STATUS OF PASTEURIZATION OF MILK. 11 



acid bacteria becomes less and less and the peptonizing group in- 

 creases until at 180 F., or above, the lactic-acid bacteria are prac- 

 tically destroyed and most of the bacteria left belong to the pep- 

 tonizing group. The heat-resistant lactic-acid bacteria which sur- 

 vive pasteurization at 145 F. for 30 minutes play* an important 

 role in the souring of commercially pasteurized milk. 



From a chemical standpoint, the advantage of the lower tempera- 

 ture is in the fact that milk pasteurized at 145 F. for 30 minutes 

 does not undergo any appreciable change which should affect its 

 nutritive value or digestibility. According to Rupp (26), the solu- 

 ble phosphates of lime and magnesia do not become insoluble and 

 the albumin does not coagulate. At 150 F. about 5 per cent of 

 the albumin is rendered insoluble, and the amount increases with 

 higher temperatures to 160 F., when about 30 per cent of the al- 

 bumin is coagulated. The heating period in Rupp's experiments 

 was 30 minutes. 



From an economic standpoint the advantages of pasteurization 

 at low temperatures is in the saving in the cost of heating and cool- 

 ing the milk. Bowen (10) has shown that the flash process of pas- 

 teurization requires approximately 17 per cent more heat than the 

 holder process. There is, of course, a correspondingly wider range 

 through -which the milk must be cooled, which also adds to the 

 cost of pasteurization. This is owing to the fact that in the holder 

 process milk may be heated to 145 F. and held for 30 minutes, 

 while to obtain the same bacteriological reduction with the flash 

 process, with one-minute heating, the milk would have to be heated 

 to 165 F., and even then the complete destruction of disease-pro- 

 ducing bacteria might be questionable. 



TEMPERATURES AND METHODS MOST SUITABLE FOR PASTEUR- 

 IZATION. 



In view of the advantages of the lower temperature for heating 

 it is believed that the temperature of pasteurization should be 

 145 F. and that the milk should be held at that temperature 

 for 30 minutes. It has been found that heating at 140 F. 

 for that length of time will destroy pathogenic bacteria, provided 

 all the milk is heated to that point and held the full length of time. 

 But it has been shown by Schorer and Rosenau (27) that it is diffi- 

 cult to do this under commercial conditions. These investigators 

 tested the destruction of pathogenic organisms by inoculating milk 

 with B. diphthei^ue, B. typhi, and B. tuberculosis and pasteurizing 

 it in 100-gallon lots under commercial conditions. They found that 



