ctivity of bacteria or to destroy them. These act more readily
on the vegetating cells than on the more resistant spores. It is of
the utmost importance that those engaged in dairy work be familiar
with these antagonistic forces since it is constantly necessary to
repress or to kill outright the bacteria in milk and other dairy
products. In many lines of dairy work it is likewise important to be
familiar with the conditions favorable for bacterial growth.

=Effect of cold.= While it is true that chilling largely prevents
fermentative action, and actual freezing stops all growth processes,
still it does not follow that exposure to low temperatures will
effectually destroy the vitality of bacteria, even in the growing
condition. Numerous non-spore-bearing species remain alive in ice
for a prolonged period, and experiments with liquid air show that
even a temperature of-310 deg. F. maintained for hours does not kill all
exposed cells.

=Effect of heat.= High temperatures, on the other hand, will destroy
any form of life, whether in the vegetative or latent spore stage.
The temperature at which the vitality of the cell is lost is known
as the _thermal death point_. This limit is dependent not only upon
the nature of the organism, but upon the time of exposure and the
condition in which the heat is applied. In a moist atmosphere, the
penetrating power of heat is great, consequently cell death occurs
at a lower temperature than in a dry atmosphere. An increase in time
of exposure lowers the temperature point at which death occurs.

For growing organisms, the thermal death point of most species
ranges from 130 deg. to 140 deg. F. for ten minutes. When spores are
present, resistance is greatly increased, some forms being able to
withstand steam at 212 deg. F. from one to three hours. In the
sterilization of milk, it is often necessary to heat for several
hours, where a single exposure is made, to destroy the resistant
spores, that seem to be more abundant under summer than winter
conditions. Steam under pressure is a much more effective agent, as
the temperature is thus raised considerably beyond 212 deg. F. An
exposure of twenty minutes, at a temperature of 230 deg. to 240 deg. F.
will kill all spores. Where heat is used in a dry state, it is much less
effective, a baking temperature of 260 deg. to 300 deg. F. for an hour
being necessary to kill spores. This condition is of the utmost
importance in the destruction of bacter