Structure, Groivth and Distribution. 15 



for a prolonged period, and experiments with liquid air 

 .show that even a temperature of 310 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 vege- 

 tative 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 temper- 

 ature 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 

 mast species ranges from 130 to 140 F. for ten min- 

 utes. When spores are present, resistance is greatly in- 

 creased, some forms being able to withstand steam . at 

 212 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 F. An exposure of 

 twenty minutes, at a temperature of 230 to 240 F. will 

 kill all spores. Where heat is used in a dry state, it is 

 much less effective, a baking temperature of 260 to 

 300 F. for an hour being necessary to kill spores. This 

 condition is of the utmost importance in the destruction 

 of bacteria in the dairy and creamery. 



Effect of drying. The spore-bearing bacteria with- 



