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. 



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. 



