Influence of moisture content on inactivation of foodborne pathogens in powdered foods by radio-frequency heating

Abstract

Radio-frequency (RF) heating is a highly appealing technology by which internal heating as a result of molecular friction is rapidly generated in response to an applied alternating electric field. It has potential for thermal processing of powdered foods which have trouble to be sterilized with conventional heating due to their low thermal conductivities. However, commercial RF heating is rather limited due to a lack of in-depth technical information. In this study, RF heating and dielectric measurement system was developed to provide information not only about reducing thermal abuse in the food product but also about maximizing process effectiveness. The influence of moisture content during RF heating on heating rate, dielectric properties, and inactivation of foodborne pathogens was investigated. The effect of RF heating on quality of red and black pepper spice within different moisture ranges was also investigated. Red peppers (12.58%, 15.16%, 19.07%, and 23.31% dry basis moisture content, db) and black pepper (10.11%, 17.17%, 23.73%, and 30.52% db) inoculated with Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium were treated in a RF heating system with a constant frequency of 27.12 MHz. The heating rate of the sample was dependent on moisture content up to 19.07% (db) of red peppers and 17.17% (db) of black pepper, but there was a significant decrease in the heating rate when the moisture content was increased beyond these levels. The dielectric properties of both samples increased with a rise in moisture content. As the moisture content increased, treatment time required to reduce E. coli O157:H7 and S. Typhimurium to below the detection limit (1 log CFU/g) decreased and then increased again when the moisture content exceeded a level corresponding to the peak heating rate. Color values and volatile flavor components of RF treated red and black pepper spice of various moisture contents were not significantly (P > 0.05) different from those of nontreated samples. RF treatment significantly (P < 0.05) reduced moisture content of red and black pepper by up to 3.01% (db) and 4.65% (db), respectively. These results suggest that RF heating can be effectively used to not only control pathogens but also reduce moisture levels in spices without affecting product quality and that the effect of inactivation is dependent on moisture content.