Inactivation of Salmonella, Listeria monocytogenes and Enterococcus faecium NRRL B-2354 in a selection of low moisture foods

• Pathogens or surrogate survived well in samples during storage (21 days at 16 °C).
• Heat resistance did not change significantly throughout the storage period.
• Viability of pathogens or surrogate was adequate for inactivation/validation studies.
• E. faecium NRRL B2354 was a suitable surrogate in tested products except confectionery.
• Pathogens were inactivated by heating to 112 °C solid foods in sealed thermal cells.

The aims of this study were to obtain data on survival and heat resistance of cocktails of Salmonella, Listeria monocytogenes and the surrogate Enterococcus faecium (NRRL B-2354) in four low moisture foods (confectionery formulation, chicken meat powder, pet food and savoury seasoning) during storage before processing. Inoculated samples were stored at 16 °C and cell viability examined at day 0, 3, 7 and 21. At each time point, the heat resistance at 80 °C was determined. The purpose was to determine a suitable storage time of inoculated foods that can be applied in heat resistance studies or process validations with similar cell viability and heat resistance characteristics. The main inactivation study was carried out within 7 days after inoculation, the heat resistance of each bacterial cocktail was evaluated in each low moisture food heated in thermal cells exposed to temperatures between 70 and 140 °C. The Weibull model and the first order kinetics (D-value) were used to express inactivation data and calculate the heating time to achieve 5 log reduction at each temperature.Results showed that the pathogens Salmonella and L. monocytogenes and the surrogate E. faecium NRRL B-2354, can survive well (maximum reduction < 0.8 log) in low moisture foods maintained at 16 °C, as simulation of warehouse raw material storage in winter and before processing. The D80 value of the pathogens and surrogate did not significantly change during the 21 day storage (p > 0.05). The inactivation kinetics of the pathogens and surrogate at temperatures between 70 and 140 °C, were different between each organism and product. E. faecium NRRL B-2354 was a suitable Salmonella surrogate for three of the low moisture foods studied, but not for the sugar-containing confectionery formulation. Heating low moisture food in moisture-tight environments (thermal cells) to 111.2, 105.3 or 111.8 °C can inactivate 5 log of Salmonella, L. monocytogenes or E. faecium NRRL B-2354 respectively.