YEAST 41 



91 C. At this temperature the "boiled" taste is not devel- 

 oped and the bacteria are killed, although the spores are not. 

 Pasteurized milk, if properly refrigerated, will keep much 

 longer than ordinary milk, because no bacteria are present 

 and the spores cannot develop in iced milk. 



Refrigeration. Refrigeration or cooling does not destroy 

 bacteria or spores but since a low temperature is not favor- 

 able to their growth it is possible by this method to keep 

 meat, fruits and vegetables for long periods. 



Sterilization. Food, clothing, medicines, surgical instru- 

 ments and apparatus are sterilized w r hen all bacteria and their 

 spores have been killed. 



Heat Sterilization. Surgical instruments and dressings, 

 apparatus, all glass containers and stoppers are readily 

 sterilized by heating in a hot air oven at a temperature of 

 160 to 170 C., or by heating in a steam bath for one-half 

 hour at a temperature of 115 to 120 C., or by boiling in 

 water for fifteen minutes. 



YEAST (Saccharomyces Cerevisioe) 



Habitat. The common yeast plant (Saccharomyces Cere- 

 vision) is found in the wild or natural state growing in the 

 juices of such ripe fruits as apples, pears and grapes. Culti- 

 vated yeast plants have been grown and used for years in 

 bakeries, breweries and distilleries. In the last few years 

 yeast has been cultivated and put on the market in a solid 

 form or yeast cake consisting of the yeast plants mixed with 

 starch and other nutritive substances. 



Histology. Yeast is a microscopic, unicellular plant. It 

 varies greatly in size, according to its age. The cellulose 

 cell wall is very thin and encloses the protoplasm and nucleus, 

 and one or more non-contractile vacuoles. When the yeast 



