evelops 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 c