88 PHYSIOLOGICAL ACTIVITIES 



acid formation is also taken advantage of in the preparation 

 of lactic acid on a commercial scale in at least one plant in 

 this country. 



Acetic acid is another common product of acid fermenta- 

 tion. However, in vinegar making the acetic acid is not 

 formed directly from the sugar in the fruit juice by bacteria. 

 The sugar is first converted into alcohol by yeasts, then the 



Fig. 65. — Filling a silo on the University farm. 



alcohol is oxidized to acid by the bacteria (Fig. 66). The 

 reaction may be represented as follows : 



Dextrose. Ethyl alcohol. Acetic acid. 



CeHizOs = 2C2H60H + 2CO2 



C2H6OH + O2 + (bacteria) = CH3COOH + H2O 



Butyric acid is generally produced where fermentation of 

 carbohydrate occurs under anaerobic conditions. Some of 

 the "strong" odor of certain kinds of cheese is due to this 

 acid which is formed partly from the milk-sugar remaining 

 in the cheese. Most of it under these conditions comes from 

 the proteins of the cheese and especially from the fat 

 (see page 90). 



