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 
