ry to expose the milk for not less than fifteen
minutes. If a higher heat is employed (and the cooked flavor
disregarded) the period of exposure may be curtailed.

~Chilling the milk.~ It is very essential in pasteurizing that the heated
milk be immediately chilled in order to prevent the germination of the
resistant spores, for if germination once occurs, growth can go on at
relatively low temperatures.

The following experiments by Marshall[150] are of interest as showing
the influence of refrigeration on germination of spores:

Cultures of organisms that had been isolated from pasteurized milk were
inoculated into bouillon. One set was left to grow at room temperature,
another was pasteurized and allowed to stand at same temperature, while
another heated set was kept in a refrigerator. The unheated cultures at
room temperature showed evidence of growth in thirty trials in an
average of 26 hours; 29 heated cultures at room temperature all
developed in an average of 50 hours, while the heated cultures kept in
refrigerator showed no growth in 45 days with but four exceptions.

Practically all of the rapid-process machines are provided with
especially constructed cooling devices. In some of them, as in the
Miller and Farrington, the cooling is effected by passing the milk
through two separate coolers that are constructed in the same general
way as the heater. With the first cooler, cold running water is
employed, the temperature often being lowered in this way to 58 deg. or
60 deg. F. Further lessening of the temperature is secured by an
additional ice water or brine cooler which brings the temperature down
to 40 deg.-50 deg. F.

In the economical use of ice the ice itself should be applied as closely
as possibly to the milk to be cooled, for the larger part of the
chilling value of ice comes from the melting of the same. To convert a
pound of ice at 32 deg. F. into a pound of water at the same temperature,
if we disregard radiation, would require as much heat as would suffice to
raise 142 pounds of water one degree F., or one pound of water 142 deg. F.
The absorptive capacity of milk for heat (specific heat) is not quite
the same as it is with water, being .847 for milk in comparison with 1.0
for water.[151] Hot milk would therefore require somewhat less ice to
cool it than would be required by any equal volume of water at the same
temperature.

~Bottling the product.~ If the milk has been properly pasteurized, it