Studies on the effect of heat shock, culture conditions, and packaging conditions on the heat resistance, recovery, and virulence of Listeria monocytogenes in ground pork

The heat resistance of two Listeria monocytogenes subspecies was determined, according to various heating conditions. In general, serotype 1 was more heat resistant than Scott A, and both subspecies were more heat resistant if packaged aerobically than anaerobically during heating. Cells exposed to 48°C for 2h prior to heating (heat shock) were more heat resistant than nonheat-shocked controls. Less survivors were detected when the cells were inoculated in pork stored for 3 months compared with fresh pork, suggesting that oxidative components can decrease cell survival during heating. Heating the samples at a slow heating rate (1.3°C/min) resulted in the highest number of survivors to heating at 62°C when compared with samples heated at a fast rate (8.0°C/min). The recovery of heat-injured Listeria during storage at 4-30°C in aerobically-packaged or vacuum-packaged ground pork was determined. Addition of 100-300 ppm of butylated hydroxyanisole (BHA) and 300-700 ppm of BHT (butylated hydroxytoluene) was examined in aerobically packaged ground pork at 7°C and 30°C. Listeria was recovered more rapidly in vacuum-packaging at 4°C, but BHA and BHT did not significantly affect cell growth at 7 and 30°C;The effect of various growth conditions on production of LLO were examined. Serotype 1 produced more LLO than Scott A and, in a fed-batch method, maximum LLO production was detected at pH 5.5-6.5 after 2h stationary phase. The production rate of LLO by heat-shocked cells increased more than 40-fold, in comparison with a two-fold rate increase for controls within 4h of incubation after the heat shock treatment. However, heat shocking inactivated existing LLO, so the levels of LLO were many times higher in controls than in heat-shocked cells after 4h of incubation;An enzyme-linked immunosorbent assay was developed for quantitative analysis of LLO by using 5 mg/ml ASO and a 1:1,000 dilution of antibody conjugated with alkaline phosphatase. The LLO was inactivated easily with heat but the protein structure was stable at temperatures below 80°C for 3 min;These studies can serve the industry to design procedures that will more effectively prevent this organism from surviving during heating and storage in ground pork products