68 MILK AND THE PUBLIC HEALTH CHAP. 



In a later report 1 they investigated the action of short 

 exposures at a higher temperature upon tubercle bacilli, this 

 being the actual conditions met with in milk pasteurised in 

 apparatus of the continuous action type. 



They carried out a large number of experiments with milk 

 heated to 160 F., 175 F., 185 F., with exposure times 

 varying from 1 to 10 minutes. Exposure for 1 minute or 

 longer at these temperatures invariably resulted in the death 

 of the tubercle bacilli as shown by failure to set up tuber- 

 culosis in inoculated animals. These results were obtained under 

 laboratory conditions, the milk being infected with tubercle 

 bacilli cultures of bovine source and exposed to the required 

 temperature in sealed glass tubes placed in a water bath. It 

 took three-quarters of a minute for the milk to reach the tem- 

 perature of the outside water. The authors consider that their 

 results approximate sufficiently to the conditions that obtain 

 in practice where continuous flow pasteurisers are used, to 

 be available for deductions for practical work. 



Eosenau 2 carried out numerous experiments, using five 

 different strains of tubercle bacilli, all of bovine origin. 

 Young cultures of tubercle bacilli in fine emulsion in milk 

 were used. All the tests were made by placing test tubes 

 containing the unwarmed infected milk in a water bath con- 

 taining water at the desired temperature (60 or 65 C.). 

 The test tubes in which the infected milk was heated were 

 open to the air, and scum formation was disregarded in all 

 instances, the object being to determine the thermal death -point 

 under natural conditions so that the results might be applied 

 with confidence to practical pasteurisation. The guinea-pig 

 test was used to ascertain the viability of the tubercle bacilli. 



A large number of very careful experiments arranged in 

 9 series were carried out. The temperature of exposure was 

 always 60 or 65 C., and the results are briefly summarised 

 in the following table : 



1 Wisconsin Agricultural Experiment Station, 21st Annual Report, 1904, 

 p. 178. 



2 Bulletin No. 42, Public Health and Marine Hospital Service, U.S.A., 1908. 



