220 PHYSICAL INFLUENCES 



Thermal Death-point op Dry and Moist Yeast 



Resistance of Spores. — The organisms most resistant to heat are 

 the spores of certain bacteria. In the chapter on moisture require- 

 ments 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 temperature of ioo° for several hours. In order to kill spores 

 in one heating the temperature must rise to about iio° for fifteen to 

 thirty minutes; this can be accomplished only by heating under pres- 

 sure. This is not always advisable for sterilizing food substances. 

 While vegetables are usually sterilized under pressure without losing 

 much of their palatability, other foods like milk are changed materially 

 in taste and appearance. 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 iod° for about fifteen min- 

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

 few spores of bacteria. After cooUng, these spores wiU 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 devel- 

 oped 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 mi^nutes' 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. 



