Preservation of Milk. 115 



necessarily higher. The organism that is invested with most 

 interest in this connection is the tubercle bacillus. On ac- 

 count of its more or less frequent occurrence in milk (see 

 p. 87) and its reputed high powers of resistance, it may 

 well be taken as a standard in pasteurizing. 



Thermal death limits of tubercle bacillus. Concerning 

 the exact temperature at which this germ is destroyed there 

 is considerable difference of opinion. Part of this arises 

 from the inherent difficulty in determining exactly when 

 the organism is killed (due to its failure to grow readily on 

 artificial media), and part from the lack of uniform condi- 

 tions of exposure. The standards that previously have 

 been most generally accepted are those of De Man, 1 who 

 found that thirty minutes exposure at 149 F., fifteen min- 

 utes at 155 F., or ten minutes at 167 F., sufficed to de- 

 stroy this germ. 



More recently it has been conclusively proven, 2 and these 

 results confirmed by different investigators, 3 that if tuber- 

 culous milk is heated in closed receptacles where the 

 scalded surface pellicle does not form, the vitality of this 

 disease germ is destroyed at 140 F. in a brief period (15 to 20 

 minutes). If the conditions of heating are such that the 

 surface of the milk is exposed to the air, the resistance of 

 bacteria is greatly increased. When heated in open vessels 

 Smith found that the tubercle organism was not killed in 

 some cases where the exposure was made for at least an 

 hour. Russell and Hastings 4 have shown an instance where 

 the thermal death-point of a micrococcus isolated from 

 pasteurized milk was increased 12.5 F., by heating it under 



De Man, Arch. f. Hyg., 1893, 18:133. 



a Th. Smith, Journ. of Expt. Med., 1899, 4: 217. 



Russell and Hastings, 17 Kept. Wis. Expt. Stat., 1900, p. 147. 



4 Russell and Hastings, 18 Kept. Ibid., 1901. 



