eedingly
simple, yet, in order to secure the best results, certain conditions
must be rigidly observed in the treatment before and after the heating
process.

It is important to select the best possible milk for pasteurizing, for
if the milk has not been milked under clean conditions, it is likely to
be rich in the spore-bearing bacteria. Old milk, or milk that has not
been kept at a low temperature, is much richer in germ-life than
perfectly fresh or thoroughly chilled milk.

The true standard for selecting milk for pasteurization should be to
determine the actual number of bacterial _spores_ that are able to
resist the heating process, but this method is impracticable under
commercial conditions.

The following method, while only approximate in its results, will be
found helpful: Assuming that the age or treatment of the milk bears a
certain relation to the presence of spores, and that the acid increases
in a general way with an increase in age or temperature, the amount of
acid present may be taken as an approximate index of the suitability of
the milk for pasteurizing purposes. Biological tests were carried out in
the author's laboratory[149] on milks having a high and low acid
content, and it was shown that the milk with the least acid was, as a
rule, the freest from spore-bearing bacteria.

This acid determination can be made at the weigh-can by employing the
Farrington alkaline tablet which is used in cream-ripening. Where milk
is pasteurized under general creamery conditions, none should be used
containing more than 0.2 per cent acidity. If only perfectly fresh milk
is used, the amount of acid will generally be about 0.15 per cent with
phenolphthalein as indicator.

[Illustration: FIG. 28. Diagram showing temperature changes in
pasteurizing, and the relation of same to bacterial growth.

Shaded zone represents limits of bacterial growth, 50 deg.-109 deg. F.
(10 deg.-43 deg. C.), the intensity of shading indicating rapidity of
development. The solid black line shows temperature of milk during the
process. The necessity for rapid cooling is evident as the milk falls
in temperature to that of growing zone.]

Emphasis has already been laid on the selection of a proper limit of
pasteurizing (p. 114). It should be kept constantly in mind that the
thermal death-point of any organism depends not alone on the temperature
used, but on the period of exposure. With the lower limits given, 140
deg. F., it is necessa