MILK AND ITS CARE. 



167 



FIG. 3S. PASTEURIZING APPARATUS. 



(2) Pasteurization. — Another method of 

 preventing the growth of bacteria in milk is 

 that of Pas- g 



teurization /7"^ 1 



(Fig. 38). This 

 consists in 

 heating milk 

 to about 160^ 

 ¥. for twenty 

 minutes, then 

 rapidly cool- 

 ing to 50° ¥. 

 This kills 

 about 99 per 

 cent, of the bacteria, and the keeping quality of 

 the milk is very much improved. 



III. Composition of Milk. 



The milk of all mammals contains practically 

 the same constituents, but varies greatly in the 

 proportion of each. The average composition 

 of cow's milk in America is as follows : Water, 

 87.5 per cent.; fat, 3.6 per cent.; casein, 2.9 per 

 cent.; albumen, .5 per cent.; milk sugar, 4.75 

 per cent.; ash, or mineral, .75 per cent. 



I. Butter Fat. — The butter fat is commer- 

 cially the most valuable part of milk. It varies 

 in amount more than any other constituent of 

 milk except water. Wide variations from the 

 average composition are constantly found. Fat 

 seldom is less than 2.5 per cent., or more than 



