Fig. 27.--Creaming of Milk.

The cylinder on the left contains raw milk; that in the center, milk
heated to 140 deg. F. for twenty minutes; on the right, milk heated to
160 deg. F. for twenty minutes. The dark line indicates the depth of the
cream after twenty-four hours. The breaking up of the fat globule
clusters delays greatly the rising of the cream.]

Heated milk has a taste unlike that of raw milk; to one not
accustomed to it the taste is objectionable. This change is due to
some extent to the expulsion of the carbon dioxide from the milk.
The insipid taste of boiled water is, in part, due to its freedom
from carbon dioxide. The production of this cooked flavor is
dependent upon the time and temperature of exposure. It has been
claimed that heated milk is less digestible than raw, and a
considerable amount of experimental work has been done, both on
animals and children, in order to determine the relative
digestibility of heated and raw milk. The results obtained have been
contradictory. It is claimed that heated milk causes such diseases
as rickets, scurvy and marasmus in children. It is probably true
that milk heated to the boiling point is less fitted as food for the
young child than raw milk, but, on the other hand, it has not been
proven that properly pasteurized milk is an unsuitable food for
children. The best evidence has been accumulated in recent years, in
many of the large cities of this country and of Europe, where
pasteurized milk has been used with the greatest success in the
feeding of children of all ages.

The heated milk does not curdle readily when rennet is added due to
the precipitation of the lime salts by heat. The curdling power can
be restored by the addition of soluble lime salts or of acids.

=Purpose of pasteurization.= There are two reasons for the
pasteurization of milk: (1) To improve the keeping quality; (2) To
destroy any pathogenic bacteria it may contain. The first may be
called the economic reason; the second, the hygienic reason fur
pasteurization. In the selection of a proper pasteurizing
temperature, two factors must be taken into account: First, the
effect of heat on milk, and second, the temperature necessary to
destroy those forms of bacteria that are of the greatest importance,
as far as the keeping properties are concerned, and the pathogenic
bacteria that might possibly be present in the milk. The lactic
acid bacteria are non-spore-bearing and are not resistant to