288 MILK AND ITS HYGIENIC RELATIONS 



flora of comparatively harmless organisms, which have gained access 

 to the milk in the process of its manipulation. 



The Thermal Death-point of Tubercle Bacilli. Much work has been 

 done upon the temperature to which milk must be raised in order 

 to destroy tubercle bacilli when they are present, and considerable 

 diversity of results has been obtained. 



Rosenau has summarised the results of investigations of 

 numerous observers, and the table on p. 289 is taken from his work. 1 



It seems hardly possible, from a consideration of the results 

 shown in that table, to arrve at a definite temperature at which 

 tubercle bacilli are killed, but it appears reasonable to suppose that 

 a temperature of 60 C. for twenty minutes may be sufficient, or a 

 short period at 95-100 C. 



Delepine in the course of his investigations found that tubercle 

 bacilli were still living in milk which had been dried by passing 

 over cylinders heated to 138-140 C. He points out, however, 

 that this temperature is not reached by the milk as it passes between 

 the rollers, and he ascertained by experiment, that the greater 

 part of the milk must have remained at a temperature below 100 C., 

 and he says that in all probability the maximum temperature reached 

 by every part of the milk did not exceed 96 C., and that the dura- 

 tion of the exposure to that temperature probably never exceeded 

 3-3 seconds. 



The Thermal Death-point of Streptococci and other Organisms. 

 Ayers and Johnson (i) investigated the temperature necessary to 

 kill various strains of streptococci. They found that the strepto- 

 cocci normally occurring in milk showed a greater resistance to 

 temperature than those obtained from the udder or from the mouth 

 or faeces of the cow. After examining the death-point of 139 strains 

 they found that 



89 or 64-03 per cent, survived 30 minutes heating at 140 R, 



46 or 33-07 145 R, 



2*58 i6oR, 



while all were killed at a temperature of 165 F. for the same period. 



Rosenow found that the virulent streptococci obtained from 

 an epidemic of sore throats were killed by a temperature of 45 C. 

 or approximately 113 F. The non-virulent ones, however, survived 

 a temperature of 60 C. or 140 F. His experiments were carried 

 out in milk. 



The thermal death-points of numerous other pathogenic 

 organisms, e.g. B. typhosus, B. diphtherae, B. cholorae, B. dysen- 

 terae, and Micrococcus melitensis, were investigated by Rosenau 

 with a view to determining the temperature to which milk must 

 be raised in order to render it safe. The general conclusion reached 

 by him was that ' milk heated to 60 C. and maintained at that 



1 For reference to the authors quoted in this table, see Rosenau's paper. 



