8 Dairy Bacteriology. 



Effect Of cold. While it is true that chilling largely pre- 

 vents 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 vegetative condition. Numerous non- 

 spore-bearing species remain alive in ice for a prolonged 

 period, and recent experiments with liquid air show that 

 even a temperature of -310 F. for hours does not effect- 

 ually 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 stage. The temperature at which the vitality of the 

 cell is lost is known as the thermal death point. This limit 

 is not only dependent upon the nature of the organism, but 

 varies with 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 vegetating forms the thermal death point of most 

 bacteria ranges from 130-140 F. where the exposure is 

 made for ten minutes which is the standard arbitrarily 

 selected. In the spore stage resistance is greatly increased, 

 some forms being able to withstand steam at 210-212 F. 

 from one to three hours. If dry heat is employed, 260- 

 300 F. for an hour is necessary to kill spores. Where steam 

 is confined under pressure, a temperature of 230-240 F. 

 for 15-20 minutes suffices to kill all spores. 



Drying. Spore-bearing bacteria like anthrax withstand 

 drying with impunity; even tuberculous material, although 

 not possessing spores retains its infectious properties for 



