A frequency-selected microwave strategy for layer-targeted heating of multilayer materials

Selective heating of multilayer materials plays a critical role in food processing, biomedical applications, and materials engineering. However, conventional microwave heating struggles to achieve both sufficient interlayer temperature differences and satisfactory temperature uniformity within the target layer. This paper proposes a frequency-selected microwave heating strategy that exploits both the frequency- and temperature-dependent dielectric loss of multilayer materials and the frequency-dependent modulation of the electromagnetic field within the cavity. By selecting an appropriate operating frequency, microwave energy can be preferentially deposited in the target layer. The dielectric properties of mashed potato were measured using an open-ended coaxial probe method, and a coupled electromagnetic and heat transfer finite element model was developed to evaluate candidate frequencies in the range of 2170–3000 MHz. A solid-state microwave source system was constructed, and the proposed strategy was validated using bilayer samples composed of mashed potato and gellan gel. Compared with conventional fixed-frequency heating at 2.45 GHz, the proposed method increased the interlayer temperature difference by 125.70%, reduced the coefficient of variation (COV) of temperature from 0.69 to 0.36, which corresponds to a 47.83% improvement in temperature uniformity. This approach offers a new method for synergistic control of selectivity and uniformity in multilayer microwave heating and has broad applicability in food and material processing.