PRESENT STATUS OF PASTEURIZATION OF MILK. dak 
In milk pasteurized at 145° F. the great increase in the propor- 
tion of the acid-coagulating and acid groups is plainly shown. The 
per cent of the alkali and peptonizing groups is reduced. At 160° F. 
the total-acid group is still the largest, but the acid-coagulating gr oup 
is made up of bacteria which coagulate very slowly. At this temper- 
ature the alkali group is greatly reduced, and the peptonizing re- 
duced to the minimum. At 170° F. the total-acid group remains 
about the same, but the organisms produce acid and coagulate the 
milk very slowly. The alkali group is practically destroyed, 
although occasionally a sample may show a fairly high per cent. 
- The most important change is in the peptonizing group. At this 
temperature the ratio of this group to the total number of bacteria 
begins to increase. The increase when milk is pasteurized at 180° I. 
is even more striking. At this temperature more than 75 per cent of 
the bacteria which survive are peptonizers. No organisms of the acid- 
coagulating group are found, and only a small per cent of the acid 
group. Occasionally a few of the alkali group may be found. At 
~ 190° F. and 200° F. the bacterial groups which survive are about the 
_ same in their relative sizes as at 180° F. 
It is very evident that when the bacterial flora of pasteurized milk 
ig under discussion the temperature of the process is of fundamental 
importance. From figure 1 the bacterial groups left in milk pasteur- 
ized at different temperatures may be seen at a glance. It must be 
remembered, however, that the relations of the bacterial groups 
represent. only average conditions and that the bacterial flora of 
every sample of milk must not be expected to conform exactly to 
these averages. Variations in methods and conditions in the produc- 
_ tion of milk may considerably influence the bacterial group rela- 
tions of an individual sample. 
The results in figure 1 may perhaps be better explained in popular 
terms. When milk is pasteurized at 145° F. for 30 minutes, the 
_ most of the bacteria (lactic-acid bacteria) left alive in it are of the 
_ kind which causes it to sour, and there are present only a few bac- 
_ teria (peptonizing) which cause it to rot. As the milk stands, the 
acid formers grow and cause the milk to sour instead of rot. When 
milk is pasteurized at 180° F. for 80 minutes, however, the bacteria 
(lactic-acid) which cause the souring of milk are practically all 
destroyed, and those which are alive (peptonizin=) continue to grow 
and cause the milk to rot. 
Since the general groups of bacteria which survive pasteurization. 
have been discussed, let us now consider a more specific group. It 
has been the custom of some authorities to consider the presence of 
streptococci in pasteurized milk an indication of an ineffective 
process. In a recent study (10) of the subject, however, it was 
