YEASTS. . 387 



Most of the saccharomyces can give rise to alcohol and are, 

 therefore, of great industrial importance. Certain species 

 induce the alcoholic fermentation at a temperature of 

 14 to 18° and are known as the top or upper yeasts, whereas 

 other forms are active at a lower temperature, 4 to 10°, and 

 are hence commonly designated as bottom or Imver yeasts. 



0^ 





Fig. 55. Yeast cells with spores (Hansen). 



The common Saccharomyces cerevisiae is a typical upper 

 yeast. It is used in brewing and in baking. The ordinary 

 compressed yeast contains this organism mixed with more 

 or less starch. It forms large, round or oval cells. " Not 

 infrequently, in brewing, this organism is contaminated 

 with other yeasts or even with bacteria, and the products 

 elaborated by these foreign organisms may greatly alter 

 the composition and general characteristics of the beer. 

 As a result of the studies of Pasteur and of Hansen, on these 

 so-called diseases of beer, it has become customary in large 

 establishments to employ only pure cultures of yeasts. 



The most common cause of the spontaneous fermenta- 

 tion of gtape and other fruit juice is the Saccharomyces 

 ellipsoideus. Secondary fermentations or "diseases" may 

 occur in wine as in beer. A group of species which fre- 

 quently show sausage-like cells are known as the S. 

 Pastorianus. 



' The torula group contains the so-called wild yeasts. 

 These rarely give rise to thread-like forms, and never pro- 

 duce spores. As a rule, they cannot produce more than 



