4o8 



Journal of Agricultural Research 



Vol. UI, No. s 



Table V. — Growth of Bacillus coli in milk heated for jo minutes at 62.8° C. {145° F.) 

 and held at 8° C. (46.4° F.) 



The relation of these results to commercial pasteurization can be 

 plainly seen. Milk is pasteurized usually at 62.8° C. (145° F.) for 30 

 minutes and in subsequent handling is kept at various temperatures 

 from low to high, depending on conditions, until it is consumed. In 

 view, therefore, of the results of our experiments, it is possible to explain 

 the presence of colon bacilli in pasteurized milk on the ground of their 

 ability to survive the process. 



These results, however, indicate that colon bacilli survive pasteuriza- 

 tion on account of the resistance of a few cells and not because the 

 cultures have a high majority thermal death point, in which case a large 

 number of cells would survive. Since it is apparent that colon bacilli 

 have a low majority thermal death point, we should not expect to find 

 large numbers of these bacteria Ln pasteurized milk immediately after 

 the heating process. If this condition is found we should believe from 

 our results that the presence of the bacilli would indicate inefficient 

 heating or a heavy reinfection. 



We must call attention to the fact that these opinions are based on a 

 study of 174 cultures of colon bacilli, and consequently, while they 

 represent a considerable number of strains of Bacillus coli, it is possible 

 that a study of still more cultures might yield different results. It is 

 not improbable that colon bacilli with a high majority thermal death 

 point do exist, and if such is the case large numbers might be found 

 immediately after pasteurization. 



SUMMARY AND CONCLUSIONS 



(i) The thermal death point of 174 cultures of colon bacilli isolated 

 from cow feces, milk and cream, human feces, flies, and cheese showed 

 considerable variation when the cultures were heated in milk for 30 

 minutes under conditions similar to pasteurization. 



At 60° C. (140° F.), the lowest pasteurizing temperature, 95 cultures, 

 or 54.59 per cent, survived ; at 62.8° C. (145° F.), the usual temperature 

 for pasteurizing, 12, or 6.89 per cent, survived. One culture was not 

 destroyed at 65.6° C. (150° F.) on the first heating, but in repeated experi- 

 ments it was always destroyed. 



