Thermoduric Organisms in Milk. Part Iv. Colony Counts of Thermoduric Organisms Obtained With Different Incubation Times at 30°

Summary: Statistical analysis of the results from four laboratories, on the effect of incubation time at 30° on the number of colonies from laboratory pasteurized milk, showed a marked increase between the second and third day, but little subsequent increase. Incubation of Petri plate cultures for 3 days at 30° was sufficiently accurate for routine purposes. In about 70% of cases the 3 day colony count exceeded 95% of the count obtained after 5 days and most samples gave values exceeding 85% of this. The same types of colonies formed within 2 days at 30° as developed subsequently. The mean shelf temperatures did not depart materially from 30° in three incubators. The general level of temperatures was much higher in the fourth incubator, where on one occasion 33° was recorded, and this may have been the reason for non‐significant differences between colony counts at the four incubation times in that case.