THE INFLUENCE OF TIME AND TEMPERATURE OF INCUBATION ON THE PLATE COUNT OF MILK1

Abstract

The geometric mean bacterial count of 131 raw milk samples, using plates incubated 48 h at 30 C, was 15% higher than the geometric mean when the bacterial counts were determined at 32 C incubation. When 72 h of incubation were used, the geometric mean of samples using the lower incubation temperature was 20% higher. The mean for all 72-h raw milk counts was 31% higher than that for 48 h counts. There was no significant difference between geometric means of 51 pasteurized milk samples using plates incubated at 30 or 32 C after 48 h incubation. After 72 h of incubation, the geometric mean obtained at 30 C was 10% higher. For all 72-h counts on pasteurized milk the mean was 55% higher than for 48 h. A number of both raw and pasteurized samples showed little or no increase in count following Preliminary Incubation at 12.8 C for 18 h while others “blew up” to high levels, suggesting undesirable contamination. The International Dairy Federation procedures tended to reflect the latter better than the SPC.