IV 



BEHAVIOUR OF BACTERIA IN MILK 63 



and bacilli enclosed in it will be protected and subjected to a 

 lower temperature than that registered by the thermometer, and 

 so may escape being killed. The physical condition of the 

 bacilli themselves is also an important factor which must be 

 taken into consideration. If the bacilli are agglutinated into 

 clumps or entangled in masses of manure or debris, those in 

 the centre will be subjected to a lower temperature than that 

 recorded or intended to be applied, and may escape destruction. 



These considerations and possible fallacies show that the 

 problem is not as simple as it appears, and that in the deter- 

 mination of thermal death -points it is very important to 

 employ bacteria in their greatest state of resistance, and under 

 conditions which allow all modifying factors to be taken into 

 consideration. Further, in deductions for practical work it is 

 advisable to allow a margin of safety sufficient to exclude errors 

 from these sources, and to make the destruction of pathogenic 

 bacteria certain. 



Since the pathogenic bacilli differ in resistance the results 

 obtained must be considered separately for each organism. 



B. typhosus. The results of the numerous experiments 

 which have been carried out by different workers are closely 

 in agreement, and show that typhoid bacilli suspended in 

 milk are, as a rule, killed when the milk is heated to 60 C., 

 provided the heating up takes several minutes, while their 

 destruction is ensured if the milk is maintained at that 

 temperature for at least 5 minutes. 



As examples of recent investigations the following may be 

 mentioned. Bassenge 1 found that when milk was heated to 

 60 C. in from 5 to 10 minutes the bacilli were killed, but 

 not all were killed if the time for heating was only 3 

 minutes. 



Kolle, Kutscher, Meinicke, and Friedel 2 found that B. 

 typhosus, B. paratyphosus, and B. enteritidis had the same re- 

 sistance to heat, and were all killed when the milk was heated 

 to 59 C. if 10 minutes was taken to heat the milk to that 

 temperature. 



Rosenau 3 carried out experiments with nine different 



1 Deutsche med. Wochenschr., 1903, xxix. p. 264. 



2 Klin. Jahrb., 1904, xiii. p. 324. 

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



