BACTERIA IN MILK 183 



teats, however, are almost never free from some germs, and 

 of course the outside skin contains many. In a diseased 

 udder there may be not only the germ causing the disease, 

 but other intruders from the outside. Bacteria come into 

 milk from the cow herself or from the outside. The latter is 

 probably the more important and the factors which must 

 be considered are the dirt on the skin, swishing of the soiled 

 tail, the soiled hands of the dairyman, and the cans, con- 

 taminated by manure or by polluted water. The ordinary 

 milk bacteria are fortunately not pathogenic, the dangerous 

 varieties from the cow being only streptococci from inflam- 

 mation of the udder, and tubercle bacilli. Those forms 

 getting into milk from the surroundings in the dairy are only 

 important in causing souring of the product. 



Milk is a capital culture medium for almost all bacteria, 

 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 tem- 

 perature 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 consists 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 Pas- 

 teurization. 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 



