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Thermal inactivation of Staphylococcus aureus in walnut shells as influenced by water activity and heating rate

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  2018 ASABE Annual International Meeting  1800604.(doi:10.13031/aim.201800604)
Keywords:   water activity; heating rate; heating block system; Staphylococcus aureus; walnut shell powder

Abstract. Water activity (aw) 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 aw and heating rate using a heating block system (HBS). The results showed that the inactivation of S. aureus ATCC 25923 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). The D-values of S. aureus ATCC 25923 increased with decreasing water activity and heating rates (<1 ºC /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 ºC /min, while the heat resistance of S. aureus ATCC 25923 were no significant difference at 1, 5 and 10 ºC /min. There was a rapid reduction of S. aureus ATCC 25923 at elevated temperatures from 26 to 56 ºC at a heating rate of 0.1 ºC /min. These results suggest that an appropriate increase in moisture content of in-shell walnuts and heating rate during heat treatment can improve the inactivation efficiency of pathogens in thermal processed low moisture foods.

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