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Microbial populations of Grade A raw milk samples from 105 individual producers and 74 bulk tank trucks (commingled) were enumerated by Standard Plate Count (SPC), psychrotrophic count (PBC), coliform count (CC), laboratory pasteurized count (LPC), thermophilic count (TBC), yeast and mold count (Y&M), and special penicillin (PEN) and crystal violet tetrazolium (CVT) agar count procedures. In addition, microbial populations were determined by the SPC, PBC, PEN, and CVT procedures after preliminary incubation (PI) of samples. Initial mean counts obtained on individual producer samples were generally lower than those for commingled samples. However, producer samples had higher mean counts after PI. Growth ratios were lower for commingled than for individual producer samples indicating slower growth during PI. Results obtained by the PBC, PEN, and CVT procedures were similar when viewed as correlation coefficients, distribution of samples according to microbial counts, mean counts, and growth ratios during PI. Before PI, the correlation between these three tests was poor and lacked statistical significance when the PBC was <50,000/ml. After PI, the tests were highly correlated (P<0.01) and the r values ranged from 0.8 to 0.9 for samples with PBC levels above 108/ml.

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Bacterial counts on 155 raw milk samples obtained with plate incubation temperatures of 27 and 32 C were closely correlated (r = 0.96) . Correlation coefficients between counts obtained at both 27 and 32 C and psychrotrophic (7 C-10 days) counts for all samples were relatively low, but statistically significant ( P < 0.01). The correlation to psychrotrophic counts was especially low and in some instances lacking in statistical significance in the sample groups with counts ( 27 and 32 C) <100,000/ml. Eighty-four of the samples had higher counts at 32 C and 62 samples had higher counts at 27 C. The mean psychrotrophic count of the samples with higher counts at 27 C was higher than the mean psychrotrophic count of the other samples. However, the correlation coefficients were higher for samples with counts higher at 32 C. Incubation at 27 C does not appear to offer significant advantages over the 32 C incubation temperature used in the Standard Plate Count.

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