Modelling the inactivation of Listeria monocytogenes and Salmonella typhimurium in simulated egg wash water

The inactivation of Listeria monocytogenes Scott A and Salmonella typhimurium in synthetic egg washwater was studied under varying conditions of temperature (38°C to 46°C), egg solids (0%, 2%), pH (9·5 to 10·5) and chlorine concentration (0 to 10 μg ml^-1 [ppm]) using a full factorial design. Survival was measured as the logarithm of the time required for a 4-log reduction in viable counts. Both pathogens were significantly affected by temperature and the presence of egg, while the survival of S. typhimurium was significantly reduced by pH and chlorine alone. Both pathogens were affected by second order interactions involving egg and either pH or chlorine. Egg was the most significant factor, reducing the survival of L. monocytogenes while promoting the survival of S. typhimurium. Two linear equations were derived to describe the decrease in survival of L. monocytogenes and S. typhimurium as a function of the 4 factors. These equations were used to estimate conditions which would reduce the time for a 4-log reduction in viable counts to <30 min. In the absence of egg solids, chlorine (20 μg ml^-1) can be used to control L. monocytogenes and S. typhimurium using moderate temperature (42°C) and pH (10·5). When egg is present in wash water, increased temperature (47·4°C) and pH (10·8) are required.