EVALUATION OF THE INCUBATED PLATE-COUNT TEST FOR PASTEURIZED MILK

Factors were examined which may influence bacterial psychrotroph count s 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 a t least five consecutive samples of milk. Increasing the number of rep licate plates or making subsamples did not decrease counting error. Wh en 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 effec t. The counts between dairies were not significantly different nor was the position in the pasteurization run from which the samples were ta ken significant. The proportion of samples which failed the test stand ard was 9.7% over the period. The bacterial flora isolated from plate count agar were identified; Gram-negative rods were the most common is olates. The spiral plater system was compared with the pour plate meth od in the preincubation test and results were found to be in close agr eement, 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 pla te counts. This suggested that preincubation conditons were critical a nd that variation in conditions even within the specified ranges of th e test could significantly alter the bacterial psychrotroph counts obt ained. Because of these results, the validity of the statutory test wa s questioned.