HIGH -TEMPERATURE PASTEURIZATION. 119 



The time occupied by the milk in passing through the machine 

 is varied at the will of the operator, from a fraction of a min- 

 ute to two minutes. The fact that this method of treating milk 

 has come into practice without consideration of its effect upon 

 the pathogenic organisms becomes evident when attention is 

 paid to the literature on the thermal death point of the tubercle 

 bacterium in milk at temperatures between 140° and 160°F. 

 At 140°F (60°C.), the shortest length of time that has been 

 found permissible is fifteen minutes (20) . Heating milk to this 

 temperature or even several degrees above for a minute or less 

 is a long way from the conditions insuring the death of the 

 tubercle organism. Hess (Ref. 9, Chap. IV) has noted the 

 deficiencies of commercially pasteurized milk as regards de- 

 struction of tubercle bacilli. It is only when the temperature 

 of the milk during the heating for brief periods approaches 

 160°F. (71°C.) that evidence is found to justify the belief that 

 pathogenic organisms are killed. 



There seems, then, to be justification for placing confidence 

 in a pasteurization process, as a means for destroying patho- 

 genic bacteria, in which all the milk is actually exposed for 

 half a minute, or even momentarily, to a temperature of 160''F. 

 (71.1°C). The statement need not necessarily imply an 

 adherence to that system of pasteurization for rendering milk 

 safe as against a system employing lower temperatures for 

 longer periods. Commercial high-temperature pasteurization 

 has narrow limits of safety bounded, on the one hand, by alter- 

 ations in the physical characteristics of the milk, and on the 

 other hand, by the danger of not killing pathogenic bacteria. 

 The temperature zone in question comprises 160" to 165°F., 

 and therefore the possibilities of negligence in running the 

 apparatus, or intentional lowering of the temperature for com- 

 mercial reasons, are obvious. There is always the possibility 

 that the machine in use may not heat all portions of the milk 

 uniformly to the temperature desired. 



Kffidc7icv. Efficiency tests of pasteurization must be inter- 

 preted with care. The bacteria remaining should not be com- 

 pared, with respect to either numbers or character, with 

 those in unheated milk. The character of the organisms in 



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