208 BACTERIA IN AIR, SOIL, WATER, AND MILK 



roundings in the dairy are only important in causing 

 souring of the product. 



Milk is a capital culture medium for almost all bac- 

 teria, and as it is warm when drawn, growth may begin 

 shortly. Unless the milk be cooled very soon, to a 

 temperature at which bacterial growth is retarded or 

 stopped, souring will occur. Perfectly fresh milk has a 

 very slight restraining influence upon the development 

 of some feebler bacteria, but this power is soon lost 

 and bacterial growth may be unlimited. It is best to 

 keep milk not above 40 F. or 5 C., but so low a 

 temperature is not always possible to maintain. The 

 consumer should strive to keep milk at the lowest 

 temperature practicable. Cities are now controlling 

 their milk supply by various regulations as to the 

 dairy management and shipping systems. The most 

 important domestic means of having clean milk con- 

 sists in receiving it in perfectly clean bottles and 

 keeping it on ice. 



Pasteurization. On account of the lack of perfect 

 municipal control of the milk supply, it is necessary 

 to resort to Pasteurization. This consists in heating 

 the milk to 60 or 70 C., 140 to 158 F., for ten to 

 twenty minutes, and then cooling rapidly. Various 

 methods are in use commercially, but this can be done 

 very easily in the home, using a double boiler and a 

 thermometer. Pasteurization kills all but the spores 

 of putrefactive bacteria, which are of little danger if 

 the milk be kept on the ice or used shortly. Some 

 persons object to the use of this heating because the 

 food value of the milk is reduced by making certain 

 chemical constituents harder to digest. The casein 



