FERMENTATION 35 



The following are the chief varieties of fermentation : 

 The alcoholic fermentation. This is mainly brought 

 about by the decomposition of sugars of the hexose group 

 (C 6 H 12 6 ), principally dextrose and Isevulose, by yeasts 

 into alcohol and carbonic acid, but some of the bacteria 

 and moulds also produce appreciable quantities of alcohol. 

 Other carbohydrates by the action of enzymes secreted by 

 the organisms may be converted into hexoses, which are 

 then fermented. The general reaction is as follows : 



C 6 H 12 6 = 2C 2 H 6 + 2C0 2 . 



As a matter of fact small amounts of by-products 

 appear in addition to the alcohol and carbonic acid, viz. 

 glycerin, succinic acid, and higher alcohols. Until 1897 

 no enzyme had been obtained which would carry out this 

 change ; it only occurred when the living yeast-cells were 

 present, but in that year Buchner, by grinding up the 

 living yeast-cells, obtained a juice which decomposed 

 dextrose with the formation of alcohol and carbonic acid. 

 This " zymase " Buchner claimed to be the alcoholic 

 enzyme of yeast. 



The lactic acid fermentation. This is brought about 

 chiefly by bacteria. Hexoses are converted into lactic 

 acid, the reaction being 



C 6 H 12 6 = 2(HC 3 H 6 3 ), 



kit it is probably not actually so simple as this, for 

 carbonic acid is given off at the same time. A familiar 

 example of this form of fermentation is the souring of 

 milk, in which the lactose is acted upon as follows : 



C 12 H 22 O n + H 2 = 4C 3 H 6 3 . 



The butyric acid fermentation. Butyric acid is formed 

 from carbohydrates by the action of bacteria, mainly the 

 Bacillus butyricus and Clostridium butyricum, the latter an 

 anaerobic organism, some by-products being formed in 



