FERMENTED MILKS 141 



two organisms and with mixtures reproduces very closely 

 the process of natural souring. 



In addition to this lactic acid fermentation, milk is 

 sometimes caused to undergo an alcohoHc fermentation. 

 This is done by fermenting the milk with yeast or with 

 a mixture of lactic acid bacteria and yeast. A well- 



Fig. 47. — Bacillus bulgaricus of the bacillary milk; X 1000 

 (Fairchild) . 



known drink, called koumiss (or kumyss), is made by 

 the Tartars from mares' milk, a small quantity of old 

 koumiss often being added to fresh milk as a starter. 

 In this country koumiss is made commercially by fer- 

 menting milk with 3^east and lactic acid bacilli. Such 

 a preparation contains not only lactic acid, but also 

 carbonic acid gas and about 1 per cent, of alcohol. 



