130 MILK HYGIENE 



degree of heat is maintained long enough. This is also 

 true of many bacteria occurring in milk which may be 

 injurious, yet which cause no specific disease. There- 

 fore, milk may be pasteurized either by a brief heating 

 at a higher temperature or a longer heating at a lower 

 temperature. But the spores of bacteria and the thermo- 

 philic forms that are specially capable of resisting heat, 

 are not destroyed by these methods, so pasteurization 

 does not have the same effect as sterilization. Milk may 

 be pasteurized in different ways. The following three 

 methods are those most commonly used : 



1. The milk is heated during one-quarter to one hour 

 at from 80 to 85 (176 to 185 P.) while it is flowing 

 through an appropriate pasteurizing apparatus ; then it 

 is cooled immediately. 



2. The milk is heated from one quarter to one hour 

 at about 65 C. (150 F.) in a tank or vat. The milk 

 flows directly from this vessel into the one in which it 

 is to be sold, or it is first passed through a cooler. 



3. The milk is poured into well cleansed or sterilized 

 bottles or metal vessels, is then heated for a consid- 

 erable time at 65 to 80 C. (150 to 176 F.), then the 

 bottles or vessels are sealed and quickly cooled. 



For sanitary reasons the last of these methods is 

 unquestionably to be preferred, but it has not been 

 adopted extensively on account of its cost. Heating 

 many small vessels requires a great amount of heat and 

 causes great loss in bottles, if such are used. The initial 

 cost, also, and the amount of labor required are con- 

 siderable. 



If one of the first two methods is employed, the pas- 

 teurized milk must be drawn into vessels (bottles or 

 metal cans) which have previously been properly 

 sterilized. 



Pasteurizing milk and then placing it in unsterilized 



