30 BULLETIN 739, U. S. DEPARTMENT OF AGRICULTURE. 



The results of the first experiments carried on in connection with 

 this point are shown in sections 2 and 3 of Table 15, where it will be 

 noted that the average percentage of the B. aerogenes type was con- 

 siderably higher in milk produced under clean conditions and han- 

 dled in sterilized utensils than the percentage found in milk pro- 

 duced under dirty conditions and handled in unsterilized utensils, 

 for any given temperature and time. The average percentage of the 

 B. aerogenes type was consistently higher in milk produced under 

 clean conditions when stored at 60° F. (15.6° C.) and 70° F. (21.1° C.) 

 up to a period of 48 hours. Further experiments with milk produced 

 under the same two sets of conditions, but held at 86° F. (30° C.) 

 showed a similar high proportion of B. aerogenes in the milk produced 

 under clean conditions, although the actual percentage of the 

 B. aerogenes in the milk produced under dirty conditions was also 

 somewhat higher than when held at the lower temperatures. These 

 results are shown in sections 4 and 5 of Table 15. 



Further analysis of the results shows that among the samples pro- 

 duced under a given condition there was a wide variation in the pro- 

 portion of the B. coli to the B. aerogenes types. Further than this, 

 the proportion of the two types after 24 hours' incubation was often 

 entirely reversed after 48 hours at the same temperature. Consid- 

 ering the higher average percentage for the B. aerogenes type in milk 

 produced under clean conditions than under dirty conditions, it 

 seemed that this might be a significant fact in tracing the conditions 

 of production. 



The results obtained, however, from a few samples of certified milk 

 as shown in section 1 of Table 15, seemed to indicate that no hard 

 and fast rule can be attached to the proportion of the two types. In 

 view of the earlier findings with milk produced under clean condi- 

 tions it was thought that certified milk would, upon holding at tem- 

 peratures above 50° F. (10° C), show a high percentage of the 

 B. aerogenes type, but the result shows that this was not the case. 



Based on these results it seems evident that both types of organ- 

 isms of the colon-aerogenes group are always present in raw milk, 

 but the differentiation into the B. coli and the B. aerogenes types is 

 of little or no value in indicating the conditions under which the milk 

 was produced or the age and temperature at which it has been held. 



The conclusions reached in regard to the value of differentiating 

 the B. coli and the B. aerogenes types are based on a study of rela- 

 tively few samples. While the results obtained indicate that the 

 proportion of B. coli to B. aerogenes types is of little value in deter- 

 mining the history of the milk, it is believed that this point deserves 

 much further study before any more definite conclusion can be 

 reached. 



