e (122 deg. F.) is sufficient to destroy the larger part
of the bacteria in the milk. Practical difficulties are encountered
in the commercial application, so that it is probable the process
will never be a commercial success.

For the preservation of composite samples of milk for analytical
purposes, such as the Babcock test, strong disinfectants, as
corrosive sublimate, are employed. This material is very poisonous,
and leaves the milk unchanged in appearance. Some coloring matter is
therefore usually mixed with the sublimate in making the
preservative tablets, so as to render their use more conspicuous.
Corrosive sublimate not only stops all bacterial growth, but quickly
destroys the life of the cells. Bichromate of potash is generally
employed in the preservation of composite samples for the Hart
casein test.

=Destruction of bacteria in milk.= Actual destruction of the life of
bacterial cells by heat is one of the most important ways for
preserving milk. Heat easily destroys the vegetating, growing
bacteria, while the spores, of which there are always a number in
milk, are very resistant. If, however, the growing organisms are
destroyed, the milk will keep much longer than if it had not been so
treated.

The process of pasteurization was first used by the French
bacteriologist, Pasteur, for the treatment of the wines of his
native district which were likely to undergo undesirable types of
fermentations due to bacteria. From the wine industry it was applied
in the brewing industry, and was later found to be of the greatest
service in the dairy industry. The process of pasteurization may be
briefly defined, as the heating of milk to temperatures, varying
from 140 deg. F. and upward for a longer or shorter time, and
subsequently cooling to a low temperature, so as to prevent the
germination of the spores that are not destroyed by the heating.

=Effect of heat on milk.= When milk is heated it undergoes more or
less profound changes, depending on the temperature and time of
heating. Some of these changes are of practical importance, since
they are more or less evident, and objectionable to the consumer.

In raw milk the fat globules are largely found in larger or smaller
aggregates, rather than uniformly distributed throughout the serum.
The surface of a mass of fat globules is smaller in proportion to
the volume of the mass than is the case with single globules, hence
globule clusters encounter less resistance i