380 MILK AND THE PUBLIC HEALTH CHAP. 



that chapter, there is not complete agreement as to these 

 thermal death-points it is better to err on the safe side, and 

 for practical working higher temperatures are recommended. 

 Temperatures of 7*7 C. (for continuous-flow pasteurisers) and 

 65 Q C. for 20 minutes are probably safe temperatures. 



The Danish law (Appendix III. p. 430) selects 80 C. as 

 the temperature to which milk must be heated. The Chicago 

 regulations enact " milk from cows reacting to tuberculin 

 shall be rejected unless it shall have first been pasteurised at 

 a temperature of 175 F. or over for 30 seconds or longer in 

 a stream not more than a quarter of an inch thick." 



Pasteurisation is efficient to kill pathogenic organisms, 

 will vastly reduce the bacterial content, and within limits will 

 increase the keeping qualities of milk. On the other hand, 

 there are a number of weighty objections to its use as a 

 routine procedure which must be considered. 



(1) While pasteurisation destroys the majority of the 

 bacteria present we have no certain evidence that it destroys 

 their toxins. The process is but rarely carried out on the / 

 farm, so that, in general, the bacteria will have had many 

 hours in which to produce their toxins. Some toxins, such as 

 those of the meat -poisoning bacilli, are very heat-resistant, 

 and although the toxins likely to be present in milk may be 

 killed at the temperature of pasteurisation, we have no certain 

 evidence upon this point. We do not know whether the 

 toxins produced in milk are prejudicial or not, but it is reason- 

 able, in the absence of proof to the contrary, to assume that 

 some of them may be harmful. 



(2) Pasteurisation does not kill all the bacteria in milk 

 but only most of them. By killing the lactic acid bacilli, 

 which are themselves harmless and which have a considerable 

 restraining influence upon the growth of any harmful bacilli 

 which may be present, it has been suggested that such milk is 

 likely to develop decidedly toxic properties when kept. 



This view arose out of Fliigge's work already mentioned 

 (p. 373), work which, however, cannot be altogether accepted 

 at the present day. If all the lactic acid bacilli are destroyed 

 it is probable that the spore-bearing forms will more rapidly 

 develop and produce their products, but it is not clear that 

 such products are toxic. 



