nd 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 f