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PHYSICAL INFLUENCES. 



Thermal Death-point of Dry and Moist Yeast. 



RESISTANCE or SPORES. The most resistant organisms to heat are 

 the spores of certain bacteria. In the chapter on moisture requirements 

 attention has been called to the great resistance of spores to drying. We 

 find the same exceptional resistance to high temperatures. Boiling heat 

 will not kill spores readily. Some bacterial spores can stand the tem- 

 perature of 100 for several hours. In order to kill spores in one heating 

 the temperature must rise to about 110 for fifteen to thirty minutes; 

 this can be accomplished only by heating under pressure. This is not 

 always advisable for sterilizing food substances. While vegetables are 

 usually sterilized under pressure without losing much of their palata- 

 bility, other foods like milk are changed materially in taste and ap- 

 pearance. To prevent these changes, discontinuous sterilization is 

 sometimes used. This is based upon the following principle: 



If milk or any other medium is heated to 100 for about fifteen min- 

 utes, all living cells of bacteria, yeasts and molds will be killed except a 

 few spores of bacteria. After cooling, these spores will germinate under 

 suitable conditions and the vegetative cells thus appearing instead of the 

 resistant spores are easily killed in a second heating. A third heating 

 is necessary in order to kill any vegetative cells which may have developed 

 from spores not yet germinated before the second heating. It is essential 

 to have the time between two heatings long enough to allow the germination 

 of spores, and not too long to permit formation of new spores. It is 

 customary to heat on three successive days for fifteen minutes each time. 

 In this case, sterilization is usually complete, while a forty-five minutes' 

 heating at once is not sufficient to guarantee sterilization. Among the 

 substances that are very easily sterilized are cider and other fruit juices, 

 while milk and soil are the most difficult materials to sterilize. 



Dry spores will resist still higher temperatures than moist spores 



