THE FUNGI 



247 



alcohol is retained in the commercial manufacture of beer, wine, 

 and all fermented liquors. The splitting of the sugar by the 

 ferment is represented by the following equation: 



Grape sugar -f zymase = alcohol + carbon dioxide + zymase. 



The above fermentation process was for a long time thought 

 to be due to the activity of the protoplast of the living yeast 

 cells, which were therefore called living ferments. It remained 

 for Buchner, a great German scientist, to demonstrate that the 

 active body in the process of fermentation is not the protoplast 

 of the yeast cell but rather a product of the cell protoplasm in 

 the form of a secretion 

 or fermenting substance, 

 which is now termed zy- 

 mase. Buchner ground 

 yeast cells in fine infu- 

 sorial earth and filtered 

 the juice pressed from 

 the yeast cells through 

 a porcelain filter. The 

 filtered extract thus ob- 

 tained caused alcoholic 

 fermentation in sugar 

 solutions without the presence of living yeast cells. As a result 

 of this experiment Buchner concluded that an active ferment 

 was secreted by the yeast cell, which passed through his filters 

 and caused fermentation in the sugar solutions to which it was 

 added. This ferment is not liberated from the yeast cell in 

 normal fermentation, but the sugar is first absorbed into the 

 cell and is then attacked by the zymase. Such a ferment, or 

 enzyme, which acts within the cell which secretes it, is called 

 an endoenzyme or ferment. In many other instances living cells 

 form exoenzymes, which are first liberated into the surrounding 

 medium before they cause active fermentation. 



The process of fermentation in yeasts is not clearly understood. 

 A distinctive feature of the process is that the ferment is neither 

 diminished in amount nor injured for further use by its activity. 



FIG. 133. Spore formation in yeast 

 After Conn 



