FERMENTATION 



537 



Alcoholic fermentation will occur in almost any moist sugar-containing 

 medium or sugar solution which is inoculated with yeast or which is left 

 exposed to the air. Since various species of wild yeasts are blown about 

 through the atmosphere inoculation of such media will occur without human 

 intervention. 



The following summary equation represents the chemical changes occur- 

 ring in alcoholic fermentation : 



Zymase 



CgHi20 



Gni2W6 



^ 2 C2H5OH + 2 CO2 + 25 kg.-cal. 



As this equation shows, fermentation of one mol of a hexose sugar results 

 in the production of two mols of ethyl alcohol and two mols of carbon dioxide, 

 energy to the amount of ap- 

 proximately 25 kg.-cal. being 

 released in the process. The 

 carbon dioxide evolved escapes 

 as a gas accounting for the 

 effervescence of a fermenting 

 liquid. Certain by-products such 

 as glycerol, succinic acid, amyl 

 alcohol and other compounds 

 are also usually produced as a 

 result of subsidiary reactions. 



Undoubtedly the reaction as 

 given above occurs in a number 

 of steps, and certain interme- 

 diate products are formed by 

 the disintegration of the sugar 



molecules which are subsequently converted into the compounds which finally 

 appear as the end products. Since zymase is known to be a complex of several 

 enzymes it seems probable that different stages in the process of alcoholic fer- 

 mentation proceed under the influence of different components of this enzyme 

 system. 



Various other types of fungi besides the yeasts can accomplish alcoholic 

 fermentation. As we shall see shortly a process also occurs in green plants 

 which is at least closely analogous to, and may be identical with alcoholic 

 fermentation. 



Yeasts can ferment glucose, fructose, galactose and mannose directly. 

 Since yeast cells also produce the enzymes sucrase and maltase, the disacchar- 

 ides sucrose and maltose can also be fermented after being hydrolyzed to 

 hexose sugars. On the other hand yeast cells cannot ferment starch because 



B 



Fig. 116. Yeast plants {Saccharomyces 

 cerevisiae). {A) a vegetative cell, (6) early 

 stage in vegetative multiplication of a yeast cell, 

 (C) ascus containing ascospores. Redrawn from 

 Gaumann and Dodge (1928) after Guillermond. 



