276 CITY MILK SUPPLY 



experiments it is manifest that 170F. is a critical temperature in pas- 

 teurizing, for in milk pasteurized above this temperature there is a tend- 

 ency for the peptonizing bacteria to predominate. Ayres concludes 

 that when the bacterial flora of pasteurized milk is under discussion the 

 temperature of the process is of fundamental importance and he cautions 

 that these summaries represent average conditions and are subject to 

 exceptions. 



Ability of B. coli to Survive Pasteurization. Certain boards of health 

 have taken the presence of more than a few colon bacteria in pasteurized 

 milk as evidence that the pasteurization has not been efficient or that 

 subsequent thereto the milk has been contaminated. The attempt to 

 establish this criterion for pasteurized milk evoked a storm of protest and 

 led to investigations of the ability of B. coli to survive pasteurization. 

 Ayres and Johnson conducted a series of experiments to determine the 

 question and reached the following conclusions: 



" 1. The thermal death point of 174 cultures of colon bacilli isolated from cow 

 feces, milk and cream, human feces, flies and cheese, showed considerable varia- 

 tion when the cultures were heated in milk for 30 min. under pasteurizing con- 

 ditions. 



"At 140F., the lowest pasteurizing temperature, 95 cultures, or 54.59 per 

 cent., survived; at 145F., the usual pasteurizing temperature 12, or 6.89 per cent., 

 survived. 



"One culture was not destroyed at 150F. on the first heating, but thereafter, 

 in repeated experiments, always was. 



"2. There is a marked difference in the effect of heating at 140F. Although 

 there is a difference of but 5, 87.3 per cent, of the cultures which survived at 

 140F. were destroyed at 145F. 



"3. Considerable variation was found in the thermal death point of the colon 

 bacilli that survived 145F. When the 12 cultures that survived were heated 

 again at the same temperature, many did not survive and different results were 

 obtained at each repeated heating. 



"145F. for 20 min. seems a critical temperature for colon bacilli. 



"4. All of the 174 cultures studied had a low majority thermal death point 

 but were able to survive pasteurizing temperatures on account of the survival 

 of a few cells. 



"5. The colon test as an index of the efficiency of pasteurization is compli- 

 cated by the ability of certain strains to survive a temperature of 145F. for 30 

 min. and to develop rapidly when the pasteurized milk is held under temperature 

 conditions that might be met during storage and delivery. 



"The presence of a large number of colon bacilli immediately after the heat- 

 ing process may indicate improper treatment of the milk. 



"6. If milk is pasteurized at 150F. or above for 30 min., the results indicate 

 that no colon bacilli would survive. Under such conditions the colon test may 

 be of value in determining the efficiency of pasteurization. However, further 

 research may discover strains of colon bacilli that will survive this temperature 

 and even higher ones." 



