cure the same without bacterial contamination. The
result of the introduction of these organisms often changes its
character materially as most bacteria cause the production of more or
less pronounced fermentative processes. Under normal conditions, milk
sours, i. e., develops lactic acid, but at times this more common
fermentation may be replaced by other changes which are marked by the
production of some other more or less undesirable flavor, odor or change
in appearance.

In referring to these changes, it is usually customary to designate them
after the most prominent by-product formed, but it must be kept in mind
that generally some other decomposition products are usually produced.
Whether the organisms producing this or that series of changes prevail
or not depends upon the initial seeding, and the conditions under which
the milk is kept. Ordinarily, the lactic acid organisms grow so
luxuriantly in the milk that they overpower all competitors and so
determine the nature of the fermentation; but occasionally the milk
becomes infected with other types of bacteria in relatively large
numbers and the conditions may be especially suitable to the development
of these forms, thereby modifying the course of the normal changes that
occur.

The kinds of bacteria that find it possible to develop in milk may be
included under two heads:

1. Those which cause no appreciable change in the milk, either in taste,
odor or appearance. While these are frequently designated as the inert
bacteria, it must not be supposed that they have absolutely no effect on
milk. It is probably true in most cases that slight changes of a
chemical nature are produced, but the nature of the changes do not
permit of ready recognition.

2. This class embraces all those organisms which, as a result of their
growth, are capable of producing evident changes. These transformations
may be such as to affect the taste, as in the sour milk or in the bitter
fermentations, or the odor, as in some of the fetid changes, or the
appearance of the milk, as in the slimy and color changes later
described.

~Souring of milk.~ Ordinarily if milk is allowed to stand for several days
at ordinary temperatures it turns sour. This is due to the formation of
lactic acid, which is produced by the decomposition of the milk-sugar.
While this change is well nigh universal, it does not occur without a
pre-existing cause, and that is the presence of certain living bacterial
fo