XIV.] MILK SUGAR 273 



lactose, which differs from ordinary sugar in its lesser 

 solubility and in its lack of sweetness. Milk sugar may 

 be obtained by evaporating down the whey after the 

 casein has been coagulated and the albumen 

 removed by heating up to boiling-point Milk sugar 

 does not ferment with ordinary yeast, " but can be 

 fermented into alcohol and carbon dioxide by certain 

 special yeasts such as those employed in making kephir 

 and koumiss — fermented liquids used by the Mongol 

 races. The most characteristic fermentation of milk 

 sugar, however, is that brought about by the lactic acid 

 bacteria, which are always about in cows' stalls, etc., and 

 find their way into the milk ; they multiply with great 

 rapidity in the warm milk, and split up the sugar into 

 lactic acid, while at the same time they oxidise some of 

 it into carbon dioxide and water. These organisms are 

 responsible for the usual natural souring of milk, in 

 which the casein is precipitated as soon as the acidity 

 reaches a certain degree. It is because the development 

 of the lactic acid .bacteria is so much accelerated by 

 warmth, that it is important to cool milk down by the 

 refrigerator immediately after it has been drawn from 

 the cow, if the milk is to be sent any distance by rail 

 and not used immediately. 



Co7nposition of Milk, — The average composition of 

 milk in England is given by Droop Richmond as 

 follows, the figures being the average of about 200,000 

 analyses : — 



