MILK 91 



ization is the destruction of the pathogenic organisms without 

 so changing the food constituents that it is less suitable for infant 

 feeding, the temperature used and the length of time of exposure 

 depend on these two points. The lowest temperature, therefore, 

 that will kill non-spore-bearing bacteria in a reasonable length of 

 time is the one chosen. An exposure to 60 C. for twenty minutes 

 or to 70 C. for five minutes has been found to be efficient. It is 

 advisable in commercial practice where milk is pasteurized in large 

 quantities to increase the temperature a few degrees and prolong 

 the heating ten to fifteen minutes. 



There are three methods in general use : 



I. The flash method consists in heating the milk to 81 C. and 

 chilling it at once. It is the quickest and cheapest 

 method, but the least reliable. 



II. The holding method consists in heating the milk to 65 C., 

 then holding it at that temperature from thirty to forty- 

 five minutes. Specially devised tanks have been con- 

 structed as " holders " that give excellent results. The 

 method has proved most satisfactory for commercial 

 purposes. 



III. Pasteurization in the bottle is the ideal method. All danger 

 of recontamination is thus eliminated. The bottles are 

 tightly stoppered, immersed in a water bath, brought to 

 the required temperature, and held there a sufficient length 

 of time. 



Whichever method of pasteurization is employed rapid cooling is 

 of great importance. 



The following experiment well illustrates the result of pasteuri- 

 zation so far as diminution in the number of bacteria is concerned. 1 

 The milk was heated to 70 C. for a half and for one minute. 



SAMPLE I 



Raw milk 600,000 bacteria per o.c. 



| minute pasteurization 2,000 bacteria per c.c. 



1 minute pasteurization 1,000 bacteria per c.c. 



1 Adapted from Park and Williams, " Pathogenic Microorganisms," p. 635. 



