e water, thus leaving
a dry powder, which on being mixed with water again will have much
the same properties as the original milk. Various methods have been
devised for the preparation of these milk powders, all of which have
been patented by the inventors. If the powder is to be kept for long
periods, skim milk must be used, since the fat slowly undergoes
changes which cause it to have a rancid odor. These dry preparations
are largely used by bakers in place of fresh milk.

CHAPTER VII.

BACTERIA AND BUTTER MAKING.

In the making of butter it is necessary to concentrate the milk fat
into a small volume. This process, known as creaming, may be
accomplished by gravity, if the milk is allowed to stand
undisturbed, the fat globules rising slowly to the surface. Much
more rapid separation may be secured, by placing the milk in a
rapidly revolving container in which it is subjected to centrifugal
force, which causes the heavier parts of the milk to pass to the
outside of the bowl, while the lighter part, the fat, collects at
the center of the revolving bowl. There is an enormous number of fat
globules in milk, over 5,000,000,000 in each cubic centimeter, and
as these move through the milk serum, they carry with them many of
the bacteria. The cream is thus much richer in bacteria than is the
skim milk, or even the milk before separation. Besides the
mechanical separation in the manner described, the method of
creaming is of importance, in determining not only the number but
also the kind of bacteria in the cream.

=Methods of creaming.= In the shallow-pan method of creaming, the milk
is kept at ordinary room temperatures. These temperatures favor
especially the growth of the acid-forming bacteria. The milk is
usually sour by the time the cream is removed from it; consequently,
the bacterial content of the cream is high. Moreover, the cream is
exposed to air contamination, and is thus seeded with molds, and
those forms of bacteria that are always found in the air. The cream
obtained in this manner is likely to contain not only numerous
bacteria, but a great variety of forms, some of which undoubtedly
are the cause of the poor keeping qualities of butter made from such
cream.

In the more modern method of gravity creaming, in which the milk is
placed in deep narrow cans kept in cold water, the conditions are
not favorable for the growth of acid-forming bacteria. If the milk
is produced under clean conditions, an