PASTEURIZATION 205 



also sometimes transmitted by milk, has not been iden- 

 tified. Of the organisms mentioned, the tubercle bacillus 

 is the most resistant to heat, with the possible exception 

 of some varieties of streptococci. The streptococci of 

 septic sore throat are destroyed by heating at 140 F. 

 (60 C.) for ao minutes (Davis) or at 145 F. (62.8 

 C.) for 20 minutes (Hamburger), and exposure to a 

 temperature of 125.6 to 143.6 F. (52 to 62 C.) for 

 30 minutes is sufficient to kill Streptococcus pyogenes 

 (Kitchens). There are some varieties of streptococci 

 which are more resistant to heat, but we have no reason 

 to believe that those which occur in milk are pathogenic. 

 It seems very probable that heating milk sufficiently to 

 destroy tubercle bacilli will also kill any pathogenic strep- 

 tococci which may be present. A temperature of 140 F. 

 (60 C.) for 2 minutes will destroy the bacilli of typhoid 

 fever and diphtheria. The question of the amount of 

 heat required to destroy the pathogenic organisms which 

 occur most frequently in milk consequently resolves it- 

 self into a question of how much heat is necessary to kill 

 the tubercle bacillus. The evidence on this point must 

 therefore be considered. 



Bang found that heating milk momentarily at 185 

 F. (85 C.) destroyed tubercle bacilli in naturally in- 

 fected milk, and Jensen reports experiments in which 

 tubercle bacilli in milk were killed by a few moments 

 exposure to 158 to 176 F. (70 to 80 C.) . On the other 

 hand, Grimmer and 6ther investigators are of the opinion 

 that, when the exposure is momentary, a temperature of 

 at least 194 ( F. (90 C.) is necessary to insure the de- 

 struction of the bacilli in all cases. 



Concerning the effects of a more prolonged exposure 

 at lower temperatures, experimental results are even 



