CONSTITUENTS OF MILK 149 



ordinary sugar in its chemical behavior and espe- 

 cially in its relations to various ferments. The 

 amount of sugar in milk varies from below 4 to 

 over 6 per cent and averages about 5 per cent. 

 Variation in the amount of sugar in different nor- 

 mal milks has little interest in connection with the 

 operations of cheese-making for the reason that 

 there is always an abundance for cheese-making 

 purposes. The milk-sugar passes largely into the 

 whey in the cheese-making process and forms a 

 large percentage of the solids in whey. The milk- 

 sugar of commerce is usually prepared by evaporat- 

 ing whey and purifying the impure product first 

 obtained. The importance of milk-sugar in cheese- 

 making depends on the fact that it is easily con- 

 verted into lactic acid by certain forms of bacteria. 

 In the making of cheddar cheese, only a small pro- 

 portion of the sugar is changed into lactic acid during 

 a considerable part of the process, but one per. cent or 

 more is so changed by the time the curd is salted. In 

 cheese made from sour milk, such as cottage cheese, 

 and in starters used in cheese-making, somewhat 

 more than one-fourth of the milk-sugar is changed 

 and there is formed in such cases about 0.7 or 0.8 

 per cent of lactic acid. When milk or whey is 

 allowed to stand for some time at ordinary tem- 

 peratures, over i per cent of lactic acid may 

 be formed. Hence, sour milk or whey, when two 

 or three days old, usually contains only 3.5 to 4 

 per cent of milk-sugar. In cheddar cheese made 

 under normal conditions, we never find any un- 

 combined or free lactic acid, since it combines with 

 calcium of certain calcium salts in the milk to form 



