Rapid Enumeration of Psychrotrophic Bacteria in Raw Milk by the Microscopic Colony Count1

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Abstract
Psychrotrophic bacteria were enumerated in raw milk by the microscopic colony count and counts were compared with those obtained by standard methods. Incubation conditions were 7 C for 10 days for the standard procedure and 7 C for 48 and 72 h or 21 C for 13.5 and 16.5 h for the microscopic colony count. Though each of the microscopic colony counts differed from the standard psychrotroph count (P < 0.01), there was a good general relationship in each instance (correlation coefficients ≥ 0.94). Wide variations in ratios of microscopic colony counts to standard psychrotroph counts made interpretations difficult. Generally, microscopic colony counts after 48 and 72 h at 7 C were less than standard psychrotroph counts. Incubation at 21 C yielded microscopic colony counts that were similar to standard psychrotroph counts for most samples; however, some microscopic colony counts at this temperature were substantially higher than the standard psychrotroph counts.

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