PREPARATION OF BACILLUS ACIDOPHILUS MILK 113 



Experiments are in progress to determine the influence of temperature 

 on coagulation time. Most rapid growth and acid production take 

 place at 35 to 37° C. Very satisfactory results are obtained also at 

 30°, though in a somewhat shorter period. At 20° slow coagulation 

 occurs, while below this temperature little or no change takes place in 

 the appearance of the milk during the first three or four days. It 

 appears from these experiments as if incubation at 30° may be substi- 

 tuted for the 37° temperature, providing the organisms are quite viable 

 in milk, as in the present instance. 



Experiments are under way also to determine whether sterilization of 

 the milk, prior to the inoculation, is absolutely necessary. Instead of 

 heating the milk in the autoclave it is heated in a double boiler or in a 

 flask deeply submerged in boiling water for forty-five minutes to one 

 hour. This treatment does not destroy spores and hence the milk is 

 not rendered sterile. Inoculation with B. acidophilus was followed by 

 incubation of duplicate samples at different temperatures, 20°, 30° 

 and 37° C. Control flasks, that is flasks which were heated in the same 

 way as the others, were incubated without being inoculated with B. 

 acidophilus, and incubated beside the others. All of the flasks were 

 examined at twelve hour intervals when whey agar plates were poured 

 and Veillon tubes were inoculated from the different flasks. 



Within twenty-four hours the milk flasks that had received the B. 

 acidophilus inoculum and were incubated at 30 and 37° underwent the 

 characteristic acidophilus coagulation, and there was no indication by 

 odor or physical appearance that any putrefactive action was in prog- 

 ress. These observations were fully corroborated by the plates and 

 Veillon tubes, which contained pure or practically pure cultures of 

 B. acidophilus. The control flasks, on the other hand, gave marked 

 evidence of putrefaction within twenty-four hours. There was partial 

 coagulation and later a very extensive liquefaction of the casein, the 

 liquid appearing watery, accompanied by offensive odors. Similar 

 changes took place in the corresponding flasks incubated at 20° C, 

 but they were very much slower. 



These experiments indicate that, while B. acidophilus does not pro- 

 duce as much acid in milk as do B. bulgaricus and Streptococcus lacti- 

 cus, it nevertheless protects the milk against undesirable decomposition 

 by so-called putrefactive organisms, and if viable and present at the 

 outset in considerable numbers, it should render milk safe against de- 

 composition harmful to the consumer. Whether boiled milk may be 

 substituted for the sterilized and the same satisfactory product can be 

 obtained has not yet been determined. Boiled or incompletely sterilized 

 milk must be inoculated immediately, however, before any bacterial 

 change has taken place. Hence, from the practical standpoint, sterili- 

 zation will be necessary to obtain constant and reliable results. 



