Pasteurization of INLilk 



21 



heating. Not until recently, however, 

 has science been in the position to ap- 

 preciate the value of the results of 

 such tests when made on a routine 

 basis. The question of the relative 

 sn.sceptibility to heat of the various 

 members of the B. Coli group and also 

 the relative frequency of them in milk 

 have been answered in part. This is 

 also true of the members of the groups 

 of streptococci. Extensive laboratory 

 and some practical studies had been 

 made by numerous workers on the 

 thermal death points, in the presence 

 of milk, of the various pathogenic 

 types of bacteria but the opportunities 

 for making tests with truly pathogenic 

 bacteria in the raw product and the 

 numbers there present known hAve 

 been extremely few. 



B. Coli as an Index of EiSciency. 

 In one series of tests made under the 

 direction of two members of the Com- 

 mittee, Dr. H. D. Pease and ^Ir. S. M. 

 Heulings, strictly pathogenic types, B. 

 typhosus (Hopkins) ; B. diphtherae ; 

 B. tuberculosis (both bovine and 

 human varieties), were added in large 

 numbers to volumes of milk possible 

 of practical treatment in an apparatus 

 which had been for several years in 

 active commercial use in a fair-sized 

 plant. Tests were applied for their 

 presence in samples collected at all 

 important stages in the process. Upon 

 all such samples, quantitative fermen- 

 tation tube tests were also made for 

 the presence or absence of those forms 

 of B. Coli which are commonly present 

 in m.ilk. It was possible, therefore, h: 

 this series of tests to make rather di- 

 rect comparisons between the suscep- 

 tibilities of each of the pathogenic 

 types named and of the milk B. Coli 

 forms, under the same conditions of 

 temperature and time period of expos- 

 ure. The results of these investiga- 

 tions are of special value as they offer 

 strong support of the use of quantita- 

 tive tests for the presence or absence 



of B. Coli in the heated product, as a 

 means of indicating pasteurization effi- 

 ciency. It can be stated that in no 

 case were any pathogenic bacteria 

 found in samples of the heated and 

 held milks when the latter did not con- 

 tain also the presence of substantial 

 numbers of living B. Coli and in many 

 of the samples in which B. Coli were 

 found present in small numbers no 

 pathogenic bacteria were detected by 

 the most careful searching. Some of 

 the pathogenic types and some of the 

 B. Coli were found living after heating 

 from 140 to 141 degrees F. and holding 

 for 15 minutes, but none of the former 

 were alive after 30 minutes, while B. 

 Coli were still present in 1 c. c. tests, 

 but were not found after 45 or 60 min- 

 utes holding at these temperatures nor 

 with 30 minutes holdings at higher 

 temperatures. In short, the greater 

 tenacity of life of the B. Coli in 

 the medium time periods of exposure 

 to the medium temperatures was clear- 

 ly indicated. 



The conclusion is certainly war- 

 ranted that the B. Coli forms com- 

 monl)- found in raw milk are at least 

 not more susceptible to the usual tem- 

 perature and time exposure conditions 

 of pasteurization than are B. typhosus 

 and B. diphtherise. In the case of B. 

 tuberculosis, the comparison must be 

 made between the effect of the heating- 

 process upon its power of infection of 

 guinea pigs as compared with the 

 growth power of the B. Coli in lactose 

 bile tubes. 



Since the time of these investiga- 

 tions (1911-1912) much work has been 

 done by Rogers and by Ayers and 

 their respective associates, on the 

 means of separation of the B. Coli 

 group into the two types of true B. 

 coli and B. aerogenes and of the rela- 

 tive frequency of them in cow manure 

 and cattle fodder as well as in milk. 



Ayers and his associates have shown 

 the relative occurrence of these two 



