way into the milk from the accumulations on the coat of
the animal.

Conn[55] has separated the rennet enzym from bacterial cultures in a
relatively pure condition, while Fermi[56] has isolated the digestive
ferment from several species.

Duclaux[57] 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 characteristic curdling and subsequent
digestion quite independent of cell development. The quantity of ferment
produced by different species differs materially in some cases. In these
digestive fermentations, the chemical transformations are profound, the
complex proteid molecule being broken down into albumoses, peptones,
amido-acids (tyrosin and leucin) and ammonia as well as fatty acids.

Not infrequently these fermentations gain the ascendency 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 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 butter" 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 organisms
find more favorable conditions for growth, not so much on account of the
lactic acid formed as in the absence 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 sterilized milk. The by-products
formed in this series of changes are quite numerous. In most cases,
butyric acid is prominent, but in addition to this, other organic acids,
as lactic, succinic, and acetic, are produced, likewise different
alcohols. Concerning the chemical origin of butyric acid there is yet
some doubt. Duclaux[58] 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
fermentation has not received the study it deserves.

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