Thermal Inactivation of Heat-Shocked Escherichia coli O157:H7, Salmonella, and Listeria monocytogenes in Dairy Compost

Abstract

Thermal resistance of heat-shocked Escherichia coli O157:H7, Salmonella, and Listeria monocytogenes was compared with that of non–heat-shocked (control) strains in finished dairy compost. A three-strain mixture of each pathogen was heat shocked at 47.5°C for 1 h and inoculated into the compost at a final concentration of 107 CFU/g. The inoculated compost was placed inside an environmental chamber set at 50, 55, or 60°C with humidity at ca. 70%. The heat-shocked E. coli O157:H7, Salmonella, and L. monocytogenes survived better (P < 0.05) at 50°C, with reductions of 2.7, 3.2, and 3.9 log CFU/g within 4 h compared with reductions of 3.6, 4.5, and 5.1 log CFU/g, respectively, in control cultures. The heat-shocked cultures of E. coli O157:H7, Salmonella, and L. monocytogenes had 1.2-, 1.9-, and 2.3-log reductions, respectively, within 1 h at 55°C, whereas the corresponding control cultures had 4-, 5.6-, and 4.8-log reductions, respectively. At 60°C, a rapid population reduction was observed during the come-up time of 14 min in control cultures of E. coli O157:H7, Salmonella, and L. monocytogenes with 4.9-, 4.8-, and 2.3-log reductions, respectively, compared with 2.6-, 2.4-, 1.7-log reductions, respectively, in heat-shocked cultures. L. monocytogenes survival curves for all three temperatures had extensive tailing. The double Weibull distribution model was a good fit for the survival curves of pathogens, with differences in the shape parameter of heat-shocked and control cultures. Our results suggest that the heat-shocked pathogens may have extended survival at lethal temperatures attained during the composting process.