PASTEURIZATION 295 



of whey, and any other features of difference between- 

 the two samples. 



(c) Pasteurization. The process of pasteurization 

 has been extensive^ applied to milk with a view of 

 increasing its keeping quality, without appreciably alter- 

 ing its natural flavour or nutritive property. It consists 

 in heating the milk to a temperature of 60 C. to 80 C. 

 (140 to 185 F.) for twenty minutes or less, and then 

 cooling it rapidly to about 10 C. (50 R). There is no 

 definite standard of temperature for the process ; that 

 perhaps most generally adopted is 85 C. at which the 

 milk is kept for three to five minutes. At 70 C. 

 (158 F.), or lower, the heating should be continued for 

 twenty minutes. In all cases for efficient pasteurization 

 it is essential to keep the milk stirred or agitated so^ as 

 to secure uniform heating. 



The heating destroys from 95 to 99 per cent of 

 bacteria present in the vegetative state ; and, if not too 

 prolonged, does not give the milk a cooked taste. 

 Cleaner milk is obtained, and at the low temperature to 

 which it is immediately cooled the few remaining or- 

 ganisms cannot grow ; the milk retains its freshness for 

 three or four days, provided that it is kept in a cool 

 place and free from dust and other contaminating sub- 

 stances. The lactic acid bacteria and many others are 

 destroyed at the temperatures indicated above, but the 

 spores and growing organisms of certain species remain 

 uninjured. According to Fliigge, the pathogenic or- 

 ganisms responsible for tuberculosis, diphtheria, and 

 typhoid fever are killed by heating the milk to 70 C. 

 (158 F.) for half an hour, although this treatment leaves 

 it with a slightly boiled taste. 



