PASTEURIZATION 209 



isms in naturally infected milk before they are adopted 

 as a standard for pasteurized milk. 



2. Effect of Heat on the Common Milk Bacteria. 

 The effect of pasteurization upon the common milk bac- 

 teria is of importance because if, for example, the lactic 

 acid bacteria are destroyed and peptonizing or gas-form- 

 ing organisms survive the milk may undergo putrefac- 

 tion instead of souring. Moreover, the putrefactive 

 changes may advance sufficiently to render the milk in- 

 jurious before the appearance or taste is altered. 



The action of different degrees of heat upon the com- 

 mon milk bacteria has been very thoroughly studied by 

 Ayers and Johnson. In their experiments, when milk 

 was heated to 170 F. (76.7 C.) or above, the majority 

 of the organisms surviving were peptonizers, but when 

 the temperature was kept below 170 F. the acid-formers 

 predominated among the surviving organisms. Acid 

 was formed slowly, however, when the temperature had 

 reached 160 F. (71.1 C.). The character of the 

 changes which the milk will undergo after pasteuriza- 

 tion will depend not only upon the kind of bacteria which 

 predominate after heating, but also upon the tempera- 

 ture at which the milk is kept. If pasteurized milk is 

 not cooled promptly and kept cool it undergoes putre- 

 faction, especially if it has been exposed to high degrees 

 of heat. Milk heated at a low temperature, 145 F. 

 (62.8 C.) for 30 minutes, when kept at 50 F. (10 C.), 

 will curdle and sour like raw milk, but much more slowly. 

 If milk treated in this manner is stored too long, it may 

 develop a strong, old taste as a result of the growth of 

 the alkali- forming bacteria which survive this amount 

 of heat. 



Pasteurization destroys or weakens the germicidal 



14 



