Effect of water activity and heating rate on Staphylococcus aureus heat resistance in walnut shells

Water activity (a_w) and heating rate have shown important effects on the thermo-tolerance of pathogens in low moisture foods during thermal treatments. In this study, three strains were selected to compare the heat resistance in walnut shell powder and finally the most heat resistant S. aureus ATCC 25923 was chosen to investigate the influence of a_w and heating rate using a heating block system (HBS). The results showed that S. aureus ATCC 25923 became more thermo-tolerant at lower a_w. The D-values of S. aureus ATCC 25923 increased with decreasing water activity and heating rates (<1°C/min). A significant increase in heat resistance of S. aureus ATCC 25923 in walnut shell powder was observed only for the heating rates of 0.2 and 0.5°C/min but not at 1, 5 and 10°C/min. There was a rapid reduction of S. aureus ATCC 25923 at elevated temperatures from 26 to 56°C at a heating rate of 0.1°C/min. The inactivation under non-isothermal conditions was better fitted by Weibull distribution (R²=0.97 to 0.99) than first-order kinetics (R²=0.88 to 0.98). These results suggest that an appropriate increase in moisture content of in-shell walnuts and heating rate during thermal process can improve the inactivation efficiency of pathogens in low moisture foods.