134 Dairy Bacteriology. 



In all pasteurized milk as it flows from the machine, 

 there will remain some living bacteria. The spores will 

 not be destroyed by any pasteurizing process, and under 

 commercial conditions, vegetating bacteria are also pres- 

 ent. If the milk is not quickly chilled after heating, 

 these forms will grow, and their development is partic- 

 ularly hastened by the destruction of the lactic bacteria, 

 the acid of which would otherwise hold them in check.. 

 The result is that, unless immediately chilled, pasteur- 

 ized milk spoils almost as rapidly as though it had not 

 been heated at all. Efficient and rapid cooling are, 

 therefore, as essential a portion of the process as the 

 heating itself. 



Care should also be taken to protect the milk from 

 contamination after treatment. Every utensil with 

 which it comes in contact should be sterilized. The bot- 

 tles should be thoroughly washed and sterilized and sub- 

 sequently protected from dust until used. 



Sterilization of milk. It is possible to render milk 

 sterile by the use of temperatures above the boiling point 

 of water, where it is heated in a closed vessel, in which 

 steam under pressure is generated. Such milk is often 

 found in the European markets. In our own country, 

 the only milk of this kind is the so-called "evaporated 

 milk." In this process sweet fresh milk is evaporated 

 in vacuum pans to about one-third of the original vol- 

 ume. This is then placed in tin cans, which are treated, 

 as in the canning of such vegetables as peas and corn, 

 by heating the milk to 230 or 240 F. for a few min- 

 utes. In this process, the bacteria (spores as well as 

 vegetating forms) are completely killed, and the milk 

 acquires a brownish tint, due to the caramelization of 

 the sugar. The appearance of the product is very sim- 



