THE EFFECT OF THE INCUBATION TIME AND TEMPERATURE ON THE DETERMINATION OF PSYCHROPHILIC BACTERIA IN MILK BY THE AGAR PLATE METHOD1*

A study has been made of the effect of incubation time and temperature on the determination of psychrophilic bacteria in milk. The incubation of agar plates at 10° C resulted in the detection of a group of thermoduric organisms which was not found when agar plates were incubated at 5° C. These organisms are not considered to be true psychrophiles. Bacterial counts on milk obtained using the 5° C and 10° C incubation temperatures did not coincide regardless of the incubation period. The counts obtained using the 10° C incubation temperature were always higher than those obtained at 5° C for a similar period. Maximum bacterial counts on milk samples stored at 5° C for 10 days or less were not obtained in less than 20 days when the agar plates were incubated at 5° C.