MARKET MILK 177 



rot (peptonizers) . When higher temperatures are used, while 

 the total number of all kinds of bacteria is reduced, the per- 

 centage of lactic-acid bacteria becomes less and less and the 

 peptonizing group increases until at 180 F., or above, when the 

 lactic-acid bacteria are practically destroyed and the most of 

 the bacteria left belong to the peptonizing group. The heat- 

 resistant lactic-acid bacteria which survive 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 low temper- 

 atures is in the fact that milk pasteurized at 145 F. for thirty 

 minutes does not undergo any appreciable change which should 

 affect its nutritive value or digestibility. According to Rupp 

 the soluble 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 albumin is coagulated. The heating 

 period in Rupp's experiment was thirty minutes. 



From an economic standpoint the advantage of pasteuri- 

 zation at low temperatures is in the saving in the cost of heat- 

 ing and cooling the milk. Bowen has shown that the flash 

 process of pasteurization 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 pasteurizing. This is 

 owing to the fact that in the holder process milk may be heated 

 to 145 F. and held for thirty minutes, while to obtain the 

 same bacteriological efficiency with the flash process, with 

 one-minute heating, the milk would have to be heated to 165 F. 

 Temperatures and methods most suitable for pasteurization (see 

 Fig. 42). 



In view of the advantages of low-temperature pasteurization, 

 it is advisable to pasteurize milk at 145 F. for thirty minutes. 



