390 Bacteria in Relation to Country Life 



140 Fahr., for fifteen to twenty minutes, or in a shorter 

 time at higher temperatures; occasionally a few of the 

 tubercle-bacteria survive this treatment, but even these 

 are rendered less virulent. 



In view of these facts, the heating of milk at 150 

 to 160 Fahr., is extensively resorted to as effective 

 for the destruction of the germs of diphtheria, typhoid 

 and tuberculosis. The process is known as pasteuriza- 

 tion, after the eminent French chemist and biologist, 

 Pasteur, who first recommended the method for ridding 

 wine of certain germs that multiplied in it and injured 

 its quality. 



When properly carried out, pasteurization reduces 

 the germ content of milk from many thousands or even 

 millions, to a few hundreds or less. Herein lie the ad- 

 vantages and disadvantages of pasteurization, as will 

 be seen from the following considerations: Under nor- 

 mal conditions, milk gradually turns sour on account 

 of the rapid development in it of the lactic-acid bacteria, 

 and the accumulation of lactic acid. The lactic-acid 

 bacteria find no difficulty in crowding out the other 

 forms, and, while the souring of milk is, in itself, ob- 

 jectionable to the^ dairyman and the consumer, it is, 

 none the less, a protection against the growth of other 

 germs whose products may not be as harmless. 



The lactic-acid bacteria do not produce spores, and 

 are all destroyed in the process of pasteurization. The 

 spore-bearing decay bacteria are now given a free 

 field, and, when the temperature conditions are favorable, 

 they multiply rapidly and produce unpleasant tastes 

 and odors in the milk, The substances thus produced 



