BACTERIA IN MILK. 2O/ 



only as showing the variety of changes which may occur in 

 milk under the influence of different bacteria. 



Alcoholic Fermentation. The alcoholic fermentation of milk 

 is one of some importance although it does not occur nor- 

 mally. Most sugar solutions are liable, under the influence of 

 yeasts, to undergo a fermentation resulting in the production 

 of alcohol. But milk sugar does not easily undergo such 

 fermentation. It is readily converted into lactic acid, but not 

 into alcohol, and, consequently, alcoholic fermentations of milk 

 are produced only by special conditions and special treatment. 

 Nevertheless alcoholic milks are frequently prepared and are 

 articles of diet of no little value, especially for invalids. There 

 are several methods of producing such fermentations. A 

 beverage called koummys, originally prepared from mare's milk 

 which readily undergoes the alcoholic fermentation, is now 

 somewhat closely imitated with cow's milk by a simple device. 

 A certain amount of cane sugar is added to the milk and yeast 

 is placed in the mixture. The yeast quickly starts an alcoholic 

 fermentation of the cane sugar, and the souring of the milk 

 that occurs at the same time adds a peculiar character to the 

 product. Another beverage called kefir is not so common and 

 requires for its manufacture some special bodies called kefir 

 grains. These are hard nodules of various sizes which have 

 the power of starting an alcoholic fermentation in ordinary 

 cow's milk. The origin of these kefir grains is unknown. 

 To-day they are handed from person to person, taken out of 

 the milk after the fermentation and dried to be used again. 

 During the fermentation in the milk they increase in size and 

 new grains may be obtained from fragments of the old ones. 

 These kefir grains prove to be a mixture of yeasts and bacteria 

 (Fig. 28). There are at least two species of bacteria present, and 

 one species of yeast, and all acting together produce the fer- 

 mentation. The bacteria appear to change the nature of the 

 milk sugar by inverting it, after which it is readily acted upon by 



