PRESENT STATUS OF THE PASTEURIZATION OF MILK 19 
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 percentage. 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 heated 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 percentage of the acid group. 
Occasionally a few of the alkali group may be found. 
At 190° and 200° F. the bacterial groups which survive are about 
the same in their relative sizes as at 180°. 
It is very evident that when the bacterial flora of pasteurized milk 
is under discussion the temperature of the process is of fundamental 
importance. In Figure 1 the proportions of the bacterial groups left 
in milk heated at different temperatures may be seen at a glance. It 
must be borne in mind, 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 influence considerably 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 142° 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 (peptonizing) 
which cause it to decompose. As the milk stands, the acid formers 
grow and cause the milk to sour instead of decompose. However, 
when milk is heated at 180° for 30 minutes, the bacteria (lactic-acid) 
which cause the souring of milk are practically all killed, 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 pasteuriza- 
tion, but some also grow at or near the pasteurizing temperature. 
These are known as thermophilic bacteria and according to Robertson 
(48) grow best between 55° and 75° C. (131° and 167° F.). Some- 
times upon long-continued heating at 140° to 145° F. for several 
hours, milk sours in the holding tanks because of 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 caused by some abnormal condition. 
Pasteurizing plants, however, frequently experience considerable 
trouble with thermophiles or bacteria causing " pin-point colonies" 
in the pasteurized milk. Yale and Breed (65, p. 1198) came to the 
following conclusion: 
Faulty plant operations proved to be more important factors in the develop- 
ment of thermophilic bacteria than the type of pasteurizing equipment. 
Hammer and Trout (32, p. 23) state: 
On plates poured with dairy products, especially those that had been subjected 
to heat, there were often yellow colonies of cocci. In general these organisms 
resisted the usual pasteurization exposures for market milk * * *. 
