Heat inactivation of Shiga toxin-producing Escherichia coli in a selection of low moisture foods

Survival during temporary storage and heat inactivation of Shiga toxin-producing Escherichia coli (STEC) were investigated in a selection of four different low moisture foods with water activities between 0.26 and 0.63. A cocktail of nine strains of O157 and non-O157 STEC was selected for this study, giving preference to those strains of public health concern associated with food consumption. The data from previous studies on STEC survival and heat resistance in wheat flour were taken into account for the selection of the relevant target serotypes and strains. No significant difference was observed on STEC viable counts among the four products stored at 16 °C for 7 days. The viable counts decreased by maximum 1 log CFU/g by day 28, as simulation of temporary storage of bulk raw materials before processing. The level of STEC inactivation by dry heat treatments varied with the food formulation. The STEC bacteria showed a lower heat resistance in savory seasoning and pet food formulations compared to confectionery formulation and chicken meat powder. We also identified the minimum heating temperature and time combinations for each food to achieve a 5 log reduction of STEC using linear and Weibull models. Our results show the capability of obtaining a 5 log reduction of STEC in pet food, savory seasoning and confectionery formulations, and chicken meat powder by heating to product temperatures such as 90.9, 92.7, 99.1, and 106.0 °C, respectively. High product temperatures can be achieved by heating food samples in thermal cells hermetically sealed and submerged in a thermo-fluid bath at temperatures of 110 °C or above. The use of thermal cells allowed to simulate in the laboratory the high processing temperatures such as those applied in food extrusion or continuous treatment without moisture loss during heating.