Thermal deformation behavior and microstructural response of pumpkin seeds during drying

Despite the widespread use of hot air drying in seed processing, the mechanisms of drying-induced deformation behavior and microstructural response in pumpkin seeds is not well understood. The moisture content evolution, strain field distribution, and cellular structural responses of seeds under varying drying temperatures (35℃, 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, and 70℃) were systematically analyzed using digital image correlation method (DIC) and cell morphology analysis. The results showed that the drying time to the safe moisture content (9.0 ± 0.5 %) at 70℃ was only 2/9 of that at 35℃. The deformation of pumpkin seeds showed obvious anisotropy, with higher absolute value of strain in the minor axis direction ${\epsilon }_x$ (from −0.017 ∼ −0.056 at 35°C to −0.011 ∼ −0.072 at 70°C) compared to the major axis ${\epsilon }_y$ (from −0.016 to −0.036 at 35°C to −0.029 ∼ −0.056 at 70°C). The pumpkin seeds were dominated by reversible elastic contraction at 50°C or less, and entered the plastic deformation stage when the drying temperature was 50–70°C. The intensified heterogeneity of the strain field reflected the synergistic interaction between the core and edge of seed. The porosity showed a continuous increasing trend with rising temperature, while the average cell area, Feret diameter, and perimeter decreased. The transition from elastic to plastic deformation was observed to occur at around 50°C, which serves as a critical threshold. This study provides a basis for optimizing the hot air drying process of pumpkin seeds, which offering practical guidance for damage-minimized seed and food drying in agricultural applications.