BEHAVIOUR OF BACTERIA IN MILK 67 



carried out at a temperature of 60° C, and only 2 to 5 minutes 

 was taken to heat up tlie milk to this temperature. The 

 viability of the tubercle bacilli was ascertained by intra- 

 peritoneal injection into guinea-pigs. In their first series they 

 carried out 30 experiments. In all the cases in which the 

 milk was kept at 60° C. for 10, 15, 20, 30, or 45 minutes, 

 no tuberculosis in the guinea-pigs occurred, but when the ex- 

 posure was for only 5 minutes tuberculosis resulted in all the 

 experiments. They concluded that a temperature of 60° C.for 

 10 minutes is sufficient to effectively destroy the vitality of 

 tubercle bacilli in milk. 



These series of experiments were carried out under con- 

 ditions in which the formation of a surface scum did not 

 readily occur. Two further series of experiments were made 

 to compare these results with those obtained by heating the 

 milk in an open vessel and without stirring. The results 

 summarised in the following table were obtained : 



— = No tuberculosis ; + = tuberculosis. Each symbol represents a. 

 separate experiment. 



Their results show clearly that when the milk was exposed 

 in a quiescent condition, as in a glass tube or bottle, the 

 tubercle bacilli were more resistant than when the milk was 

 in a closed receiver and agitated. The cause of the differ- 

 ence is probably to be ascribed to the formation of a surface 

 scum. 



The authors recommend, " in order to thoroughly pasteurise 

 milk so as to destroy any tubercle bacilli which it may contain, 

 without in any way injuring its creaming properties or con- 

 sistency, to heat the same in closed pasteurisers for a period 

 of not less than 20 minutes at 140° F. (60° C.)." 



