Enhancing Food Quality Through Radio Frequency Drying of In-Shell Oil Crops: A Critical Review of Mechanisms, Advantages, and Challenges.

The postharvest drying of in-shell oil crops is severely constrained by their shell-kernel structure, which creates stubborn heat and mass transfer resistance, leading to prolonged processing times, high energy consumption, and rapid quality deterioration, including lipid oxidation and color loss. Radio frequency (RF) drying offers an inside-out volumetric heating strategy that bypasses shell resistance and directly heats the kernel, thus reducing drying time and preventing quality deterioration. This review aimed to systematically clarify the mechanism of RF selective heating induced by dielectric differences of the shell-kernel system, which is associated with the vapor pump effect (the phenomenon of internal pressure increase caused by the accumulation of water vapor in the shell) and enhanced moisture migration driven by internal pressure. The shell not only acted as a dual barrier of dielectric and physical properties but also led to enzyme inactivation under RF treatment. Beyond drying, RF treatment improved oil extractability and promoted the formation of desirable volatile compounds (e.g., pyrazines, aldehydes, and furans). However, previous studies focused on extremely few crop species, lacked dynamic dielectric data under realistic drying conditions, left subcellular mechanisms largely unexplored, and failed to monitor the real‑time microenvironment inside the shell. These limitations contributed to persistent industrial challenges, including nonuniform heating, thermal runaway, mechanical damage, and poor process control. Future studies should focus on multiphysics modeling, intelligent synergistic drying technologies (RF-vacuum and RF-hot air), smart sensing and adaptive control systems, and integrated processing approaches combining drying, disinfection, and mycotoxin degradation. Based on the theories of selective heating induced by shell-kernel dielectric differences and internal pressure‑driven moisture migration resulting from the steam pump effect, this review provides technical support for the process design and industrial scale‑up of RF drying for in‑shell oil crops.