512 MILK 



germs in milk. This must be considered the ultimate object of 

 modern methods of pasteurization. 



The thermal death-point of those pathogenic organisms which 

 have been known to infect milk-supphes serves as a basis for cal- 

 culating pasteurization temperatures. These bacteria are the 

 bacilli of tuberculosis, typhoid fever, dysentery, diphtheria, 

 Malta fever, the spirillum of Asiatic cholera, and the unknown 

 organisms of scarlet fever and foot-and-mouth disease. Except 

 Bacillus tuberculosis, all these micro-organisms are destroyed by 

 exposure to 60° C. for ten minutes. In fact, the great majority 

 of cells are killed before 60° C. is actually reached. 



The thermal death-point of the tubercle bacillus has led to 

 contradictory results. According to some reports the bacilli were 

 not killed by one or even two boiUngs, while others indicated 

 that they were destroyed at much lower temperatures. Some 

 conclusions were based on applying the specific stain for tubercle 

 bacilli to the milk after heating. This method may lead to er- 

 roneous results, since staining does not distinguish tubercle from 

 acid-proof bacilli commonly found in milk, and does not distin- 

 guish between living and dead bacilli. The criterion adopted 

 by later investigators is the production of tuberculous lesions in 

 guinea-pigs. Even this method, as pointed out by Rosenau, 

 leads to reliable results only if doubtful lesions are transferred to 

 another guinea-pig, since dead tubercle bacilli sometimes form 

 lesions closely resembling those produced by hving bacilli. Theo- 

 bald Smith has called attention to another soiu-ce of error. When 

 milk is heated a membrane forms on the surface, which in con- 

 tact with the air cools rapidly and protects bacteria from destruc- 

 tion. Smith found that when this source of error is ehminated 

 tubercle bacilli are destroyed by twenty minutes' exposure to 

 60° C, if infection of guinea-pigs is taken as the index. His 

 work has been confirmed by Russell and Hastings, Rosenau, and 

 others. Tubercle bacilli are evidently somewhat more resistant 

 to heat than other non-sporing baciUi, and for that reason their 

 thermal death-point should be taken as a basis for milk pasteuriza- 

 tion. As a general rule it may be stated that the higher the tem- 

 perature, the shorter the time necessary for destruction of bacteria. 



The effect of pasteurization on mold spores in milk has been 

 investigated by Thom and Ayers, who state that in milk heated 

 to 145° F. (62.8° C.) for thirty minutes the conidia of every species 

 investigated were killed except those of Aspergillus repens, A. 

 flavus, and A. fumigatus. The molds which survived are rarely 

 found in milk. In milk heated to 165° F. (73.9° C.) for thirty 

 seconds the spores of all molds tested were destroyed, except many 

 spores of one form and occasional spores of three forms. In 



