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 elance. 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 1s 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 
é 
