the only agents capable of pro-
ducing such changes were enzyms, and they found that it
was possible to concentrate this milk enzyin in centrifuge
slime. The addition of these slime extracts to boiled milk
started anew the digestive process, and produced by-prod-
ucts identical with those occurring in normal cheese. This
enzym, called by them galactase, on account of its origin
in milk, is somewhat closely related to the tryptic type

1 Grorini, Abs. in Expt. Stat. Rec.. 11:388.

a Babcock and Cussell, 14 Kept. Wis. Expt. Stat., 1897, p. 161.

176 Dairy Bacteriology.

of ferments, but yet sufficiently distinct to be separated
from trypsin. Jensen1 has also shown that the addition of
pancreatic extracts to cheese accelerated the formation of
soluble nitrogenous products.

The action of galactase in milk and cheese has been con-
firmed by Freudenreich2 and Jensen,8 as well as by Ameri-
can investigators, and this euzym is now quite generally
accepted as the cause of the decomposition of the casein.
Freudenreich admits that it plays a role in converting the
casein into albumose and peptones, but that the lactic-acid
bacteria are chiefly responsible for the further decomposi-
tion of the nitrogen to amid form.

Failure before to recognize the presence of galactase in
milk is attributable t0 the fact that all attempts to secure
sterile milk had been made by heating the same, in which
case galactase was necessarily destroyed. A brief exposure
at 176° F. is sufficient to destroy its activity, and even an
exposure at lower temperatures weakens its action consider-
ably, especially if the reaction of the medium is acid. This
undoubtedly explains the contradictory results obtained in
the ripening of cheese from pasteurized milk, such cheese
occasionally breaking down in an abnormal manner.

The results mentioned on page 172, in which cheese failed
to ripen when treated with disinfectants, — experiments
which were supposed at that time to be the foundation of
the bacterial theory of casein digestion — are now explica-
ble on an entirely different basis. In these cases the casein
was not peptonized, because these strong disinfectants de-
stroyed the activity of the enzyms as well as the bacteria.

» Jensen, Cent f. Bact. II Abt., 3:752.

• Freudenreich, Cent, f . Bakt., It Abt., 1900, 6:882.

• Jensen, Ibid., 1900, 0:734.

Bacteria in Cheese. 177

Influence of rennet on ripening. The addition of in-
creased quantities of rennet extract facilitate the breaking
down of the casein in cheese. This is due to the fact that
such extracts always contain pepsin, the digestive enzym
found in the stomach. This enzym exerts a digestive ef-
fect on the casein of cheese, producing those decomposi-
tion products (albumoses and higher peptones) that charac-
terize peptic action. This effect is dependent upon the
acidity of the milk, and does not become marked until
there is about 0.3 per cent of lactic acid, which is about
the amount developed in the cheddar process. The effect
of the rennet then is to aid the galactase in the peptoniza-
tion of the casein.1

Conditions determining: quality. In determining the qual-
ity of cheese, several factors are to be taken into considera-
tion. First and foremost is the flavor, which determines
more than anything else the value of the product. This
should be mild and pleasant, although with age the inten-
sity of the same generally increases but at no time should
it have any bitter, sour, or otherwise undesirable taste or

FIG. 32. Showing texture of cheese cured at different temperatures. Right
hand, 60° F., crumbly texture; left hand, 40° F., waxy texture.

» 17 Kept. Wis. Expt. Stat,, 1900, p. 103.
13

178 Dairy Bacteriology.

aroma. Texture registers more accurately the physical
nature of the ripening. The cheese should not be curdy
and harsh, but should yield quite readily to pressure under
the thumb, becoming on manipulation waxy and plastic
instead of crumbly or mealy. Body refers to the openness
or closeness of the curd particles, a close, compact mass
being most desirable. The color of cheese should be even,
not wavy, streaked or bleached.

For a cheese to possess all of these characteristics in an
optimum degree is to be perfect in every respect — a condi-
tion that is rarely reached.

So many factors influence this condition that the problem
of making a perfect cheese becomes exceedingly difficult.
Not only must the quality of the milk — the raw material
to be used in the manufacture — be perfectly satisfactory,
but the factory management while the curds are in the vat
demands great skill and careful attention; and finally, the
long period of curing in which variation in temperature
or moisture conditions may seriously affect the quality, —
all of these stages, more or less critical, must be successfully
gone through, before the product reaches its highest state
of development.

It is of course true that many phases of this complex
series of processes have no direct relation to bacteria, yet
it frequently happens that the result attained is influ-
enced at some preceding stage by the action of bac-
teria in one way or another. Thus the influence of the
acidity developed in the curds is felt throughout the whole
life of the cheese, an over-development of lactic-acid bac-
teria producing a sour condition that leaves its impress not
only on flavor but texture. An insufficient development
of acid fails to soften the curd-particles so as to permit of

Bacteria in Cheese. 179

close matting, the consequence being that the body of the
cheese remains loose and open, a condition favorable to the
development of gas-generating organisms.

Production Of flavor. The importance of flavor as deter-
mining the quality of cheese makes it imperative that the
nature of the substances that confer on cheese its peculiar
aromatic qualities and taste be thoroughly understood. .It
is to be regretted that the results obtained so far are not
more satisfactory, for improvement in technique is hardly
to be expected until the reason for the process is thoroughly
understood.

The view that is most generally accepted is that this
most important phase of cheese curing is dependent upon
bacterial activity, but the organisms that are concerned in
this process have not as yet been satisfactorily determined.
In a number of cases, different species of bacteria have
been separated from milk and cheese that have the power
of producing aromatic compounds that resemble, in some
cases, the peculiar flavors and odors that characterize some
of the foreign kinds of cheese; but an introduction of these
into curd has not resulted in the production of the peculiar
variety, even though the methods of manufacture and cur-
ing were closely followed. The similarity in germ content
in different varieties of cheese made in the same locality
has perhaps a bearing on this question of flavor as related
to bacteria. Of the nine different species of bacteria found
in Emmenthaler cheese by Adametz, eight of them were
also present in ripened Hauskase. If specific flavors are
solely the result of specific bacterial action, it might natu-
rally be expected that the character of the flora would differ.

Some suggestive experiments were made by Babcockand
Russell on the question of flavor as related to bacterial

ISO Dairy Bacteriology.

growth, by changing the nature of the environment in
cheese by washing the curds on the racks with warm
water. In this way the sugar and most of the ash were re-
moved. Under such conditions the character of the bac-
terial flora was materially modified. While the liquefying
type of bacteria was very sparse in normal cheddar, they
developed luxuriantly in the washed cheese. The flavor
at the same time was markedly affected. The control ched-
dar was of good quality, while that made from the washed
curds was decidedly off, and in the course of ripening be-
came vile. It may be these two results are simply coinci-
dences, but other data1 bear out the view that the flavor
was to some extent related to the nature of the bacteria
developing in the cheese. This was strengthened materi-
ally by adding different sugars to washed curds, in which
case it was found that the flavor was much improved, while
the more normal lactic-acid type of bacteria again became
predominant.

Ripening Of moldy Cheese. In a number of foreign
cheeses, the peculiar flavor obtained is in part due to the
action of various fungi which grow in the cheese, and there
produce certain by-products that flavor the cheese. Among
the most important of these are the Roquefort cheese of
France, Stilton of England, and Gorgonzola of Italy.

Roquefort cheese is made from goat's or cow's milk, and
in order to introduce the desired mold, which is the ordi-
nary bread-mold, Penicillium glaucum, carefully-prepared
moldy bread-crumbs are added to the curd.

At ordinary temperatures this organism develops too
rapidly, so that the cheese to ripen properly must be kept
at a low temperature. The town of Roquefort is situated

i Bibcock and Russell, 18 Kept. Wis. Expt. Stat., 1901.

Bacteria in Cheese. 181

in a limestone country, in a region full of caves, and it is
in these natural caves that most of the ripening is done.
These caverns are always very moist and have a tempera-
ture ranging from 35° to 44° F., so that the growth of the
fungus is retarded considerably. The spread of the mold
throughout the ripening mass is also assisted in a mechan-
ical way. The partially-matured cheese are run through
a machine that pricks them full of small holes. These
slender canals allow the mold organism to penetrate the
whole mass more thoroughly, the moldy straw matting
upon which the ripening cheese are placed helping to fur-
nish an abundant seeding of the desired germ.

When new factories are constructed it is of advantage
to introduce this necessary germ in quantities, and the
practice is sometimes followed of rubbing the walls and
cellars of the new location with material taken from the
old established factory. In this custom, developed in purely
an empirical manner, is to be seen a striking illustration
of a bacteriological process crudely carried out.

In the Stilton cheese, one of the highly prized moldy
cheeses of England, the desired mold fungus is introduced
into the green cheese by exchanging plugs taken with a
cheese trier from a ripe Stilton.

Ripening of soft cheese. The type of ripening which
takes place in the soft cheeses is materially different from
that which occurs in the hard type. In many cases, the
peptoiiizing action does not go on uniformly through-
out the cheese, but is hastened on the exterior by the de-
velopment of organisms that exert a solvent effect on the
casein. For this reason, soft cheeses are usually made up
in small sizes, so that this action may be facilitated. The
bacteria that take part in this process are those that are

182 Dairy Bacteriology.

able to form enzyms (similar in their action to tr37psinr
galactase, etc.), and these soluble ferments gradually diffuse
from the outside through the cheese.

Most of these peptonizing bacteria are hindered in their
growth by the presence of lactic acid, so that in many cases
the appearance of the digesting organisms on the surface
is delayed until the acidity of the mass is reduced to the
proper point by the development of other organisms, prin-
cipally molds, which prefer an acid substratum for their
growth.

In Brie cheese a blue coating of mold develops on the
surface. In the course of a few weeks, a white felting ap-
pears which later changes to red. This slimy coat below
the mold layer is made up of diverse species of bacteria and
fungi that are able to grow after the acid is reduced by the
blue mold. The organisms in the red slimy coat act upon
the casein, producing an alkaline reaction that is unfavor-
able to the growth of the blue mold. Two sets of organ-
isms are, therefore essential in the ripening process, one
preparing the soil for the ferment that later produces the
requisite ripening changes. As ordinarily carried on, the
process is an empirical one, and if the red coat does not de-
velop as expected, the maker resorts to all kinds of devices
to bring out the desired ferment. The appearance of the
right form is dependent, however, upon the proper reaction
of the cheese, and if this is not suitable, the wished-for
growth will not appear.

INFLUENCE OF BACTERIA IN ABNORMAL CHEESE PROCESSES.

The reason why cheese is more subject to abnormal fer-
mentation than butter is because its high nitrogen content
favors the continued development of bacteria for some time

Bacteria in Cheese. 183

after it is made. It must be borne in mind, in considering
the more important of these changes, that not all defective
conditions in cheese are attributable to the influence of
living organisms. Troubles frequently arise from errors in
manufacturing details, as too prolonged cooking of curds,
too high heating, or the development of insufficient or too
much acid. Then again, the production of undesirable
flavors or impairment in texture may arise from imperfect
curing conditions.

Our knowledge regarding the exact nature of these indefi-
nite faults is as yet too inadequate to enable many of these
undesirable conditions to be traced to their proper source;
but in many cases the taints observed in a factory are due
to the abnormal development of certain bacteria, capable
of evolving unpleasant or even putrid odors. Most of them
are seeded in the milk before it comes to the factory and
are due to careless manipulation of the milk while it is still
on the farm. Others gain access to the milk in the fac-
tory, owing to unclean conditions of one sort or another.
Sometimes the cheese-maker is able to overcome these
taints by vigorous treatment, but often they pass on into
the cheese, only to detract from the market value of the
product. Most frequently these "off" flavors appear in
cheese that are cured at too high temperatures, say above
65° F.

"Gassy" fermentations in cheese. One of the worst and
at the same time most common troubles in cheese-making
is where the cheese undergoes a fermentation marked by
the evolution of gas. The presence of gas is recognized by
the appearance either of spherical or lens-shaped holes of
various sizes in the green cheese; often they appear in
the curd before it is put to press. Usually in this condi-

184

Dairy Bacteriology.

tion the curds look as if they had been punctured with a
pin, and are known as upin holey" curds. Where the gas
holes are larger, they are known as "Swiss holes" from
their resemblance to the normal holes in the Swiss pro-
duct. If the development of gas is abundant, these holes
are restricted in size. Often the formation of gas may be
so intense as to cause the curds to float on the surface of
the whey before they are removed. Such curds are known
as "floaters" or "bloatars."

If " gassy " curds are put to press, the abnormal fermen-
tation may continue. The further production of gas causes
the green cheese to " huff" or swell, until it may be con-
siderably distorted as in Fig. 33. In such cases the texture

Fio. 33. Cheese made from gassy milk.

of the cheese is greatly injured, and the flavor is generally
impaired.

Such abnormal changes may occur at any season of the
year, but the trouble is most common in summer, espe-
cially in the latter part.

This defect is less likely to occur in cheese that is well

Bacteria in Cheese. 185

cheddared than in sweet curd cheese. When acidity is pro-
duced, these gassy fermentations ar^ checked, and in good
cheddar the body is so close and firm as not readily to per-
mit of gaseous changes.

In Swiss cheese, which is essentially a sweet curd cheese,
these fermentations are very troublesome. Where large
holes are formed in abundance (blahen), the trouble reaches
its maximum. If the gas holes are very numerous and

FIG. 34. Block Swiss cheese showing " gassy " fermentation. '

therefore small it is called a " nissler." Sometimes the
normal " eyes " are even wanting when it is said to be
itblind"ora"glasler."

One method of procedure which is likely to cause trouble
in Swiss factories is often produced by the use of sour,
fermented whey in which to soak the natural rennets.
Freudenreich and Steinegger J have shown that a much more
uniform quality of cheese can be made with rennet extract
if it is prepared with a starter made from a pure lactic fer-
ment.

The cause of the difficulty has long been charged to va-

i Cent. f. Bakt. 1899, p. 14.

186 Dairy Bacteriology.

rious sources, such as a lack of aeration, improper feeding,
retention of animal gases, etc., but in all these cases it was
nothing more than a surmise. Very often the milk does
not betray any visible symptom of fermentation when re-
ceived, and the trouble is not to be recognized until the
process of cheese-making is well advanced.

Studies from a biological standpoint have, howevert
thrown much light on this troublesome problem; and it
is now known that the formation of gas, either in the
curd or after it has been put to press, is due entirely
to the breaking down of certain elements, such as the
sugar of milk, due to the influence of various living germs.
This trouble is, then, a type fermentation, and is, therefore,
much more widely distributed than it would be if it was
caused by a single specific organism. These gas-produc-
ing organisms are to be found, sparingly at least, in al-
most all milks, but are normally held in check by the
ordinary lactic species. Among them are a large number
of the bacteria, although yeasts and allied germs are often
present and are likewise able to set up fermentative changes
of this sort. In these cases the milk-sugar is decomposed
in such a way as to give off CO 9 and H, and in some casesr
alcohol.

According to Guillebeau, a close relation exists between
those germs that are able to produce an infectious inflam-
mation (mastitis) in the udder of the cow and some forms
capable of gas evolution. Several outbreaks of " gassy "
milk have been traced directly to animals suffering from
an acute udder inflammation in which it has been shown
that the organisms producing this disease were the direct
cause of the gas production in the milk.

If pure cultures of these gas-producing bacteria are added

Bacteria in Cheese. 187

to perfectly sweet milk, it is possible to artificially produce
the conditions in cheese that so frequently appear in prac-
tice.

Treatment of " pin-holey " curds. When this type of
fermentation appears during the manufacture of the cheese,
the maker can control it in part within certain limits.
These methods of treatment are, as a rule, purely mechan-
ical, as when the curds are piled and turned, and subse-
quently ground in a curd mill. After the gas has been
forced out, the curds are then put to press and the whole
mats into a compact mass.

Another method of treatment based upon bacteriological
principles is the addition of a starter to induce the for-
mation of acid. Where acid is developed as a result of
the growth of the lactic-acid bacteria, the gas-producing
species do not readily thrive. Another reason why acid
aids in repressing the development of gas is that the curd
particles are partially softened or digested by the action of
the acid. This causes them to mat together more closely,
and there is not left in the cheese the irregular mechan-
ical openings in which the developing gas may find lodg-
ment.

Another method that is also useful with these curds is to
employ salt. This represses gaseous fermentations, and the
use of more salt than usual in making the cheese will very
often restrain the production of gas. Tendency to form
gas in Edam cheese is controlled by the addition of a starter
prepared from slimy whey (lange wei) which is caused by
the development of an acid-forming organism.

Some have recommended the custom of washing the
curds to remove the whey and the gas-producing bacteria
contained therein. Care must be taken not to carry this

188 Dairy Bacteriology.

too far, for the removal of the sugar permits taint-produc-
ing organisms to thrive.1

The temperature at which the cheese is cured also mate-
rially affects the development of gas. At high curing tem-
peratures, gas-producing organisms develop rapidly; there-
fore more trouble is experienced in summer than at other
seasons.

If milks which are prone to undergo " gassy " develop-
ment are excluded from the general supply, it would be
possible to eliminate the source of the entire trouble. To
aid in the early recognition of such milks that are not ap-
parently affected when brought to the factory, fermenta-
tion or curd tests (p. 76) are of great value. The use of
this test in the hands of the factory operator often enables
him to detect the exact source of the trouble, which may
frequently be confined to the milk delivered by a single
patron.

"Fruity" or "sweet" flavor. Not infrequently the
product of a factory may acquire during the process of
ripening what is known as a " sweet" or u fruity" flavor.
This flavor resembles the odor of fermented fruit or the
bouquet of certain kinds of wine. It has been noted in
widely different sections of the country and its presence
bears no relation to the other qualities of the cheese. The
cause of this trouble has recently been traced2 to the pres-
ence of various kinds of yeasts. Ordinarily yeasts are
rarely present in good cheese, but in cheese affected with
this trouble they abound. The addition of starters made
from yeast cultures resulted in the production of the unde-
sirable condition.

1 Babcock and Russell, 18 Kept. Wis. Expt. Stat., 1901.

3 Harding, Rogers and Smith, Bull. 183, N. Y. (Geneva) Expt. Stat., Dec., 1900.

Bacteria in Cheese. 189

Mottled Cheese. The color of cheese is sometimes cut to
that extent that the cheese presents a wavy or mottled ap-
pearance. This condition is apt to appear if the ripening
temperature is somewhat high, or larger quantities of ren-
net used than usual. The cause of the defect is obscure,
but it has been demonstrated that the same is communi-
cable if a starter is made by grating some of this mottled
cheese into milk. The bacteriology of the trouble has not
yet been worked out, but the defect is undoubtedly due to
an organism that is able to grow in the ripening cheese.
It has been claimed that the use of a pure lactic ferment
as a starter enables one to overcome this defect.

Bitter Cheese. Bitter flavors are sometimes developed in
cheese especially where the ripening process has not been
fully completed, or where improper temperatures have been
maintained for a considerable length of time. Several or-
ganisms associated with this abnormal fermentation have
been noted.

Guillebeau1 isolated several forms from Emmenthaler
cheese which he connected with udder inflammation that
were able to produce a bitter substance in cheese.

Von Freudenreich 2 has described a new form Micrococ-
cus easel amari (micrococcus of bitter cheese) that was
found in a sample of bitter cheese. This germ is closely
related to Conn's micrococcus of bitter milk. It develops
lactic acid rapidly, coagulating the milk and producing an
intensely bitter taste in the course of one to three days.
When milk infected with this organism is made into
cheese, there is formed in a few days a decomposition prod-
uct that imparts a marked bitter flavor to the cheese.

1 Guillebeau, Landw. Jahr., 1890, p. 27.

2 Freudenreich, Fuehl. Landw. Ztg., 43:361.

190 Dairy Bacteriology.

It is peculiar that some of the organisms that are able to
produce hitter products in milk do not retain this property
when the milk is worked up into cheese.

Putrid or rotten Cheese. Sometimes cheese undergoes
a putrefactive decomposition in which the texture is pro-
foundly modified and various foul smelling gases are
evolved. These often hegin on the exterior as small cir-
cumscrihed spots that slowly extend into the cheese, chang-
ing the casein into a soft slimy mass. Then, again, the
interior of the cheese undergoes this slimy decomposition.
The soft varieties are more prone toward this fermentation
than the hard, although the firm cheeses are by no means
exempt from the trouble. The " Verlaufen " or " running"
of lim burger cheese is a fermentation allied to this. It is
where the inside of the cheese breaks down into a soft
semi-fluid mass. In severe cases, the rind may even be
ruptured, in which case the whole interior of the cheese
flows out as a thick slimy mass, having sometimes a putrid
odor. The conditions favoring this putrid decomposition
are usually associated with an excess of moisture, and an
abnormally low ripening temperature.

Rusty Spot. This name is applied to the development
of small yellowish-red or orange spots that are formed
sometimes throughout the whole mass of cheddar cheese.
A close inspection shows the colored points to be located
along the edges of the curd particles. According to Hard-
ing,1 this trouble is most common in spring and fall. The
cause of the difficulty has been traced by Connell 8 to the
development of a chromogenic bacterium, Bacillus ruden-
sis. The organism can be most readily isolated on a po-

i Bull. 183, N. Y. (Geneva) Expt. Stat., Dec. 1900.
'Connell, Bull. Canadian Dept. of Agr., 1897.

Bacteria in Cheese. 191

tato surface rather than with the usual isolating media,
agar or gelatin.

Other pigment Changes. Occasionally, with the hard
type of cheese, but more frequently with the softer foreign
varieties, various abnormal conditions arise that are marked
by the production of different figments in or on the cheese.
More frequently these are merely superficial and affect only
the outer layers of the cheese. Generally they are attrib-
utable to the development of certain chromogenic organ-
isms (bacteria, molds and yeasts), although occasionally
due to other causes, as in the case of a blue discoloration
sometimes noted in foreign cheese made in copper kettles.1

De Vries2 has described a blue condition that is found
in Edam cheese. It appears first as a small blue spot on
the inside, increasing rapidly in size until the whole mass
is affected. This defect he was able to show was produced
by a pigment-forming organism, B. cyaneo-fuscus. By the
use of slimy whey (lange wei) this abnormal change was
controlled.

Moldy Cheese. With many varieties of cheese, especially
some of the foreign types, the presence of mold on the ex-
terior is not regarded as detrimental; in fact a limited de-
velopment is much desired. In hard rennet cheese as
cheddar or Swiss, the market demands a product free from
mold, although it should be said that this condition is im-
posed by the desire to secure a good-looking cheese rather
than any injury in flavor that the mold causes. Mold
spores are so widely distributed that, if proper temperature
and moisture conditions prevail, these spores will always
develop. At temperatures in the neighborhood of 40° P.

» SchmOger, Milch Zeit., 1883, p. 483.
2 De Vries, Milch Zeit., 1888, pp. 861. 885.

192 Dairy Bacteriology.

and below, mold growth is exceedingly slow, and often fructi-
fication does not occur, the only evidence of the mold being
the white, felt-like covering that is made up of the vegetat-
ing filaments. The use of paraffin has been suggested as a
means of overcoming this growth, the cheese being dipped
at an early stage into melted paraffin. Recent experiments
have shown that "off" flavors are apt to develop where
cheese are paraffined directly from the press. Furthermore,
the paraffin has a tendency to crack and separate from the
rind, thus allowing molds to develop beneath the paraffin
coat, where the conditions are ideal as to moisture, for
evaporation is excluded and the air consequently saturated.
The use of formalin (2$ solution) has been suggested as a
wash for the outside of the cheese. This substance or
sulfur is also applied in a gaseous form. Double bandag-
ing is also resorted to as a means of making the cheese
more presentable through the removal of the outer bandage.
The nature of these molds has not been thoroughly
studied as yet. • The ordinary blue-green bread mold,
Penicillium glaucum, is most frequently found, but there
are numerous other forms that appear, especially at low
temperatures.

Poisonous Cheese* Cases of acute poisoning arising from
the ingestion of cheese are reported from time to time.
Vaughan has succeeded in showing that this condition is
due to the formation of a highly poisonous alkaloid which
he has isolated, and which he calls tyrotoxicon.1 This poison-
ous ptomaine has also been demonstrated in milk and other
milk products, and is undoubtedly due to the development
of various putrefactive bacteria that find their way into
the milk. It seems quite probable that the development

»Zeit. f. physiol. theinie, 10:146.

Bacteria in Cheese. 193

of these toxic organisms can also go on in the cheese after
it is taken from the press.

Prevention or cheese .defects* The defective conditions
previously referred to can rarely be overcome in cheese
so as to improve the affected product, for they only be-
come manifest in most cases during the later stages of the
curing process. The only remedy against future loss is to
recognize the conditions that are apt to prevail during the
occurrence of an outbreak and see that the cheese are han-
dled in such a way as to prevent a recurrence of the diffi-
culty.

Many abnormal and undesirable results are incident to
the manufacture of the product, such as u sour " or
41 mealy " cheese, conditions due to the development of too
much acid in the milk or too high a " cook." These are
under the direct control of the maker and for them he
alone is responsible. The development of taints due to
the growth of unwelcome bacteria that have gained access
to the milk while it is yet on the farm are generally be-
yond the control of the cheese maker, unless they are so
pronounced as to appear during the handling of the curds.
If this does occur he is sometimes able, through the inter-
vention of a starter or by varying some detail in making,
to handle the milk in such a way as to minimize the trou-
ble, but rarely is he able to eliminate it entirely.

One of the most strenuous duties which the maker must
perform at all times is to point out to his patrons the ab-
solute necessity of their handling the milk in such a way
as to prevent the introduction of organisms of a baleful
type.

13

INDEX.

Acid, effect of, on churning, 137; in
butter-making, 138.

Acid test, 51.

Aeration of milk, 59.

Aerobic bacteria, 7.

Alcoholic fermentation in milk, 72.

Anaerobic bacteria, 7.

Animal, influence of, on milk infec-
tion, 42.

Animal odor, 56.

Anthrax, 94.

Antiseptics, 9, 88.

Aroma, of butter, 140.

Bacillus: definition of, 2.

acidi lactici, 64; cyaneo-fuscus,
188; cyanogenus, 74; foetidus lactis,
157; lactis aerogenes, 65; lactis
erythrogenes, 74; lactis saponacei,
67; lactis viscosus, 71; nobilis, 162,
174; prodigiosus, 74; rudensis, 188;
synxanthus, 75 ; tuberculosis, 84.

Bacteria: on hairs, 34; kinds in milk, 64;
in barn air, 42; in milk pails, 27; in
butter, 154; classification of, 4; in
cheese, 160; culture of, 17; in cream,
128; discovery of, 1; external con-
ditions affecting, 8; form of, 2; in
butter, 142; in butter- making, 127;
in centrifuge slime, 40; in fore
milk, 30; in rennet, 163; in sepa-
rator slime, 40; manure, 33; num-
ber of, in milk, 49.

Distribution of: milk of American
cities, 49; European cities, 49; in re-
lation to cheese, 168.
Of disease: anthrax, 94; cholera, 98;
diphtheria, 99; lockjaw, 94; toxic,

100; tuberculosis, 84; typhoid fever,
98.

Methods of study of: culture, 15;
culture media, 13; isolation, 14.

Bitter butter, 158; cheese, 189; milk, 72.

Bloody milk, 74.

Blue cheese, 191; milk, 74.

Bovine tuberculosis, 84.

Brie cheese, 182.

Butter: bacteria in, 154; bitter, 158;
"cowy," 157; fishy, 159; lardy,
157; moldy, 158; mottled, 156; oily,
158; putrid, 156; rancid, 155; tal-
lowy, 157; turnip flavor in, 157.
Making: aroma, 140; flavor in, 140;
pure culture, 143; in ripening of
cream, 136.

Butyric acid fermentation, 69.

By-products of factory, methods of pre-
serving, 25.

Casease, 68

Caseone, 68.

Centrifugal force, cleaning milk by,
39, 105.

Cheese: bacterial flora of, 168: bitter,
189; blue, 187; Brie, 182; Edam, 72,
162; Emmen thaler, 185; flavor of,
179; gassy fermentations in, 183;
Gorgonzola, 180; molds on, 191;
mottled, 189; "nissler," 185; poi-
sonous, 192; putrid, 190; ripening
of moldy, 180; ripening of soft, 181;
Koquefort, 180; rusty spot in, 188;
Stilton, 180; Swiss, 185.
Making and curing: chemical changes
in curing, 166; influence of temper-
ature on curing, 169; influence of

196

Index.

rennet, 177; physical changes in
curing, 165; prevention of defects,
193; starters in, 161; temperature
in relation to bacterial influence,
169.

Theories of curing: digestive, 173;
galactase, 175, 177; lactic acid, 174

Chemical changes in cheese-ripening,
166.

Chemical disinfectants in milk: bleach-
ing powder, 81; corrosive subli-
mate, 81; formalin, 80; sulfur, 80;
whitewash, 81 ; vitriol, 81.

Chemical preservatives, 80.

Children, milk for, 45.

Cholera in milk, 98.

Coccus, definition of, 2.

Cold, influence on bacteria, 8, 47.

Contamination of milk through disease
germs, 95, 191.

Coolers, 181.

Cooling milk, 123.

Cream, bacterial changes in, 135; me-
chanical causes for bacteria in, 135;
pasteurized, 118; restoration of con-
sistency of pasteurized, 119.
Ripening of, 136; advantage of pure
cultures in, 144; by natural start-
ers, 142; characteristics of pure
cultures in, 145; objections to pure
cultures in, 146; principles of pure
cultures in, 143; propagation of
pure cultures, 151; home-made
starters in, 146.

Creaming methods, 134.

Curd test, 76.

Dairy utensils a source of contamina-
tion, 21.

Diarrhoeal diseases, 100.
Digesting bacteria, 67.
Digestibility of heated milk, lit
Diphtheria, 99.

Dirt in milk, 34.

Dirt, exclusion of, 36.

Disease germs in milk, 95; effect of
heat on, 91 ; origin of, 83.

Disinfectants, 9: carbolic acid, 81; chlo-
ride of lime, 81; corrosive subli-
mate, 81; formalin, 80; sulfur, 80;
vitriol salts, 81 ; whitewash, 79.

Disinfectants in milk: alkaline salts,
106; boracic acid, 106; formalin,
106; preservaline, 107; salicylic acid,
106.

Domestic pasteurizing apparatus, 125.

Drugs, taints in milk due to, 5(5.

Drying, effect of, 8.

Edam cheese, 72, 162.
Emmen thaler cheese, 185.
Endospores, 3.
Enzyms, 10.

Factory by-products, 25; treatment of,
25.

Farrington alkaline tablet, 52.

Fecal bacteria, effect of, on butter, 35.

Fermentation:
In cheese: gassy, 183.
In milk: alcoholic, 72; bitter, 72;
blue, 74; butyric. 69; digesting, 67;
slimy, 69; gassy, 66; kephir, 72;
koumiss, 72; lactic acid, 63; lange-
wei, 72; red, 74; ropy, 69; slimy,
69; soapy, 73; souring, 63; sweet
curdling, 67; treatment of, 75.
Tests, 76; Gerber's, 76; Walther's,
76; Wisconsin curd, 76.

Filtration of milk, 104.

Fishy butter, 159.

Flavor: of butter, 140; of cheese, 179.

Foot and mouth disease, 93.

Fore milk, 29.

Formaldehyde, 80.

Formalin, 80.

Fruity flavor hi cheese, 188.

Index.

197

Galactase in cheese, 175,

Gassy fermentations: in cheese, 183;
in milk, 66; in Swiss cheese, 167,

Glasler, 185.

Gorgonzola cheese, 180.

Growth of bacteria, essential condi-
tions for, 4.

Hair, bacteria on, 34.

Heat, influence on bacterial growth, 8.

Heated .milk: characteristics of, 109;
action toward rennet, 112; body,
110; digestibility, 111; fermentative
changes, 111; flavor, 110; hydrogen
peroxid test in, 23; Storch's test, 23.

Hygienic milk, bacteria in, 44.

Infection of milk: animal, 82; dairy
utensils, 8; fore milk, 29; milker, 35.
Isolation of bacteria, methods of, 14.

Kephir, 72.
Koumiss, 72.

Lactic acid: fermentation in milk, 63;

theory in cheese-curing, 1?4.
Lange-wei, 72.
Lardy butter, 157,,
Light, action on bacteria, 9.

Manure, bacteria in, 33.

Methods: of isolation, 14; culture, 15.

Micrococcus casei amari, 187.

Microscope, use of, 17.

Milk: a bacterial food medium, 19; bac-
teria in, 48.

Disease organisms in: anthrax, 94;
cholera, 98; diphtheria, 99; foot
and mouth disease, 93 ; poisonous,
101; ptomaines, 101; scarlet fever,
99; tuberculosis, 84; typhoid fever,
98.

Contamination, 21: from air, 40; from
animal odors, 55; dirt, 32; distinc-
tion between bacterial and non-

bacterial, 58; fore milk, 28; infec-
tion in factory, 59; milker, 35; rel-
ative importance of various kinds,
42; utensils, 21.

Milk fermentations: alcoholic, 72; bit-
ter, 72; bloody, 74; blue, 74; buty-
ric acid, 69; gassy, 66, 167; kephir,
61; koumiss, 67; lactic acid, 63; red,
72; ropy, 69; slimy, 69, soapy, 74;
souring, 63; sweet curdling, 67;
tests for, 76; treatment of, 70; yel-
low, 70.

Milk, heated: action towards rennet,
112; digestibility, 111; flavor of, 110;
fermentative changes in, 111; hy-
drogen peroxid test, 110.

Milking machines, influence of, on
germ content, 36.

Milk preservation: chemical agents in,
106; condensation, 107; freezing,
108; heat, 108; pasteurization, 112;
sterilization, 113.

Milk-sugar as bacterial food, 19.

Mold, in butter, 158; in cheese, 191.

Mottled cheese, 189.

" Nissler " cheese, 185.

Odors, direct absorption of, in milk, 55.
Oidium lactis, 159.
Oily butter, 158.

Parasitic bacteria, 5.

Pasteurization of milk: acid test in,
123; bacteriological study of, 117,
133, 149; for butter, 147; for cheese,
162; for direct use, 113; of skim
milk, 25; details of, 112; tempera-
ture and time limit in, 114.

Pasteurizing apparatus: continuous
flow, 127; coolers, 131; Danish, 149;
domestic, 125; intermittent flow,
129; Potts, 131; Reid, 150; Russell,
130.

Pathogenic bacteria in milk, 82.

198

Index.

Penicillium glaucum, 159, 180, 190.

Pepsin, 10.

Physical changes in cheese-ripening,
165.

Poisonous bacteria: in cheese, 192; in
milk, 100, 101.

Preservaline, 167.

Preservation of milk: by exclusion, 103;
chemical agents, 106; condensing,
107; filtration, 104; freezing , 108;
pasteurization, 112; physical
agents, 107; sterilization, 112.

Ptomaine poisoning, 101.

Pure cultures, 15.

Pure culture starters: advantages of
144; characteristics of, 145; home-
made cultures compared with, 146;
propagation of, 157.

Putrid cheese, 190; butter, 156.

Rancidity in butter, 155.

Red milk, 74.

Rennet: action in heated milk, 112;
bacteria in, 163; influence of, on
cheese-ripening, 177.

Restoration of consistency in pasteur-
ized cream, 119.

Ripening of cheese: moldy cheese, 180;

soft cheese, 181.

Of cream, 136; artificial starters, 143;
natural starters, 142; principles of
pure culture starters in, 143.

Ropy milk, 69.

Roquefort cheese, 180.

Rusty spot in cheese, 190.

Rusty cans: effect of, on acidity, 53.

Sanitary milk, 44.
Sanitary pails, 39.
Saprophytic bacteria, 5.
Scarlet fever in milk, 99.
Separator slime: bacteria in, 40; tuber-
cle bacillus in, 93.
Size of bacteria, 2.

Skim-milk, a distributor of disease,
96.

Slimy milk, 69.

Soapy milk, 74.

Soft cheese, ripening of, 186.

Sources of contamination in milk: barn
air, 40; dairy utensils, 21; dirt from
animals, 32; factory cans, 27; fore-
milk, 29; milker, 35.

Souring of milk, 63.

Spirillum, definition of, 2.

Spores, 3.

Starters: in cheese-making, 161; in
butter-making, 142; propagation
of, 151; pure cultures in cream-
ripening, 143.

Sterilization of milk, 112.

Streptococcus Hollandicus, 72, 162.

Stilton cheese, 181.

Starch's test, 23.

Sulfur as a disinfectant, 81.

Sweet curdling milk, 68.

Sweet flavor in cheese. 188.

Swiss cheese, 177; gassy fermentations
in, 185.

Taints, absorption of, 55.

Taints, bacterial vs. physical, 58.

Taints in milk, absorption of, 55.

Taints, use of starters in overcoming,
79.

Taints in butter: putrid, 156; rancidity,
155; turnip flavor, 157.

Tallowy butter, 157.

Temperature: effect on bacterial de-
velopment, 6, 46; effect of low, 108;
effect of high, 108; and time limit
in milk pasteurization, 114.

Tests for milk: fermentation, 76;
Storch's, 23.

Theories in cheese-curing: digestive,
171; galactase, 175, 177; lactic acid,
174.

Trypsin, 10.

Index.

199

Tubercle bacillus: in milk, 90; in sep-
arator slime, 93; thermal death
limits, 115.

Tuberculin test, 86.

Tuberculosis, bovine, 84.

Turnip flavor in butter, 157.

Typhoid fever, 98.

Tyrogen, 162.

Tyrotoxicon, 101, 190.

Udder: milk germ-free in, 19; infection
of, 38; washing, 37; tuberculosis in,
87.

Viscogen, 119.

Water: as a source of infection, 61.
Whey vats, pollution of, 23; method of

preserving, 125; treatment of, in

vats, 123.
Whitewash, 81.
Wisconsin curd test, 76.

Yeasts: alcoholic ferments in milk, 72;
kephir, 72; fruity flavor in cheese,
186.

92011

THE UNIVERSITY OF CALIFORNIA LIBRARY