98 PLANT PHYSIOLOGY 



series of transformations of the sugar molecule, excretes alcohol 

 and carbon dioxide as waste products. A series of investigations 

 proved, however, that fermentation and the general vital activity 

 of yeast are not identical. If the yeast cells are given pure 

 sugar without nitrogen and mineral substances, they will cause 

 fermentation but the cells will not multiply. On the other hand, 

 in the presence of free oxygen, fermentation will be slightly 

 retarded, but the reproduction of yeast cells is favored by the 

 presence of the free oxygen. Therefore, for obtaining alcohol, 

 it is more advantageous to limit the access of oxygen; while for 

 purposes of increasing the quantity of yeast, e.g., in yeast fac- 

 tories, it is more advantageous to give abundant aeration of the 

 liquid medium. 



The most convincing evidence that fermentation may be 

 separated from other manifestations of the vital activity of yeast 

 cells has been given in the experiments of Buchner. In grinding 

 yeast with sand and subjecting the mass obtained to a pressure 

 of 300 to 400 atmospheres in a hydrauHc press, he succeeded in 

 securing a transparent juice not containing any living cells that 

 nevertheless possessed the capacity of fermenting sugars. The 

 active part of this juice could be precipitated by either acetone 

 or alcohol and then again dissolved without loss of its capacity 

 of producing fermentation. For this reason, it was considered 

 an enzyme and was termed "zymase." Later, simpler methods 

 of obtaining zymase were elaborated, e.g., by means of treating 

 the yeast cells with acetone or extraction with water after careful 

 drying. This greatly facilitated the study of the enzyme. 

 Further investigations proved that zymase represents not a single 

 enzyme but a mixture of enzymes; and accordingly the reaction 

 of alcoholic fermentation proves to be very comphcated, proceed- 

 ing in several separate phases. 



These phases have not yet been definitely estabhshed, though 

 the recent investigations of Neubauer and Kostytchev and 

 their coworkers have brought us close to the solution of this 

 problem. According to these investigations, the chief intermedi- 

 ate products of fermentation are pyruvic acid, CH3COCOOH, 

 and acetaldehyde, CH3CHO. 



Under normal conditions, these intermediate products do not 

 accumulate in the fermenting liquid; but by introducing various 

 substances, it is possible to change the course of the reaction 



