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 b