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

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.