pearance of normal milk showing
the fat-globules aggregated in clusters.]

~Use of high temperatures.~ Heat has long been used as a preserving agent.
Milk has been scalded or cooked to keep it from time immemorial. Heat
may be used at different temperatures, and when so applied exerts a
varying effect, depending upon temperature employed. All methods of
preservation by heat rest, however, upon the application of the heat
under the following conditions:

1. A temperature above the maximum growing-point (105 deg.-115 deg. F.)
and below the thermal death-point (130 deg.-140 deg. F.) will prevent
further growth, and consequently fermentative action.

2. A temperature above the thermal death-point destroys bacteria, and
thereby stops all changes. This temperature varies, however, with the
condition of the bacteria, and for spores is much higher than for
vegetative forms.

Attempts have been made to employ the first principle in shipping milk
by rail, viz., prolonged heating above growing temperature, but when
milk is so heated, its physical appearance is changed.[132] The methods
of heating most satisfactorily used are known as sterilization and
pasteurization, in which a degree of temperature is used approximating
the boiling and scalding points respectively.

[Illustration: FIG. 23. Microscopic appearance of milk heated above 140
deg. F., showing the homogeneous distribution of fat-globules. The
physical change noted in comparison with Fig. 22 causes the diminished
consistency of pasteurized cream.]

~Effect of heat on milk.~ When milk is subjected to the action of heat, a
number of changes in its physical and chemical properties are to be
noted.

_1. Diminished "body."_ When milk, but more especially cream, is heated
to 140 deg. F. or above, it becomes thinner in consistency or "body," a
condition which is due to a change in the grouping of the fat globules.
In normal milk, the butter fat for the most part is massed in
microscopic clots as (Fig. 22). When exposed to 140 deg. F. or above for
ten minutes these fat-globule clots break down, and the globules become
homogeneously distributed (Fig. 23). A _momentary_ exposure to heat as
high as 158 deg.-160 deg. may be made without serious effect on the cream
lime; but above this the cream rises so poorly and slowly that it gives
the impression of thinner milk.

_2. Cooked Taste._ If milk is heated for some minutes to 160 deg. F., it
acquires a cooked taste that becom