=Temperature.= One of the most important conditions influencing the rate of growth of bacteria is the temperature. Each form has a _minimum_ temperature below which growth can not take place; also a _maximum_ above which growth is again impossible. For the majority of species the minimum temperature ranges from 40 to 45° F. the maximum from 105 to 110° F. Growth takes place most rapidly at the 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° 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° 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° 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° 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.