Decoupling microwave heating efficiency from food property variations: A reduced-gradient index metamaterial strategy for superior efficiency and uniformity

Microwave heating is recognized as a green processing technology; however, its energy conversion efficiency is often limited in practice due to variations in the shape, volume, and dielectric properties of heated samples, which hinder proper impedance matching. In this study, we propose a metamaterial-assisted microwave oven design that incorporates a wedge-shaped ceramic strip within the cavity to improve energy utilization and heating uniformity. The wedge-shaped ceramic, designed based on transformation optics theory, manipulate electromagnetic waves by converting waveguide-propagating waves into surface waves, thereby facilitating more efficient energy coupling into the food. A finite element-based multi-physics model for microwave heating was implemented in COMSOL software, and the effects of food rotation were simulated through co-simulation with MATLAB. Both simulation and experimental results demonstrated that the proposed method enables efficient heating of foods with varying geometries and components. For ready-to-eat foods, including curry rice, pasta, and congee with preserved egg and lean pork, microwave heating efficiency was maintained above 90 %. Compared to conventional turntable microwave ovens, this approach demonstrated improved heating uniformity, effectively reducing cold and hot spots during the heating process and resulting in higher overall food quality.