252 PATHOGENIC BACTERIA IN MILK 



organism is able to resist the action of heat at 140° F. (60^ C.) for 

 considerably longer periods of time. 



3. Efficient pasteurisation can be more readily accomplished in a 

 closed receptacle, such as is most frequently used in the commercial 

 treatment of milk, than where the milk is heated in open bottles or 

 open vats. 



4. It is recommended in order thoroughly to pasteurise milk so 

 as to destroy any tubercle bacilli which it may contain, without in 

 any way injuring its creaming properties or consistency, to heat 

 the same in closed pasteurisers for a period of not less than twenty 

 minutes at 140° F. (60° C). 



Under these conditions one may be certain that disease 

 bacteria, such as the tubercle bacillus, will be destroyed without the 

 milk or cream being injured in any way. For over a year this 

 new standard has been in constant use in the University of Wis- 

 consin Creamery, and the results, from a purely practical point of 

 view, reported last year by Farrington and Russell,^ have been 

 abundantly confirmed. 



Whilst these results seem definite and clear, it should not be 

 forgotten that other recent workers have failed to obtain such 

 satisfactory results, and many authorities still hold that, the only 

 practicable standard upon which absolute reliance can be placed is 

 to raise the milk to the boiling point. 



(ii.) Low temperatures. — Low temperatures per se appear to 

 have little or no effect upon the bacillus tuberculosis. Cadeac and 

 Malet kept portions of a tuberculous lung in a frozen condition for 

 four months, and found that, at the end of this time, the substance 

 was still virulent. Experiments carried out by Swithinbank in the 

 year 1900^ are of interest upon this point {see Plate 23). Virulent 

 cultures of the organism were exposed to the temperature of liquid 

 air (— i93°C.)for continuous periods varying from six hours to 

 forty-two days, with the result that there was no apparent modifi- 

 cation of either vitality or virulence. It was shown, however, that 

 successive alternations of extreme cold and normal temperatures 

 had a decidedly destructive effect. 



(iii.) Light. — Pure cultures of the tubercle bacillus are very 

 susceptible to the effect of direct sunlight, a comparatively short 

 exposure being sufficient to destroy their virulence (Koch, Ransome, 



* Sixteenth Annual Report of the Wisconsin Agricultural Expt. Sta., 1899, 

 p. 129. 



^ Proc. of Royal Society, 1901, p. 498, and Trans. Brit. Cong, on Tuberculosis, 

 1901, vol. iii., p. 657. 



