PRESENT STATUS OF PASTEURIZATION OF MILK. 11 



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 100° F. 

 the total-acid group is still the largest, but the acid-coagulating group 

 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° F. 

 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 

 is 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 30 minutes, however, the bacteria 

 (lactic-acid) which cause the souring of milk are practically all 

 destroyed, and those which are alive (peptonizing) 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 



