Mechanistic Insights into Texture and Microstructure Evolution During Intermittent Microwave-Convective Drying of Cellular Plant-Based Food Material

Abstract The physical or textural transformations during drying of plant-based food materials are common scenarios and can be well correlated with the dynamically changing mechanical properties such as hardness, elastic modulus, and fracture stress. Despite their importance, studies that relate these mechanical properties to moisture variation are underexplored and often lack analysis under actual drying conditions. In particular, the evolution of fracture stress with decreasing moisture content, together with comprehensive analyses of external and internal microstructure and case-hardening effects under different drying conditions, remain largely unexplored. Addressing these issues, this study systematically investigates the relationships between moisture content and key mechanical properties—hardness, elastic modulus, and fracture stress during both convective (CD) and intermittent microwave-convective drying (IMCD), while also evaluating case-hardening behaviour of apple sample. The results show that hardness and elastic modulus initially decrease with moisture loss, then increase sharply at low moisture levels (1.06 kg/kg and 0.971 kg/kg dry bases for CD and IMCD, respectively), reflecting the onset of structural hardening. Notably, fracture occurred only during the early stages of CD (with degree of compression ranging from 50 to 65% of samples’ instantaneous height and up to a moisture content of 2.85 kg/kg dry bases), whereas IMCD samples exhibited no fracture throughout the drying process, demonstrating superior structural resilience. Case-hardening analysis further revealed thicker crusts at 45 °C compared to 60 °C in both methods, with IMCD consistently yielding thinner and less rigid crusts than CD. These findings provide new insights into the coupled evolution of mechanical properties, fracture behaviour, and microstructure during drying, offering a robust basis for texture-based quality evaluation and the optimisation of energy-efficient drying processes for plant-based food materials.