STATUS OF PASTEURIZATION OF MILK 
13 
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 proc- 
ess due to some abnormal condition. 
STKYIYAL OF STBEPTOCOCCI 
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 Pas- 
teurized milk an indication of an in- 
effective process. As already pointed 
out, pathogenic streptococci are read- 
ily destroyed by proper Pasteurization. 
In a study of the subject (4), however, 
it was found that certain strains of 
streptococci are able to survive Pas- 
teurizing temperatures. 
The thermal death points of 139 
cultures of streptococci isolated from 
cow feces, from the udder and mouth, 
and from milk and cream, showed a 
wide variation when the milk was 
heated for 30 minutes under condi- 
tions similar to Pasteurization. At 
140° F., the lowest Pasteurizing tem- 
perature, 89 cultures, or 64.03 per 
cent, survived ; at 145° F.. the usual 
temperature for Pasteurizing, 46, or 
33.07 per cent, survived ; and at 160° 
F.. 3 cultures, or 2.16 per cent, sur- 
vived : all these were destroyed at 
165° F. The streptococci from the 
udder, on the whole, were less re- 
sistant and those from milk and cream 
more resistant to heat than those from 
the mouths and feces of the cows. 
Two classes of streptococci seem to 
survive Pasteurization : ( 1 ) Strepto- 
cocci which have a low majority ther- 
mal death point (the temperature at 
which a majority of the bacteria are 
killed), but among which a few cells 
are able to survive the Pasteurizing 
temperature. This ability of a few 
bacteria may be due to certain re- 
sistant characteristics peculiar to them 
or it may be caused by some protec- 
tive influence in the milk. (2) Strep- 
tococci which have a high majority 
thermal death point, and which, when 
such is the case, survive because this 
point is above the temperature of 
Pasteurization. This ability to resist 
destruction by heating is a permanent 
characteristic of certain strains of 
streptococci. 
These streptococci which have a 
high thermal death point above the 
Pasteurizing temperature undoubtedly 
play an important part in the occa- 
sional high counts found in Pasteur- 
ized milk. Such counts are some- 
times observed when the count of the 
raw milk runs the same as usual. As 
the proportion of these heat-resistant 
types varies in milk their numbers may 
at times reach such figures that their 
survival of the Pasteurizing process 
gives an abnormally high-count prod- 
uct. The presence and variation of 
their numbers in milk, therefore, are 
matters which must be given consider- 
ation in connection with bacteria 
standards for Pasteurized milk. 
It is evident that certain varieties 
of streptococci are able to survive 
Pasteurization, while others are prob- 
ably always destroyed. Numerous in- 
vestigators have studied the thermal 
death point of streptococci isolated 
from patients having septic sore throat 
and have found that the organism 
was destroyed by Pasteurization at 
145° for 20 minutes. These results, 
together with the protection which 
proper Pasteurization seems to afford 
against epidemics of that disease 
caused by milk supplies, indicate that 
the varieties of streptococci associated 
with or responsible for the disease are 
among the varieties which have a low 
thermal death point. 
THE COLON TEST FOE EFFI- 
CIENCY OF PASTEURIZATION 
In a study (6) of the ability of colon 
bacilli to survive Pasteurization it was 
found that certain strains could sur- 
vive Pasteurization at 145° F. for 30 
minutes. On examining 174 cultures 
of colon bacilli it was found that at 
140° F., the lowest Pasteurizing tem- 
perature, 95 cultures survived ; at 145° 
F., the usual temperature for Pasteur- 
ization, 12 survived. In each case the 
heating period was 30 minutes. Con- 
siderable variation was observed in 
the thermal death point of the colon 
bacilli which survived at 145° F. 
When the cultures which withstood 
the first heating were again heated it 
was found that many did not survive, 
and in each subsequent heating dif- 
ferent results were obtained. Colon 
bacilli have a low majority thermal 
death point but on account of the re- 
sistance of a few cells, they may sur- 
vive the Pasteurizing process. 
The colon test as an index of the 
efficiency of the process of Pasteuriza- 
tion is complicated by the ability of 
