Growth characteristics of post-pasteurization contaminants isolated from pasteurized milk

Abstract

SummaryNine organisms were isolated from separate pasteurized milk samples after they had been incubated at 6 °C for 5 d (European Union preincubated count, PIC), four from high count samples (> 5 x 106cfu/ml) and five from low count samples (< 103cfu/ml). When the organisms were harvested without overt stress being applied and subjected to a simulated PIC using UHT whole milk, all except one isolate gave comparatively high (> 106cfu/ml) counts. The imposition of a heat stress at 50 °C prior to a simulated PIC resulted in a segregation of the isolates into those giving high and those giving low counts, which reflected the PIC values of the milk samples from which they were originally isolated. When the isolates were subjected to a cold stress (25 to 4 °C) and inoculated into nutrient broth at 4 °C, the high count isolates were found to have significantly (P< 0·05) shorter lag phases than the low count isolates.