FERMENTATION OF CARBOHYDRATES 



83 



group and are indispensable in many commercial processes. 

 In the making of butter the cream is first "ripened," as is 

 the milk from which many kinds of cheese are made (Fig. 58) . 

 The chief feature of this "ripening" is the formation of lac- 

 tic acid from the milk-sugar by the action of bacteria. A 

 similar change occurs in the. popular "Bulgarian fermented 

 milk." The reaction is usually represented by the equa- 

 tion: 



Milk-sugar. Lactic acid. 



C12H22O11 + H2O + (bacteria) = 4C3H6O3 



Fig. 58. — A cream ripener. In this apparatus cream is "ripened," i. e. 

 undergoes lactic acid fermentation preparatory to making it into butter. 



It is not probable that the change occurs quantitatively as 

 indicated, because a number of other substances are also 

 formed. Some of these are acetic and succinic acids and 

 alcohol. Another industrial use of this acid fermentation 

 is in the preparation of "sauer kraut." These bacteria are 

 chiefly anaerobic and grow best in a relatively high salt 

 concentration. They occur naturally on the cabbage leaves. 

 In the formation of ensilage (Fig. 59) the lactic acid 

 bacteria play a very important part, as they do also in " sour- 



