18 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 



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 pas- 

 teurized 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 production of 

 milk may considerably influence the bacterial group relations 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, most 

 of the bacteria . (lactic-acid) left alive in it are of the kind which 

 cause it to sour, and there are present only a few bacteria (pep- 

 tonizing) which cause it to decompose. As the milk stands, the acid 

 formers grow and cause the milk to sour instead of decompose. 

 When milk is pasteurized at 180 F. for 30 minutes, however, the bac- 

 teria (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 decompose. 



Not only do certain types of lactic- acid bacteria survive pas- 

 teurization but some also grow at the pasteurizing temperature. 

 Sometimes upon long-continued heating at 140 to 145 F. for 

 several hours, milk sours in the holding tanks due to the growth 

 of these organisms. The ordinary period of holding does not provide 

 sufficient time for their development, so this type of souring is not 

 encountered in milk plants except when there is an interruption 

 in the pasteurizing process due to some abnormal condition. 



SURVIVAL OF STREPTOCOCCI. 



Since the general groups of bacteria which survive pasteuriza- 

 tion 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. As already pointed out, pathogenic streptococci are readily 



