CHAPTER VI. 



PASTEURIZATION OF MILK. 



General definition. Pasteurization is a generic term refer- 

 ring to various processes of heating milk, followed by cooling. 

 The temperatures involved vary from 140°P. (60"C. ) to 185°F. 

 (85°C.) and the length of time of exposure to these tempera- 

 tures varies from a few seconds to thirty minutes. A consider- 

 able number of pasteurizing machines designed to accomplish 

 the heating of the milk in various ways are in use, and in 

 actual practice do their work with varying degrees of uni- 

 formity. The temperature to which the milk is finally cooled, 

 an important factor in the efficiency and value of the process, 

 likewise varies. Another element leading to diversity of 

 results is the bacteriological quality of the milk treated by the 

 processes. Notwithstanding the widely differing processes 

 designated by " pasteurization," the term unfortunately is con- 

 stantly used without qualification to indicate the precise process 

 designated. 



Sterilization, a term frequently used in the discussion of the 

 subject of heated milk, properlj^ refers to a process by which 

 all bacteria and their spores are killed by heat. This in con- 

 nection with the preservation of milk involves an exposure to 

 heat above the boiling point. It is employed in the preser- 

 vation of canned milk and never in connection with ordinary 

 market milk. 



Incentives foi- pasteurization. The destruction of pathogenic 

 bacteria has been the chief incentive to the development of 

 pasteurizing processes. The effect of heating in enhancing 

 the keeping quality of milk is a valuable result which also has 

 stimulated interest in the process. Methods for preserving 

 milk are not necessarily the same as for killing pathogenic bac- 

 teria, although the converse is true. 



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