utes
or more), the maximum temperature which can be used without producing
this change is about 140 deg. F.; if the exposure is made for a very
brief time, a minute or less, the milk may be heated to 158 deg.-160 F.
deg. without injuring the creaming property.

_3. No diminution in cream "body."_ Coincident with this change which
takes place in the creaming of the milk is the change in body or
consistency which is noted where cream is pasteurized at too high a
temperature. For the same reason as given under (2) cream heated above
these temperatures is reduced in apparent thickness and appears to
contain less butter-fat. Of course the pasteurizing process does not
change the fat content, but its "body" is apparently so affected. Thus a
25 per cent. cream may seem to be no thicker or heavier than an 18 per
cent. raw cream. This real reduction in consistency naturally affects
the readiness with which the cream can be whipped.

~Biological requirements.~ _1. Enhanced keeping quality._ In commercial
practice the essential biological requirement is expressed in the
enhanced keeping quality of the pasteurized milk. This expresses in a
practical way the reduction in germ life accomplished by the
pasteurizing process. The improvement in keeping quality depends upon
the temperature and time of exposure, but fully as much also on the way
in which the pasteurized product is handled after heating. The lowest
temperature which can be used with success to kill the active,
vegetative bacteria is about 140 deg. F., at which point it requires
about ten minutes exposure. If this period is curtailed the temperature
must be raised accordingly. An exposure to a temperature of 175 deg. F.
for a minute has approximately the same effect as the lower degree of
heat for the longer time.

The following bacteriological studies as to the effect which a variation
in temperature exerts on bacterial life in milk are of importance as
indicating the foundation for the selection of the proper limits. In the
following table the exposures were made for a uniform period (20
minutes):

_The bacterial content of milk heated at different temperatures._

Number of bacteria per cc. in milk.
45 deg. C. 50 deg. C. 55 deg. C. 60 deg. C. 65 deg. C. 70 deg. C.
Unheated 113 deg. F. 122 deg. F. 131 deg. F. 140 deg. F. 149 deg. F. 158 deg. F.
Series I. 2,895,000 ----