Structure, Growth and Distribution. 13 



below which growth can not take place ; also a maximum 

 above which growth is again impossible. For the ma- 

 jority 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 tem- 

 perature at which growth can occur. Most of the bac- 

 teria 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 ani- 

 mal, 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 pos- 

 sess the power of taking oxygen from organic com- 

 pounds 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 bac- 

 teria are able to live either in the presence or in the ab- 

 sence of free oxygen. Most of the bacteria of impor- 

 tance in the dairy are of this nature. 



Rate of growth. When there is an abundant supply 

 of food and when the temperature conditions are favor- 

 able, the bacteria increase in numbers with astounding 

 rapidity. It has been determined by actual experiment 



