Thermal Death Point of Bacteria. 197 



of the heat by the formation of a surface scum and by coagulation. 

 De Jong concludes from his experiments (bottle sterilization) : 



1. That heating for a half hour at 71-72 deg. C. is not always 

 sufficient to destroy the tubercle bacilli mixed with the milk. 



2. That heating even to a higher degree does not always give 

 satisfactory results since the resisting power of the tubercle 

 bacillus varies. 



3. The designation "free from disease-bacteria" for pas- 

 teurized milk is false. 



4. Those who desire milk free from tubercle bacilli must 

 purchase sterilized milk, provided it is not obtained from herds 

 free from tuberculosis. Bang, de Mann and Forster obtained 

 evidence that heating to 85 deg. C., for three minutes destroys the 

 tubercle bacilli, a fact which has also been confirmed by the work 

 of Weigmann and by the experiments of Tjaden, Koske and Her- 

 tel under conditions which prevail in large distributing plants with 

 milk from tuberculous udders. Other non-spore-containing dis- 

 ease-producers in milk are also destroyed at this temperature pro- 

 vided that certain conditions do not prevent proper heating. 



[According to the experiments of Rosenau, it is evident that 

 the tubercle bacillus in milk loses its infective properties for guinea 

 pigs when heated to 60 C. and maintained at that temperature 

 for 20 minutes or to 65 C. for a much shorter time. It should be 

 remembered that the milk in the tests of Rosenau was very heavily 

 infected with virulent cultures, which was indicated by the prompt 

 deaths of the control animals. Milk would practically never con- 

 tain such an enormous amount of infection under natural condi- 

 tions. It is therefore justifiable to assume that if 60 C. for twenty 

 minutes is sufficient to destroy the infectiveness of such milk when 

 injected into the peritoneal cavity of a guinea pig, any ordinary 

 market milk after such treatment would be safe for human use by 

 the mouth as far as tubercle bacilli are concerned. These results 

 are substantiated by the findings of Versin, Bonhoff, Th. Smith, 

 Schroeder, Russell and Hastings and Hesse. 



Relative to the thermal death point of other organisms Rose- 

 nau found that typhoid bacilli are killed in milk when heated to 

 60 deg. C. and maintained at that temperature for two minutes. 

 The great majority of these organisms are killed by the time the 

 temperature reaches 59 C. and few survive to 60 C. 



The diphtheria bacillus succumbs at comparatively low tem- 

 peratures. Oftentimes it fails to grow after heating to 55 C. 

 Some occasionally survive until the milk reaches 60 C. 



The cholera vibrio is similar to the diphtheria bacillus so far 

 as its thermal death point is concerned. It is usually destroyed 

 when the milk reaches 55 C., only once did it survive to 60 C. 

 under the conditions of the experiments. 



The dysentery bacillus is somewhat more resistant to heat 

 than the typhoid bacillus. It sometimes withstands heating at 



