from a sanitary point of view, and whenever the milk supply is
of unknown purity, it should be pasteurized.[38] After the milk has been
thus treated, the same care should be exercised in keeping it protected
to prevent fresh inoculation or contamination, as though it were
unpasteurized milk. For family use milk can be pasteurized in small
amounts in the following way: Before receiving the milk, the receptacle
should be thoroughly cleaned and sterilized with boiling water or dry
heat, as in an oven. The milk is loosely covered and placed in a pan of
water, a false bottom being in the pan so as to prevent unequal heating.
The water surrounding the milk is gradually heated until a temperature
of 159 deg. F. is registered, and the milk is kept at this temperature for
about ten minutes. It is then cooled and placed in the refrigerator.

[Illustration: FIG. 24.--PASTEURIZING MILK.]

105. Tyrotoxicon.--Tyrotoxicon is a chemical compound produced by a
ferment body which finds its way into milk when kept in unsanitary
surroundings. It induces digestion disorders similar to cholera, and
when present in large amounts, may prove fatal. It sometimes develops in
cream, ice cream, or cheese, but only when they have been kept in
unclean places or produced from infected milk.

601. Color of Milk is often taken as a guide to its purity and
richness in fat. While a yellow tinge is usually characteristic of
milks rich in fat, it is not a hard and fast rule, for frequently
light-colored milks are richer in fat than yellow-tinged ones. The
coloring material is independent of the percentage of fat, and it is not
always safe to judge the richness of milk on the basis of color.

107. Souring of Milk.--Souring of milk is due to the action of the
lactic acid organism, which finds its way into the milk through
particles of dust carried in the air or from unclean receptacles which
contain the spores of the organism.[39] When milk sours, a small amount
of sugar is changed to lactic acid which reacts upon the casein,
converting it from a soluble to an insoluble condition. When milk is
exposed to the air at a temperature of from 70 deg. to 90 deg. F., lactic
acid fermentation readily takes place. At a low temperature the process
is checked, and at a high temperature the organisms and spores are
destroyed. In addition to lactic acid ferments, there are large numbers
of others which develop in milk, changing the different compounds of
which milk is