FERMENTATIONS OF MILK.
Milk, under normal conditions, is always contaminated with bacteria
coming from the most varied sources. If it is produced under clean
conditions, the number of bacteria will be small, but in any case,
the number of kinds of bacteria that find their way into milk will
be large. Many of them find in milk at ordinary temperatures
suitable conditions for growth; they use a portion of some of the
constituents of the milk as food, producing certain other compounds
that are known as "by-products." These by-products impart to milk a
taste and odor that is not found in fresh milk. The effect of the
action of bacteria may also be made evident by the change in the
appearance of the milk. When these various changes become evident to
the senses, either by taste, smell or sight, the milk usually is so
modified as to be unfit for many ordinary purposes. The preservation
of milk, a subject to be treated later, is a study of the ways of
preventing or retarding the growth of bacteria in milk, and thus
delaying the time when evidences of their action first become
apparent.
Each class of bacteria produces more or less specific changes
in the milk as a result of their growth. Certain bacteria are
of the greatest benefit to the butter and cheese maker, while
others are distinctly harmful to the manufacturer of dairy
products. The changes produced by the different bacteria are called
"fermentations" of milk, each being most commonly named from the
most important by-product formed.
=Acid fermentation of milk.= Fresh milk has a sweet taste and little
or no odor, but if it is allowed to stand at ordinary temperatures,
it sours; the taste is no longer sweet because the sweetness of the
sugar of the milk is masked by the acid produced from the
decomposition of a portion of the sugar by the bacteria. The change
in odor and taste of milk is apparent long before the appearance is
altered and increases in intensity as the acid-fermentation
progresses. The first alteration in appearance is most usually one
of consistency; the liquid milk is transformed into a semi-solid
mass. The terms "curdling" and "sour" are usually synonymous. Milk
is, however, often said to be sour as soon as the acid fermentation
has progressed to a point where it is evident to taste or smell.
This process of souring, or the acid fermentation is so common a
change that raw milk which does not show this type of fermentation
is looked upon with suspicion, and, usually, justly so. The process
in the past was thought to be something inherent in the milk, a
natural and inevitable change. It is now known that this is not so,
but that it is due to certain kinds of bacteria, and that if these
are prevented from getting into milk, it will not sour, but will
undergo some other less desirable type of decomposition.
The acid-forming bacteria comprise but a very small part of the
total number of organisms that find their way into the milk during
its production on the farm, yet in sour milk scarcely any other
kinds of bacteria can be found. At ordinary air temperatures, the
acid-forming bacteria grow more rapidly in milk than do any other
forms, and the acid produced by them renders the milk an unfavorable
medium for the growth of other bacteria. This is the reason why milk
practically always undergoes the acid fermentation, although it is
contaminated with a host of other kinds of bacteria. If a mixture of
seeds is sown on low wet ground, certain kinds will grow best; if
the same mixture is sown on drier land, other types will find most
favorable conditions for growth, and the plants which appeared on
the low land will not appear. The same condition is found in milk
where the environment is most favorable for the acid-forming
bacteria.
=Amount of acid formed in milk.= In this country the acidity of milk
is expressed as so many per cent of lactic acid. A milk that shows
an acidity of one per cent should, theoretically, contain one pound
of lactic acid in each one hundred pounds of milk. The acid
determined does not actually represent lactic acid, as there are
other substances in milk which act as acids, with the reagents used
in the present methods of determining the acidity of milk. For
instance, perfectly fresh milk has an apparent acidity of 0.13 to
0.18 per cent, although no fermentation has occurred. Other acids
than lactic are formed in the acid fermentation, but the entire acid
content is referred to as lactic when speaking of the acidity of
milk. When the developing acidity of milk reaches 0.25 to 0.3 per
cent, a sour taste becomes evident and the milk will curdle on
heating. When the acidity increases to 0.6 to 0.7 per cent, the milk
curdles at ordinary temperatures. The acidity continues, however, to
increase until it reaches about 1 per cent, which is the maximum
amount that will be produced in milk by the ordinary acid-forming
bacteria. Milk contains about 4 per cent of milk sugar, all of which
is fermentable. If this were all decomposed by bacteria, the acidity
of the milk would actually exceed 4 per cent. It is thus evident
that the reason why more acid is not formed in milk is not because
of any lack of sugar. The bacteria, like all other kinds of living
things, are injured by their own by-products, unless these are
constantly removed in some way; in milk the bacteria cannot escape
the action of the acid which they themselves have formed,
consequently growth ceases. The amount of acid formed is dependent
on the kind of bacteria present and on the composition of the milk.
Certain bacteria will not produce enough acid to cause the curdling
of the milk; still others will form 2 or even 3 per cent. These
types, however, do not play any important part in the spontaneous
souring of milk.
In milk the acid first formed combines with the ash constituents and
the casein to form salts which do not seriously affect the growth of
the bacteria. Ultimately, the limit of the ash and casein to take up
acid is reached, and free lactic acid which is harmful to bacterial
growth appears. If the content of casein and ash constituents is
high, a higher degree of acidity will be reached than in a milk with
a lower content. If a large part of the volume of the milk is made
up of a compound that has no role whatever in the acid fermentation,
such as the butter fat in cream, the amount of acid formed per unit
volume of milk will be reduced, since in determining the acidity, a
definite volume of milk is taken, and the acidity is expressed, as
such a per cent of this amount.
=Types of acid-forming bacteria.= When substances undergo
decomposition, it is a common belief that compounds offensive to the
odor and taste are formed; but such is not necessarily the case. The
products of the decomposition may be as agreeable and as harmless as
the compounds decomposed. Whether the decomposition products of any
substance are offensive or not is dependent on the kinds of
micro-organisms acting on it. There are forms of acid-producing
bacteria that change milk in odor, taste, and appearance, yet the
sour milk is not offensive in any sense of the word. Other bacteria
also sour the milk, but produce offensive odors and a disagreeable
taste. Thus, the acid-forming bacteria may be divided into two main
groups, which may be designated as desirable and undesirable. This
division is of importance to the butter and cheese maker and to the
consumer of milk.
=Desirable acid-forming bacteria.= If milk is produced under clean
conditions, it is not likely to have a disagreeable odor or taste at
any time, even when it is sour; rather the taste is agreeable like
that of good butter milk. The curd is perfectly homogeneous, showing
no holes or rents, due to the development of gas, and there is but
little tendency for the whey to be expressed from the curd. This
type of fermentation is largely produced by the group of bacteria to
which has been given the name, _Bacillus lactis acidi_.
The main by-product of this group of bacteria is lactic acid; small
amounts of acetic acid and alcohol, with traces of other compounds,
are also formed. The agreeable odor and to some extent the flavor of
milk fermented by these bacteria is due to other by-products than
lactic acid, for this has no odor and only a sour taste. The acid
fermentation of milk is often called the lactic acid fermentation.
In reality only the fermentation produced by the desirable group in
which lactic acid is the most evident by-product should be thus
called.
[Illustration: Fig. 20.--Different Types of Curds.
On the left a solid, homogeneous curd produced by desirable
bacteria; on the right, the curd produced by harmful bacteria. Note
the gas holes and free whey.]
The bacteria of this group may enter the milk from the dust coming
from the coat of the cow. They are also found in the barn dust and
on cultivated plants. Under ordinary farm conditions, the larger
part of those found in milk come directly from the utensils. If the
milk is drawn under extremely clean conditions and care is taken to
sterilize the utensils, but few acid-forming bacteria of any kind
will enter the milk; under such conditions most of the acid-forming
bacteria will belong to the group in question. They find, however,
such favorable conditions for growth in milk that they develop more
rapidly than most other types with which milk becomes seeded;
consequently under normal conditions, they gain the ascendency and
so control the type of fermentation.
The desirable type of acid-forming bacteria do not form spores;
hence, are easily killed by heating the milk. They can grow in the
presence or in the absence of free oxygen. In the bottom of a can of
milk or in the middle of a cheese, there is no air, yet these
bacteria grow as well under these conditions, as in milk exposed to
the air. The range of temperature for growth varies from 50 deg. to 100
deg. F. but development is most rapid at 90 deg. to 95 deg. F. and about
1 per cent of acid is formed.
Another group of bacteria which may be classed among the desirable
acid-forming organisms is constantly found in milk. They have little
to do with the ordinary acid fermentation as they grow very slowly
at ordinary temperatures. If a sample of raw milk is placed at the
temperature of the animal body, the acidity will reach 1 per cent in
a few hours. Thereafter the acidity will increase slowly and may
reach three per cent or above. The continued increase in acid is due
to the growth of long rods of the _Bacillus Bulgaricus_ type,
which apparently enter the milk with the fecal matter. The nature of
the change produced by them in milk is very similar to that caused
by _Bact. lactis acidi_ in that lactic acid is the chief product; no
gas is produced and hence the curd is uniform in appearance.
Temperatures from 100 deg. to 110 deg. F. favor their development.
Organisms belonging to this group are used in the preparation of the
fermented milks now so widely sold in the cities.
These desirable, acid-forming bacteria are of the greatest service
in every branch of the dairy industry, whether in butter or in
cheese making, or in the sale of milk in the city. The dairy
industry is dependent upon fermentative activity, as much as the
manufacture of beer or wine, and the main basis of this is the acid
fermentation of the milk by these desirable types of bacteria.
Although milk contains a large amount of nitrogenous substances
(casein and albumen), it does not undergo putrid decomposition, as
do meat and eggs, not because it is not fitted for the growth of the
bacteria causing that type of change, but because the acid formed in
it stops the growth of the putrefactive bacteria. If a sample of
milk is placed in a stoppered bottle, it will have much the same
taste and odor at the end of several months as at the end of a few
days. The acid acts as a preservative, like the vinegar in pickles,
or the acid in silage and in sauerkraut. Meat placed in a stoppered
bottle which is then filled with milk will be preserved.
The products formed in the decomposition of meat and eggs are not
only offensive but may also be injurious to the health of the
consumer. Milk that has been fermented by the desirable kinds of
acid-forming bacteria is not harmful. It is consumed in a variety of
forms (buttermilk, cottage cheese) as a common article of food and
its use is rapidly increasing. The preparation of the pure culture
buttermilks or artificially soured milks that are now so frequently
recommended for digestive troubles rests upon an acid fermentation
of this type.
=Undesirable acid-forming bacteria.= Other types of bacteria capable
of forming substances that impart to milk an offensive odor and a
disagreeable taste not infrequently appear instead of the desirable
group. Instead of producing from the sugar of milk large quantities
of lactic acid, these types generate other acids, such as acetic and
formic, which impart a sharp taste to the milk. Besides the acids
the bacteria of this group form gases from the sugar of the milk.
Some produce small amounts of gas; others so much that the curd will
be spongy and will float on the surface of the whey. The
fermentation caused by them is often called a "gassy fermentation"
and is dreaded by butter and cheese makers since the gas is
indicative of bad flavors that will appear in the product. Gas may
also be produced in other types of fermentations to be discussed
later.
This class of bacteria enters the milk with the dust, dirt, and
manure, in which materials they are especially abundant. No spores
are formed; hence they are easily killed by heating the milk. They
grow both in the presence and in the absence of free oxygen. High
temperatures favor their growth, most rapid development taking
place at 100 deg. to 103 deg. F.
=Spontaneous fermentation of milk.= The normal souring of milk is due
to a mixture of these two groups of bacteria. The relative
proportions existing between the two in any sample of milk is
dependent on a number of factors, most important of which is the
degree of cleanliness exercised in the production of the milk. Where
careless conditions obtain under which dust and manure particles
find their way into milk, it becomes more abundantly seeded with
gas-generating bacteria, and consequently, the type of fermentation
is undesirable. If, however, the milk is drawn into clean utensils
and care is taken to exclude dirt, the pure lactic acid types are
able to control the character of the changes produced, and a clean,
pleasant tasting liquid results. It will be seen that things are
well arranged by nature; one of the most important food products
undergoes a type of decomposition that is not offensive and when
produced under clean conditions, the sour milk is as healthful a
food as is the fresh product. Thus there is every reason for
cleanliness in the production of milk, for cleanliness' sake and
because clean milk means better products, and greater returns to
everyone, producer and dealer.
There are other kinds of acid-forming bacteria in milk but they are
of small importance compared with those just discussed. Some of the
bacteria derived from the inside of the udder of the cow form acid,
but these forms grow very slowly in milk at ordinary temperatures,
and have no influence on the keeping quality.
[Illustration: Fig. 21.--Different Types of Curds.
The flask on the left shows the soft curd produced by the bacteria
that curdle the milk without the production of acid. The flask on
the right shows the gassy curd formed by butyric acid bacteria in
heated milk.]
=Sweet curdling fermentation of milk.= Samples of milk are sometimes
found that are curdled, but which do not taste sour, or have the
normal odor of sour milk. The curd is usually soft and the taste
bitter. It is evident that the curdling cannot be due to the same
factors as in the normal souring of milk. Such a change is similar
to the action of rennet which is used to curdle the milk in cheese
making. This ferment will curdle perfectly sweet milk, producing a
curd that looks like that formed in the acid fermentation of milk.
The cause of these sweet curdling milks, which appear from time to
time, is due to the introduction of certain bacteria which have the
power of secreting an enzyme resembling that found in rennet. In
such cases the milks curdle prematurely especially when warmed. The
curd may gradually disappear, for the bacteria also produce another
enzyme that digests the curd, and thus renders it soluble. When this
advanced phase becomes evident, it is often called the _digestive
fermentation_ of milk. This change is produced largely by
putrefactive bacteria of various kinds that find their way into milk
with dust and dirt. Many of them are spore formers; hence, are not
killed when milk is heated, as in pasteurization, while the
acid-formers are destroyed. Pasteurized milk is thus likely to
undergo the sweet-curdling fermentation, if it is kept for
any length of time. Raw milk rarely undergoes this type of
decomposition, since the rennet-forming bacteria under ordinary
conditions are unable to develop in competition with the
acid-forming bacteria.
=Butyric acid fermentation of milk.= A fermentation that is much less
frequently noted than the two previously discussed is known as the
butyric fermentation, since butyric acid is the principal
by-product. The causal bacteria cannot compete with the ordinary
acid-forming bacteria in raw milk; hence it is most frequently noted
in pasteurized milk, since the organisms produce spores and are not
killed by the heating. Pasteurized milk under the action of the
butyric acid bacteria undergoes a gassy fermentation, developing a
pronounced acidity and the disagreeable odor of butyric acid, which
resembles that of rancid butter. The butyric acid bacteria are
anaerobic, and thus can grow in butter and cheese away from the air.
=Slimy or ropy fermentation of milk.= A slimy or ropy condition of
milk is frequently noted on the farm and in the dairy. Several
causes for this abnormal condition exist. Sometimes the milk may be
slimy when milked from the cow. This occurs most frequently in the
case of inflammation of the udder which may or may not be due to
bacteria. The direct cause of the abnormal condition in milk is the
presence of fibrin and white corpuscles from the blood which form
masses of slimy material; in such cases the trouble does not
increase in intensity with age, nor can it be propogated by
transference to another sample of fresh milk.
[Illustration: Fig. 22.--Slimy Milk.
It does not mix with water when poured into it.]
Another type of slimy milk is produced by the growth of certain
types of bacteria which enter the milk after it is drawn from the
udder. These may come from various sources. The bacteria concerned
belong to two groups: (1) those that grow best in the air and do not
form acid; (2) those that grow in the absence of air, throughout the
entire mass of milk and which form acid. The slimy condition is
noted in the milk only after the milk has been stored for some time;
it usually increases with the age of the milk and can be produced
in a second sample by transferring a little of the slimy milk to it.
The fermentation produced by the aerobic bacteria is most often met
in bottled milk and cream during the warmer times of the year. On
account of their relation to oxygen, the growth is confined to the
surface of the milk and only the upper layer becomes slimy; thus
when the cream is removed, the abnormal condition is noted. The
sliminess is due to the mass of bacterial growth rather than to the
production of any specific substance in the milk. This trouble may
be of considerable economic importance to the dealer, as such
abnormal milk is objectionable for ordinary use, but as far as is
known, it is incapable of affecting the health of the consumer.
In numerous outbreaks of this trouble the source of contamination
has been traced to infection from well water or a stream, as the
organisms causing the trouble are found naturally in water. Keeping
the milk in a tank in the pump house sometimes permits of troubles
of this sort, the water used for cooling giving opportunity for
contamination. Cattle wading in a stream sometimes pollute their
udders and so indirectly infect the milk. Such outbreaks rarely
persist for any considerable length of time as the common acid
organisms soon regain the ascendency.
Creameries and cheese factories are sometimes troubled with
sliminess in starters. This seems to be due to some change which the
ordinary lactic acid bacteria undergo on long propagation rather
than to contamination of the starter. There are, however, types of
acid-producing bacteria that are able to form specific substances in
milk that are slimy in character. Two of these forms of slimy milk
are of economic importance. The slimy whey (lange Wei) of Holland
is added to milk in the manufacture of Edam cheese, apparently
serving the same purpose as the addition of the pure culture starter
in cheddar cheese making. In Norway, a sour, slimy milk
(taettemjolk) is used as food. It is produced by the addition of
some previously fermented milk. This beverage is also used in some
of the Norwegian settlements of Wisconsin, the original seed having
been brought from Norway, and the bacteria maintained by constant
propagation from one sample of milk to another. The milk has the
odor and taste of butter milk, but is not especially appetizing in
appearance to any one not accustomed to it; it is, however, as
harmless to health as is any other form of sour milk. It is not
known that any of these forms of slimy milk are distinctly harmful
to the quality of butter or cheese.
=Alcoholic fermentation of milk.= The bacteria as a class are
incapable of producing alcohol in appreciable amounts. The alcoholic
beverages, beer, wine, and cider, are produced by the growth of
yeast, in such sugar containing liquids as fruit juices, extracts of
grains, etc. The common types of yeasts are incapable of acting on
milk sugar, but they can ferment glucose, maltose, and cane sugar,
forming equal amounts of alcohol and carbonic acid gas, which causes
the effervescence of fermented and carbonated drinks. There are,
however, some types of yeasts found in milk and its products that
are able to ferment milk sugar.
All yeasts grow best in an acid medium, hence those fermenting milk
sugar find suitable conditions for growth in sour milk or whey. They
may at times become of economic importance in the cheese industry,
because of the contamination of the milk with large numbers of
them. The arrangement of the whey vat is often such that it cannot
be completely emptied and cleaned; the sour whey thus presents
favorable conditions for the growth of the lactose-fermenting
yeasts. The return of the whey to the farm in the milk can that is
often imperfectly cleaned may serve to contaminate the milk with the
yeast. In the making of Swiss cheese the whey is often so handled as
to favor especially the growth of such yeasts, and since this type
of cheese is prepared from sweet milk, the competition between the
yeast and the acid-forming bacteria is not so sharp as in the making
of cheddar cheese. The writers have found several instances where
considerable loss was occasioned in the Swiss cheese industry
through the development of gassy cheese due to this type of
fermentation.
The yeasty or alcoholic fermentation may also be of importance in
butter making. In many sections of the country the milk is separated
on the farm and the cream is forwarded to the creamery at more or
less infrequent intervals. It becomes sour and if it has become
contaminated with yeasts, they will find favorable conditions for
growth in the acid medium. A large amount of carbon dioxide gas is
produced. Cans of gathered cream often foam to such an extent as to
run over, and in some cases actual explosions have occurred on
account of the great pressure caused by the gas.
=Bitter fermentation of milk.= Bitterness in milk may be due to
bacteria that enter the milk after it is drawn from the cow, or it
may be caused by the feed consumed by the animal. It has been
previously shown that certain specific substances contained in the
food may be absorbed and reappear in the milk. If the animal eats
ragweed, lupines, or other plants containing bitter substances, the
milk is likely to have a bitter taste, which will be noticeable at
the time the milk is drawn. The milk of cows at certain advanced
stages of lactation may show a bitter taste, due to a change in the
ash constituents of the milk in which the lime salts are largely
replaced by salts of sodium.
There are many bacteria that will impart to milk a bitter taste.
Milk that has undergone the sweet-curdling fermentation is likely to
be bitter, as is the ease with pasteurized milk. Some of the
acid-forming bacteria are able to develop a bitter principle, the
milk retaining a pleasant odor and having the normal amount of acid,
while the taste is intensely bitter. One of the authors (H) found in
the case of a Wisconsin brick cheese factory, that the usual acid
organism was almost wholly replaced by a bitter type.
Storage of milk at very low temperatures is conducive to the
appearance of a bitter taste in milk, the explanation in this case
being that the acid-forming bacteria are unable to grow at a low
temperature, while some of the putrefactive forms can multiply and
develop these astringent or bitter by-products.
=Miscellaneous fermentations of milk.= There are a number of other
abnormal fermentations in milk that occur so rarely as to be of but
little economic importance. Some, as the colored milks, are however,
quite striking, and on this account have had much attention directed
to them in the past. There are bacteria that are able to produce
various colored substances, such as red, yellow, and blue. In case
milk becomes seeded with large numbers of any of these kinds, it is
very likely to be colored by the growth. Red milk may be due to
bacteria, but more frequently is caused by the actual presence of
blood in the milk, due to a wound in the udder, or the effect of a
severe case of inflammation of this gland. Such a condition may be
readily distinguished by allowing the milk to stand for a short
time, in which case, if due to blood, the red corpuscles will soon
settle to the bottom of the container, while bacterial troubles
producing a red coloration are more evident on the surface.
It is also claimed that certain bacteria may impart a soapy taste or
turnip flavor to milk.
=Cycle of fermentations in milk.= If a sample of milk is allowed to
stand, it will undergo a certain sequence of fermentations that well
illustrates the principle that one type of organisms is dependent on
some other type to furnish suitable conditions for its development.
This cycle of changes that normally occurs in milk is as follows:
(1) The bacteria that come from the interior of the udder are the
first to develop, but usually the change they produce is not
evident.
(2) Of the types that gain admission, subsequent to the milking, the
acid-producing species are able to adjust themselves most perfectly
to the conditions that obtain in milk. Within a few hours they
greatly predominate and soon the milk curdles under the production
of acid. Their growth, however, is soon stopped by the accumulation
of their own by-products.
(3) The semi-solid curdled milk, on account of its acid reaction
then becomes a favorable medium for the growth of molds; a prevalent
form, known as _Oidium lactis_ usually develops as a white velvety
layer. The molds in their growth form alkaline by-products, which
tend to neutralize the acid reaction, so that in the course of two
to three weeks, if the layer of the milk is not too deep (an inch or
less), the chemical reaction of the milk becomes neutral or
alkaline.
(4) The putrefactive bacteria which found their way into milk when
it was first drawn, and which have remained dormant in the sour
milk, now find favorable conditions for growth. As a result of their
activity, the milk soon undergoes a putrid decomposition, which is
marked by offensive odors.
If the milk is placed under such conditions as will exclude the
growth of the mold, such as where the air is excluded from the
surface, the sour milk will remain in that condition for an
indefinite period, since the putrefactive bacteria are inhibited in
their development by the acid, in a manner comparable to the
preservation of pickles in vinegar, or the keeping of silage because
of the acid that is produced as a result of the changes that the
plant tissue undergoes when excluded from the air. The preservative
effect of acids is of much importance in the case of certain dairy
products (see Chapter VIII).
=Fermented drinks from milk.= Within the last few years a great deal
of attention has been directed toward the preparation of various
kinds of drinks from milk. The use of such beverages has rapidly
increased. Butter milk is one which meets with the greatest
approval. The true butter milk from cream that has been soured by
the desirable acid-forming bacteria has a mild agreeable acid taste,
wholly free from any sharpness that is often noted in butter milk
made from cream in which considerable numbers of the undesirable
acid-forming bacteria have grown. Butter milk made from pasteurized
cream soured with pure cultures will have good keeping qualities and
is a most healthful drink for all classes of people, even for young
children.
Butter milk is also prepared by allowing milk to sour and then
breaking up the curd by stirring. If the type of fermentation is
controlled as may be done (see Chapter VII), such a form of
fermented milk is a most desirable drink. It is probably as
healthful and has all the therapeutic properties that are ascribed
to other forms of fermented milks such as the Bulgarian "Yoghurt."
This type of fermented milk is produced by an acid-forming organism
that can form large amounts of acid, 2.0 or 3.0 per cent. The casein
is dissolved to some extent and the remainder so changed, that it
will remain in suspension for a long time in a finely divided form,
after the curd has been broken up. Such milk is sold under various
names at home and abroad. One of the authors (H) has found such
organisms in practically all milks examined. If raw milk is kept
warm (98 deg. to 100 deg. F.) in a stoppered bottle which is filled
full, the acidity will be found to increase slowly from day to day,
reaching a maximum in ten to fourteen days. If the milk is then
examined, it will be found to contain large numbers of an
acid-forming organism very different in appearance from the bacteria
causing the rapid souring of milk at ordinary temperatures. This
organism is very similar if not identical with the one found in the
Bulgarian milk to which the name _B. Bulgaricus_ has been given. The
use of the milk fermented by this organism has spread rapidly
because it is claimed by certain European bacteriologists that it
has a favorable effect on the health of people, especially those
suffering from intestinal troubles. It is not at all certain
that ordinary sour milk or butter milk will not have the same
effect; in fact in many of the fermented milks sold in Europe, _B.
Bulgaricus_ has not been found, but only the ordinary lactic acid
bacteria.
Several alcoholic drinks made from milk, such as kefir and koumiss,
have been originated among the nomadic tribes of Western Asia. Kefir
is prepared from cow's milk by adding the kefir ferment in the form
of grains which contain a number of kinds of bacteria and a yeast.
The acid-forming bacteria impart a sour taste to the fermented milk,
while the yeast forms carbon dioxide and about two per cent of
alcohol. If the milk is allowed to ferment in stoppered bottles, the
resulting product will be an acid effervescing drink, which is
claimed to be more easily digested than sweet milk. This drink is
used frequently in the treatment of invalids but it is improbable
that it is more easily digested than ordinary soured milk or butter
milk. The grains are removed from the fermented milk, and are then
added to a quantity of fresh milk, or they may be dried and kept for
future use. When needed again, they are soaked in water, then added
to the milk.
Koumiss is made in Russia from mare's milk and has much the same
composition as kefir. In America and Europe it is made from cow's
milk, by adding cane sugar and compressed yeast. The yeast ferments
the cane sugar while the acid-forming bacteria ferment the milk
sugar. There is thus obtained a drink that is similar in composition
to the real koumiss, in which both the acid and the alcohol come
from the fermentation of the milk sugar. In koumiss and kefir the
curd is very finely divided and will remain in suspension for a long
time as with butter milk.
=Determination of the cause of taints in milk.= It is often of the
greatest importance to be able to locate the cause of abnormal
odors or tastes in milk, since methods for overcoming the trouble
can be intelligently applied only when the actual cause is known. An
abnormal condition may be caused either by the direct absorption of
odors before or after the milk is drawn from the animal, or it may
be due to bacteria. If the milk appears bad-flavored when first
drawn, and if such taint becomes less pronounced as the milk becomes
older, it is likely that the trouble is due to some characteristic
of the feed. Certain feeds, like green rye, rape, cabbage, and
certain of the root crops, like turnips, impart a strong odor to
milk, if the same are fed shortly before milking. If the tainted
condition appears only some time after the milk is drawn, it may be
due to the direct absorption of taints from the surroundings in
which the milk is kept, or it may be caused by bacteria. These
causes can often be differentiated, by noting whether the taint
tends to increase in intensity with age. If such is the case, it is
likely that the cause is of germ origin, but if the reverse is true,
it cannot be ascribed with certainty to bacteria and recourse must
be had to other methods, such as the transfer of a small quantity of
the tainted milk to a sample of perfectly fresh milk, or preferably
to some milk that has been heated to the boiling point and then
cooled. In the case of an odor due to direct physical absorption, it
will not appear in the inoculated sample, since the small amount
transferred is not sufficient to be noted. If it is due to living
organisms, the inoculation of the smallest quantity into a fresh
sample is likely to reproduce the same change as originally noted.
=Tests for the bacteriological condition of milk.= Within certain
limits milk can be indirectly examined as to its bacterial content
without any special equipment. Milk when drawn from the cow has an
apparent acidity ranging from 0.16 to 0.18 per cent. By the use of
any of the methods of determining acidity in milk, much can be told
concerning the number of bacteria in the milk, and hence concerning
its keeping quality. Milk that has an acidity of over 0.2 per cent
is certain to contain many bacteria, and consequently will keep
poorly. Such milk is of low value for market milk, but may not be
objectionable for butter or cheese making. If the acidity is below
0.2 per cent, but little can be told as to the numbers of bacteria,
since any increase in acid is always preceded by an enormous
increase in the numbers of acid-forming bacteria.
A more important test than the acid test, from the standpoint of the
butter and cheese maker, and even the milk dealer, is the
fermentation test. In its simplest form, it consists in placing a
sample of the milk to be tested in a warm place and noting the time
required to curdle and the type of curd formed. In this country the
fermentation test has been largely supplanted by the Wisconsin curd
test which possesses the advantage of detecting the presence of
bacteria harmful in cheese making, especially the gas forming
bacteria.
The curd test is helpful in detecting the source of an abnormal
condition in a milk supply coming from diverse sources. The milk
furnished by each patron can be tested separately and the trouble
located, perhaps in an individual herd; the offending herd
determined, the test may then be used on the milk of individual
cows. The cheese maker and the milk dealer should be able not only
to detect which of the patrons furnish him poor milk, but he should
be able to give the patron definite instructions how to avoid the
sources of such trouble. This information can be given only when
the source is positively known.
[Illustration: Fig. 23.--Curd Test.
A good curd obtained from milk containing no harmful bacteria but
many desirable acid-forming organisms.]
The Wisconsin curd test is made as follows: Samples of the milk to
be tested are placed in sterile pint fruit jars. The milk is warmed
to 90 deg. F., ten drops of rennet are added to each sample, and as soon
as the curd is solid, it is cut into small pieces with a case knife
so as to facilitate the expulsion of the whey. As the curd settles
to the bottom of the vessel, the whey is poured off at intervals so
that a pat of firm curd is left. As the milk curdles the bacteria
are enmeshed and are carried with the curd. The jars are kept at a
temperature of 100 deg. to 105 deg. F., since this temperature is
favorable to growth of the bacteria that are sought, the gas-forming
organisms. At the end of ten to twelve hours, the jars are examined;
if the curd is solid, the texture firm, not mushy or slimy on the
surface, if the odor is agreeable, it indicates that the milk
contains few or none of the undesirable forms of bacteria. If the
curd is full of gas holes, it is apparent that undesirable bacteria
are present and under such circumstances the curd will not have an
agreeable odor. If the gas-forming bacteria are numerous, the curd
may even be spongy from the abundance of gas holes, and the
undesirable odor more pronounced. Such curds are tough and rubbery.
In some cases a bad flavor or odor is apparent even though the
texture of the curd is not open and full of holes. The curd, the
surface of which is slimy indicates undesirable organisms. A solid
curd of agreeable odor is indicative of the presence of the
desirable acid-forming bacteria. Such a milk is excellent from the
standpoint of the butter or cheese maker, but may not be so
desirable from the standpoint of the milk dealer on account of its
poor keeping qualities. On the other hand a milk suitable from the
standpoint of the milk dealer, on account of its low germ content,
and hence good keeping quality, may give a poor curd test. It is
certain to contain some bacteria, especially those from the interior
of the udder while it may contain none of the desirable acid-forming
organisms without which a curd of good texture and flavor can not be
obtained. The bacteria in the clean milk will grow rapidly at the
high temperatures at which the curds are kept and the changes they
will produce as to flavor and odor may be undesirable. The milk
might be judged as poor when in reality it might be a most excellent
sample, and if kept at the ordinary storage temperatures, it might
keep for days. The test when used for market milk should be
interpreted with this in mind.
[Illustration: Fig. 24.--Curd Test.
The curd obtained from milk containing many gas-forming bacteria.
The irregular, angular holes are mechanical, due to the imperfect
fusion of the pieces of curd.]
If the results are to be of any value, the test must be made with
care to avoid all sources of error; the tester must know that the
bacteria causing the gas and bad flavors in the sample were
originally present in the milk at the time the sample was taken, and
that they have not come from the containers used or from other
sources. To insure these conditions the jars must be thoroughly
cleaned and then sterilized just before use by placing them in cold
water and bringing them to the boiling point, or sterilized by a
thorough steaming. The sample of milk of a patron must be taken so
as to avoid contamination from the milk of the other patrons. This
can best be done by filling the jars as the milk is poured from the
patron's can into the weigh can. In cutting the curds, the knife
used must be dipped in hot water between each test to cleanse the
same. In short, the test should be carried out with great care so
that the tester is certain of the results obtained.
Other tests for the bacteriological condition of milk will be
described in Chapter IX.
=Overcoming abnormal fermentations.= The lactic acid bacteria are
often looked upon as normal to milk, and it is certain that they are
to be classed as harmful, only as they injure the keeping qualities
of milk. In milk designed for butter and cheese their presence is
necessary. At times these desirable forms of bacteria may disappear,
and be replaced by less desirable types. In one case it was observed
that the usual lactic bacteria had been replaced in a cheese factory
supply by an acid-forming organism that produced an intensely bitter
taste in the milk, thus rendering the cheese of no value. When such
harmful forms appear, they must be overcome, and the normal types of
bacteria replaced. A thorough cleaning of the milk utensils,
attention to the cattle and all places from which such bacteria may
find their way into the milk is often sufficient to cause a
disappearance of the trouble. If the acid-forming bacteria have
disappeared, the inoculation of the milk with cultures in ways later
to be discussed is often of advantage. At times more stringent
measures must be employed in order to destroy the harmful bacteria,
such as the use of strong disinfectants.
=Disinfection and disinfectants.= If any building or room becomes
infected with disease-producing bacteria, or if organisms causing
abnormal fermentations become established in a factory, the use of a
disinfectant that will destroy with great rapidity the life of
bacteria is necessary. The disinfection of all types of dairy
apparatus and utensils can be accomplished by thorough cleansing,
and by the use of steam or boiling water. The disinfection of rooms
and stables cannot be so readily accomplished.
Consideration must always be given to the resistance of the organism
it is desired to destroy. Those that form spores are very resistant
toward all chemical agents, while those that do not produce these
resistant bodies are easily killed. In the dairy and factory, it is
often necessary to destroy the organisms that develop in decomposing
organic matter. Here, as in all disinfection, a thorough cleaning
should precede the application of any disinfectant. Some chemicals
act as deodorants, _i.e._, destroy the offensive odor, without
removing the cause. It is impossible effectually to destroy bacteria
embedded in a mass of organic matter, and through the removal of the
material itself, the larger part of the bacteria will be removed.
The disinfectant then comes in direct contact with the surface to be
disinfected, consequently destroys the bacteria not removed in the
cleaning.
All places in which dairy work of any kind is done should be
provided with an abundance of light and air. The direct rays of
the sun have a powerful disinfecting action, and light makes evident
accumulations of dirt that in a darker room would be unnoticed.
Ventilation keeps the rooms dry and thus prevents the growth of mold
and the development of a musty odor.
Disinfectants are divided into two classes: (1) solid materials used
in suspension, or in watery solutions; (2) gaseous substances. The
latter are preferable for room disinfection when their use is
permissible, for the gas penetrates to every part of the space, even
into the cracks. Gaseous disinfectants can only be used when the
space is tightly closed, for the gas must be confined for several
hours in the room, in order to make the process effective. Such
disinfectants can often be used to advantage in the treatment of
refrigerators and cheese rooms to destroy mold spores. In less
tightly closed spaces, reliance must be placed on the use of the
solid or liquid disinfectants.
=Lime.= Quick lime or stone lime has a considerable disinfecting
action. On exposure to the air, quick lime becomes air slaked, and
then has no disinfecting action whatever. Water-slaked lime used in
the form of white wash, lime water, or the powder is effective.
Air-slaked and water-slaked lime are similar in appearance, but a
difference can be noted by placing a particle of each on the tongue;
the air-slaked tastes like chalk while the water-slaked material
causes the tongue to burn.
White wash is one of the most effective agents that can be used in
the disinfection of barns, milkrooms, etc. Besides being a fairly
strong disinfectant, it has a tendency to absorb odor, it encrusts
the walls and lightens the interior of rooms. It can be applied with
a brush or with a spray pump.
=Carbolic acid and cresol compounds.= These substances are among the
cheapest and best disinfectants, but their use in the dairy is not
advisable, on account of the penetrating and lasting odor. They can
be used to advantage on the farm. Some of the proprietary compounds,
as Zenoleum, Kresol, etc., are easily applied, since they mix
readily with water in all proportions, forming a milky-white
emulsion that can be easily applied. They are less caustic and less
poisonous than carbolic acid.
=Corrosive sublimate.= Corrosive sublimate is the most efficient
disinfectant under ordinary conditions. It is such an intense poison
that it must be used with caution in places to which stock have
access, or in the dairy. A solution of one part of the salt to a
thousand parts of water (half ounce to 4 gallons of water) is the
standard generally used.
For gutters, drains, and waste pipes in factories, ferrous sulphate
(green vitriol), and copper sulphate (blue vitriol), can be used to
advantage. They are to be classed as deodorants rather than as true
disinfectants. Since they have no odor of their own, they can be
used in any amount in the dairy.
=Sulphur= can be used to advantage in the destruction of mold spores
in cheese rooms, but the effect of the vapors of burning sulphur on
germ life is relatively slight, unless there is an abundant supply
of moisture in the air of the enclosed space, in which case
sulphurous acid is formed which has a much greater effect. To have
the desired effect sulphur should be burned at the rate of three
pounds to each one thousand cubic feet of space, and the room kept
sealed for at least twelve hours. If the sulphur is placed in an
iron kettle which is set in a vessel of water, danger from fire
will be avoided, and the heat generated by the burning sulphur will
evaporate sufficient water to increase the effect of the fumes.
=Formalin.= Another disinfectant that may be used as a liquid or as a
gas is formalin, which is a watery solution of the gas,
formaldehyde. It is much more powerful in its action than sulphur,
and has a great advantage over corrosive sublimate and other strong
disinfectants in that it is not so poisonous to animals as it is to
bacteria and fungi.
It can be used as a solution (one to five per cent) for the washing
of woodwork, or for the treatment of any object, since it has no
corrosive action. It can also be employed as a gaseous disinfectant
for the treatment of rooms. It is most conveniently applied by
suspending large cloths in the room and spraying them with the
solution, then closing the room for a number of hours.
=Bleaching powder.= Chloride of lime, or bleaching powder as it is
often called, is a good disinfectant, as well as a deodorant. It is
used as a wash in the proportion of four to six ounces to a gallon
of water. It must be used with care in factories since the free
chlorine that is given off has a penetrating odor.