BACTERIOLOGY OF MILK 135 



unfit for food. In addition to the curdling and the 

 souring, there are produced by some of these bacteria 

 toxic substances which are especially badly borne by 

 infants, and, as it is chiefly as an infant food that milk 

 must be considered, it becomes necessary to devise means 

 of keeping the milk sweet. 



Safeguarding the Milk Supply by Pasteurization. — 

 Much can be done at the dairy, by keeping stables and 

 animals very clean, and by using due precautions when 

 milking to keep out dust and dirt. Then, the warm milk 

 must be rapidly cooled to about 40° to 50° F., bottled, 

 and shipped in ice to the consumer. But such condi- 

 tions prevail at comparatively few dairies, and naturally 

 increase the cost of production considerably, bringing 

 such milk beyond the reach of the great masses. When, 

 therefore, for any reason the bacterial count of milk is 

 high, it is best to resort to sterilizing by heat. This may 

 mean boiling for five to ten minutes. The objections 

 to this are: First, the coagulation of some of the pro- 

 teins, which may render them less digestible, and, 

 second, a pecuHar and rather unpleasant taste and odor 

 which are imparted to the milk. Therefore, pasteuriza- 

 tion has largely superseded boiling. This means heating 

 to 140°-160° F. for fifteen minutes or more. When done 

 on a large scale, the milk to be pasteurized is allowed to 

 run in a thin stream over a metal surface, which is 

 heated by steam to the required temperature, or it may 

 run through a coil of tubes, which are kept in hot water, 

 bringing the temperature of all the milk up to the re- 

 quired degree. It is important that all the milk be thus 

 heated, since otherwise bacteria remain alive in some 

 of the milk, and will develop rapidly throughout the 

 whole lot of milk. Commercial pasteurization has 



