Thawing in a microwave cavity: Comprehensive understanding of inverter and cycled heating

• A coupled thermal-electromagnetic model was developed for thawing in a microwave oven.
• Thermal and dielectric properties varied with space and time.
• As material thaws, total energy absorption decreases due to the thawed outer shell.
• Total energy absorption changes with size due to shift in cavity resonant frequency.
• Inverter heating produced only minimal differences in temperature or color.

A comprehensive understanding of the electromagnetics and thermal aspects of the thawing of frozen foods in a domestic microwave oven cavity was developed through a model that two-way couples Maxwell's equations with the heat conduction equation with continuous variation of dielectric and thermal properties with temperature. Tylose, a material with properties similar to those of lean beef, was used to experimentally validate the model. Temperatures were measured using fiber optic and infrared sensors. Various salt contents that were used in formulating the tylose represented important variations in dielectric properties, latent heat release, and initial freezing point. Spatial and time variations of all important parameters are used to explain the results. As material thaws, total energy absorption decreases, a novel result that is explained by reference to continuous changes in dielectric properties. Shifts in the resonant frequency of the cavity as load size changes explain the changes in total energy absorption with size. Inverter heating, a newer technology that claims superiority over ordinary cycled heating, produced only minimal differences in temperature uniformity or color.