Effect of changes in microwave frequency on heating patterns of foods in a microwave assisted thermal sterilization system

•The generators showed frequency shifts response to power settings.•The heating pattern did not changed within a possible frequency range of 900-920MHz.•Lower loss factor of circulation water increased the heating rate of food.•The loss factor of circulating water had no effect on the heating pattern of food. This research studied the influence of frequency variation on heating patterns within prepackaged foods in a 915MHz single-mode microwave assisted sterilization (MATS) system consisting of four microwave heating cavities. The frequencies of the four generators powering the MATS system at Washington State University were measured at different power levels over one year. The effect of frequency shifts in the generators on heating patterns within a model food (whey protein gel, WPG) was studied through computer simulation. The simulated heating patterns were experimentally validated using a chemical marker. Our measurement results showed that a 0.5kW increase in the microwave power caused the operating frequencies of the generators to increase by 0.25–0.75MHz. The simulation results suggested that the heating pattern of WPG processed by the MATS system was not affected by the varying frequencies of generators within the operating frequency bandwidth (900–920MHz). In addition, the simulation results revealed that using deionized water as the circulation medium in the MATS system resulted in a 23–37% increase in the temperature of WPG as compared with that when using normal tap water, but did not alter the heating pattern.