he
optimum temperature, which, for each species, lies close to the
maximum temperature at which growth can occur. Most of the bacteria
of importance in the dairy grow well at from 70 to 100 deg. F.

There are forms that can grow below the freezing point of water when
they are in solutions that do not freeze at this temperature. There
are still other bacteria that can grow at 140 deg. F. a temperature that
is quickly fatal to most forms. These are of importance in the dairy
since they limit the temperatures at which milk can be stored for
long periods of time.

=Air supply.= Living organisms, both plant and animal, require air or
oxygen for the combustion of their food and for the production of
energy. Most bacteria use, as do the green plants and animals, the
free oxygen of the air for their respiration. Such organisms are
called _aerobic_ or air-living. A much smaller group possess the
power of taking oxygen from organic compounds such as sugar and the
like and therefore are able to live under conditions where air is
excluded. These are called _anaerobic_ bacteria. A large number of
bacteria are able to live either in the presence or in the absence
of free oxygen. Most of the bacteria of importance in the dairy are
of this nature.

=Rate of growth.= When there is an abundant supply of food and when
the temperature conditions are favorable, the bacteria increase in
numbers with astounding rapidity. It has been determined by actual
experiment that the process of cell division under favorable
conditions takes place in a few moments. Barber has shown that one
of the forms of bacteria constantly found in milk will divide in 17
minutes at 98 deg. F. and that a single organism kept at this
temperature for ten hours would increase to 1,240,000,000. If the
temperature is reduced to 50 deg. F., the time required for division is
increased to several hours. The explanation for the rapid spoiling
of milk that is not well cooled is thus apparent. The initial rapid
rate of increase cannot be maintained for any length of time as the
conditions become more and more unfavorable as growth continues, due
to the accumulation of the by-products of the cell activity. Thus,
the growth of acid-forming organisms in milk becomes checked by the
formation of acid from the fermentation of the sugar.

=Detrimental effect of external conditions.= Environmental conditions
of a detrimental character are constantly at work tending to repress
the a