8 
BULLETIN" 342, XJ. S. DEPARTMENT OF AGRICULTURE 
use of lower temperature has become 
more common. As a general rule, 
when the holder process is used milk 
is heated to about 145° F. for from 
20 to 30 minutes and to at least 160° 
F. for 1 minute when the flash process 
is used. From bacteriological, chemi- 
cal, and economical standpoints it is 
highly desirable that milk be Pas- 
teurized at the lower temperature. 
From a bacteriological standpoint. 
Pasteurization at 145° F. for 30 
minutes gives assurance, so far as we 
know, of a complete destruction of 
nonspore-forming disease-producing 
bacteria and at the same time leaves 
in the Pasteurized milk the maximum 
percentage of the bacteria that cause 
milk to sour (lactic-acid bacteria) and 
only a small percentage of those that 
cause it to decompose (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 in- 
creases until at 180° F., or above, the 
lactic-acid bacteria are practically de- 
stroyed and 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 ad- 
vantage of the lower temperature 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 digestibil- 
ity. According to Rupp (26), the solu- 
ble phosphates of lime and magnesia 
do not become insoluble and the albu- 
min does not coagulate. At 150° F. 
about 5 per cent of the albumin is ren- 
dered insoluble, and the amount in- 
creases with higher temperatures to 
160° F., when about 30 per cent of the 
albumin is coagulated. The heating 
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 (10) has shown that the flash 
process of Pasteurization requires ap- 
proximately 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 Pas- 
teurization. This is owing to the fact 
that in the holder process milk may be 
heated to 145° F. and held for 30 min- 
utes, while to obtain the same bacteri- 
ological reduction with the flash proc- 
ess, with 1-minute heating, the milk 
would have to be heated to 165° F.. 
and even then the complete destruc- 
tion of disease-producing 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 be- 
lieved that the temperature of Pas- 
teurization should be 145° F. and that 
the milk should be held at that tem- 
perature 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 heat- 
ed to that point and held the full 
length of time. But it has been shown 
by Schorer and Rosenau (27) that it 
is difficult to do this under commercial 
conditions. These investigators tested 
the destruction of pathogenic organ- 
isms by inoculating milk with B. diph- 
therice, B. typhi, and B. tuberculosis 
and Pasteurizing it in 100-gallon lots 
under commercial conditions. They 
found that sometimes the organisms 
were not all destroyed, and in this con- 
nection state: 
Nothing- in our experiment throws any 
doubt upon the thermal death points of the 
microorganisms tested. We are sure that if 
the milk reaches 140° F. and is held there 
for 20 minutes it will kill tubercle, typhoid, 
and diphtheria bacilli. Our experiments 
show that milk Pasteurized at this tem- 
perature for the specific time may not 
always, in practice, reach these minimum 
requirements. It is therefore evident that 
a liberal factor of safety is necessary in 
the operation of this type of Pasteurizer 
under commercial conditions. 
They state further : 
Perhaps the best temperature to meet 
practical conditions is 145° F. and the 
milk should be held from 30 to 45 minutes. 
This should give sufficient leeway. If the 
Pasteurizer is set at 145° F. care will prob- 
ably be taken that it does not go above 
148° F. on account of destroying the cream 
line, and it is not likely that the mixed 
milk in the holding tank would drop below 
140° F., which is the minimum. 
Other experiments are reported by 
Pease and Heulings. (in the Report of 
the Committee on Milk Supply of 
American Public Health Association, 
1920), in which the destruction of 
pathogenic organisms was tested under 
commercial conditions of Pasteuriza- 
tion. Some of the pathogenic types 
were found living after heating to 
from 140° to 141° F. and holding for 
15 minutes, but none were found alive 
after 30 minutes' holding. Here again 
is evidence of the narrow margin of 
