Comparative study on drying kinetics and thermodynamic characteristics of Indica rice under microwave and hot-air drying

Abstract

To overcome mass-transfer bottlenecks, such as case hardening, in traditional grain dehydration, this study systematically investigated the drying kinetics and thermodynamic mechanisms of Indica rice subjected to hot-air drying (HAD) and microwave drying (MD). Dehydration experiments were conducted across various operating parameters, fitted to thin-layer kinetic models, and evaluated for effective moisture diffusivity (Deff) and activation energy (Ea). The results demonstrated that the Midilli et al.’s model best predicted the drying behavior (R2>0.99). The volumetric heating inherent to microwave drying alleviated case-hardening-like resistance by inducing internal vapor pressure-driven moisture migration. To balance high dehydration efficiency with the prevention of thermal damage, a power level of 350 W and a loading mass of 40 g were identified as the optimal microwave drying conditions. Thermodynamically, HAD exhibited a V-shaped Ea trend (30.05, 15.18, and 37.63 kJ/mol) with increasing loading mass, driven by the competing effects of case hardening and macroscopic physical resistance. Conversely, MD displayed a monotonic decrease in Ea (from 35.21 to 17.04 kJ/mol), reflecting a transition toward internal pressure-driven mass transfer. Notably, MD drastically reduced the equivalent activation energy to 7.95 W/g. Under the optimal conditions (350 W, 40 g), MD reduced drying time by approximately 87.5–90.0%, increased Deff by about one order of magnitude, and achieved a higher drying-rate constant compared to HAD. In conclusion, dielectric heating effectively bypasses conventional thermal barriers, offering a thermodynamically superior and highly efficient pathway for quality-preserving industrial grain processing.