STATUS OF PASTEURIZATION OF MILK. 19 
destroyed by proper pasteurization. In a study of the subject (4), 
however, it was found that certain strains of streptococci are able 
to survive pasteurizing 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 conditions similar to pasteurization. At 140° F., the lowest 
pasteurizing temperature, 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, survived ; 
all these were destroyed at 165° F. The streptococci from the udder, 
on the whole, were less resistant 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) 
Streptococci which have a low majority thermal 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 tem- 
perature. This ability of a few bacteria may be due to certain 
resistant characteristics peculiar to them or it may be caused by 
some protective influence in the milk. (2) Streptococci which have 
a high majority thermal death point, and which, when such is the 
case, survive because this point is above the temperature of pas- 
teurization. This ability to resist destruction by heating is a per- 
manent 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 occasional high counts found in pasteurized milk. Such 
counts are sometimes observed when the count of the raw milk runs 
the same as usual. As the proportion of these heat-resistant types 
vary in milk their numbers may at times reach such figures that 
their survival of the pasteurizing process gives an abnormally high- 
count product. The presence and variation of their numbers in milk 
therefore is a matter which must be given consideration in connec- 
tion with bacteria standards for pasteurized milk. 1 
It is evident that certain varieties of streptococci are able to survive 
pasteurization, while others are probably always destroyed. Numer- 
ous investigators 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 pasteuriza- 
tion 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. 
