Evaluation of the incubated plate count test for pasteurized milk

Abstract

Factors were examined which may influence bacterial psychrotroph counts obtained by the European Community's directed preincubation test for pasteurized milk. Studies to determine the best protocol for sampling milk indicated that the least counting error was obtained by taking at least five consecutive samples of milk. Increasing the number of replicate plates or making subsamples did not decrease counting error. When milk samples from five commercial dairies were examined over an 11 month period there was a significant difference (p < 0·001) in counts obtained over the period, but this did not seem to be a seasonal effect. The counts between dairies were not significantly different nor was the position in the pasteurization run from which the samples were taken significant. The proportion of samples which failed the test standard was 9.7% over the period. The bacterial flora isolated from plate count agar were identified; Gram­negative rods were the most common isolates. The spiral plater system was compared with the pour plate method in the preincubation test and results were found to be in close agreement, suggesting that the spiral plater could be used to improve the current official methods. Varying the time and temperature of sample preincubation was found to give significantly different results in plate counts. This suggested that preincubation conditons were critical and that variation in conditions even within the specified ranges of the test could significantly alter the bacterial psychrotroph counts obtained. Because of these results, the validity of the statutory test was questioned.

Growth characteristics of post-pasteurization contaminants isolated from pasteurized milk

“…These species have been isolated by other workers (Schroder, 1984;Wilson & Gilmour, 1990;Sutherland et al 1993;Ternstrom et al 1993), who carried out flora analyses of pasteurized milk. We assume that all these organisms gained access as post-pasteurization contaminants and are also similar to those organisms described in previously published work since they failed to survive a laboratory pasteurization procedure, although there is evidence that certain strains of Pseudomonas spp.…”

“…| Three different heat stress treatments were chosen in order to equalize the sublethal injury to each organism, and these are indicated in parentheses: treatment 1, 50 °C for 30 min; treatment 2, 50 °C for 15 min; treatment 3, 50 °C for 5 min. The spiral plater system was used as a viable count method because it is rapid and has been shown to give results statistically comparable to the pour plate and other counting techniques (Sutherland et al 1993).…”