84 



PHYSIOLOGICAL ACTIVITIES 



mash" distilling and in many kinds of natural "pickling." 

 In fact, whenever green vegetable material "sours" spon- 

 taneously, lactic acid bacteria are always present and 

 account for a large part of the acid. This property of lactic 

 acid formation is also taken advantage of in the preparation 

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

 this countrv. 



Fig. 59.— Filling a silo on the University farm. 



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 bac- 

 teria. The sugar is first converted into alcohol by yeasts, 

 then the alcohol is oxidized to acid by the bacteria (Fig. 60). 

 The reaction may be represented as follows: 



Dextrose. Ethyl alcohol. Acetic acid. 



CeHizOe = 2C2H5OH + 2CO2 



C2H5OH + O2 + (bacteria) = CH3COOH + H2O 



Butyric acid is generally produced where fermentation of 

 carbohydrates occurs under anaerobic conditions. Some of 



