SUBSTITUTION PRODUCTS OF THE HYDROCARBONS 33 



the life process of the yeast plant was directly connected with 

 alcoholic fermentation. Pure yeast is able to ferment only 

 certain sugars, the two most common ones being glucose, or 

 grape sugar, and fructose, or fruit sugar. In the grape juice both 

 glucose and the yeast cells are present, the latter occurring 

 naturally on the bloom of the grape. 



Enzymes, Zymase. — The recent work of Buchner has shown 

 that the fermentation is due not to the living action of the yeast 

 cell but to a substance known as zymase, which is secreted by 

 the cell. A number of substances are known which act catalyti- 

 cally in producing chemical changes of the same nature, and 

 which are termed in general fermentations. Ptyalin, the active 

 substance in saliva, which converts starch into sugar; pepsin, 

 the active substance in gastric juice converting proteins into 

 simpler compounds; and diastase, a constituent of sprouting 

 grain which also converts starch into sugar, — are examples of 

 these substances. They are known 2iS ferments. Because alco- 

 holic fermentation, which is the most common process of this 

 nature, was supposed, until Buchner's time, to be due to a living 

 cell, these other substances which could be obtained in a more 

 or less pure condition were distinguished from the yeast plant 

 ferment by the name unorganized ferment and later as enzymes^ 

 the alcoholic ferment being known as an organized ferment. 



Buchner, however, proved that the living yeast cell could be 

 entirely destroyed and an unorganized ferment which he called 

 zymase obtained from it which in itself possessed the power of 

 fermenting grape sugar. Thus alcoholic fermentation is of the 

 same nature as these other fermentations, and is due like them 

 to the catalytic action of an unorganized ferment or enzyme. 

 Thus both of the old views of Liebig and Pasteur may be con- 

 sidered as in a way true. The action, as Liebig claimed, is 

 catalytic, i.e. depending upon the mere presence or contact of 

 the enzyme, not upon its mass, while the living yeast cell is 

 necessary, not directly to the fermentation itself, as Pasteur 

 claimed, but to the formation of the enzyme, a chemical sub- 

 stance which produces the fermentation. 



