THE PATHOGENIC FUNGI 981 



The yeasts have been studied most extensively in connection with fermen- 

 tation and are industrially of great importance in the production of wine 

 and beer. Much of our knowledge of the life-processes of bacteria is based 

 on these early investigations of the yeasts. Although Schwann, as early 

 as 1837, recognized the fact that many fermentations could not occur without 

 the presence of yeast, it was not until considerably later that the study 

 of such fermentations was put upon a scientific basis. The typical fermen- 



A 



FIG. 109. YEAST CELLS. Young culture unstained. (After Zettnow.) 



tative action consists in the transformation of sugar into ethyl alcohol accord- 

 ing to the following formula': 



C 6 H 12 O 8 = 2 C 2 H 5 OH -f 2 CO 2 . 



The enzyme by which this fermentation is produced is known as "zymase," 

 and is, according to Buchner, in most cases, an endo-enzyme which may be 

 procured from the cell by expression in a hydraulic press. In addition to 

 this, however, the various yeasts also produce other ferments by means 

 of which they may split higher carbohydrates, such as saccharose, maltose, 

 and even starch, and prepare them for action of the zymase. The manner 

 in which this is accomplished, and the by-products which are formed during 

 the process, vary among different species, and it is for this reason that 

 the employment of pure cultures is of such great importance in the wine 

 and beer industries where differences in flavor and other qualities may be 

 directly dependent upon the particular species of yeast employed for the 



2 Pasteur, Etudes sur la biere, Paris, 1876. 



