6 BULLETIN 342, U. S. DEPARTMENT OF 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 Rupp (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. Bowen (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 80 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. 
