PASTEURIZATION 195 



death-point of pathogenic bacteria. These must be surely 

 killed so as to eliminate the danger. So far as we 

 are able to judge from our present knowledge the best 

 temperature is 60° C continued twenty minutes. A higher 

 degree of heat for a shorter time is just as effective so far 

 as the destruction of the bacteria is concerned. 



It may be stated with confidence that the tubercle 

 bacillus, the typhoid and the dysentery bacilli, the diph- 

 theria bacillus, the cholera vibrio, the specific cause of 

 scarlet fever, the streptococci, and other harmful micro- 

 organisms which get into the milk are rendered harmless 

 by heating to 60° C. for twenty minutes. I am dogmatic 

 upon this point because I have made it the subject of much 

 study m the laboratory. Further, the experience and ex- 

 periments of many scientists who have investigated this 

 subject are very clear upon this point. It is fortunate that 

 the thermal death-points of the pathogenic bacteria that 

 most concern us are below those of the ferments in milk, 

 for in this way the infections may be destroyed without 

 seriously altering the quality of the milk.^ 



Different times and temperatures, however, are used to 

 pasteurize milk. The historical development of this part 

 of the subject is interesting. At first steriUzation was at- 

 tempted at or above the boiling-point. It was soon shown 

 that boiling does not sterilize milk, and further that a high 

 degree of heat is not necessary. A more precise knowledge 

 of the objects to be attained has gradually resulted in 

 lowering the temperature and shortening the time. Tem- 

 peratures varying from 95° to 60° C, and periods varying 

 from a moment to two hours, have been variously recom- 

 mended for the pasteurization of milk. 



The temperature and time recommended by various 

 authorities for milk pasteurization follow (page 196) : — 



» That is, 140° F. 



