6 BULLETIN 342, U. S. DEPARTMENT OP AGRICULTURE. 



those that cause it to rot (peptonizers). When higher temperatures 

 are used, while the total number of all kinds of bacteria is reduced, 

 the percentage 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 pas- 

 teurized milk. 



From a chemical standpoint the advantage of low temperatures 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 Kupp (7) 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 heat- 

 ing period in Rupp's experiments was 30 minutes. 



From an economic standpoint the advantage of pasteurization at 

 low temperatures is in the saving in the cost of heating and cooling 

 the milk. Bo wen (8) has shown that the flash process of pasteuriza- 

 tion 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 pasteur- 

 izing. 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 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. 



In view of the advantages of low -temperature pasteurization, it is 

 advisable to pasteurize milk at 145° F. for 30 minutes. It has been 

 found that heating at 140° F. for that length of time will destroy 

 pathogenic bacteria, but in practice it is advisable to use a tempera- 

 ture several degrees above the limit of safety. During extensive 

 studies of the effect of different temperatures it has been shown that 

 an increase of 5 degrees above 140° F. produces a great increase in 

 the destruction of bacteria in milk. 



The holder process, as previously described, is entirely satisfac- 

 tory when properly used. Considerable attention is necessary, how- 

 ever, to see that the milk is not contaminated during cooling and 

 capping. 



Pasteurization in bottles eliminates the danger of reinfection, 

 provided no water is introduced into the bottles during cooling. 



