174 



MANUAL OF MILK PRODUCTS 



It will be seen that of nine cities with a population of more 

 than 500,000 each, in seven more than 50 per cent of the milk 

 is pasteurized ; in fact, the proportion is much higher, as the 

 next table shows. Since these figures were obtained the percent- 

 age of milk pasteurized has probably increased in these cities. 



Proportion of Total Milk Supply Pasteurized in Certain 



Cities l 



City 



Pek Cent 

 Pasteur- 



IZED 



City 



Per Cent 

 Pasteur- 

 ized 



Boston, Mass. . . . 



Chicago, 111 



Detroit, Mich. ... . 

 New York, N. Y. 



80 

 80 

 57 



88 



Philadelphia, Pa. . . 

 Pittsburgh, Pa. . . . 

 St. Louis, Mo. . . . 



85 

 95 

 70 



Methods of pasteurization. 



At present there are three processes of pasteurization prac- 

 ticed in this country. The first is known as the flash, or con- 

 tinuous, process; the second, the holder, or holding, process; 

 and the third is known as pasteurization in the bottle. 



The flash process consists in heating rapidly to the pasteur- 

 izing temperature, then cooling quickly. In this process the 

 milk is heated from 30 seconds to 1 minute only, usually at a 

 temperature of 160° F. or above. 



In the holder process the milk is heated rapidly to temper- 

 atures of from 140° to 150° F. and held for approximately 30 

 minutes, after which it is rapidly cooled. Sometimes the milk, 

 instead of being held at a certain temperature in one tank for 

 30 minutes, is merely retarded in its passage through several 

 tanks so that the length of time is required for the milk to pass 

 through. In such cases, however, there is no assurance that 

 all the milk is held for the desired time. The holder process, 



1 In the small cities the per cent of milk pasteurized is much lower. 



