Moisture-dependent mechanical anisotropy of oil sunflower seed tissues: Insights from puncture, shear, and compression tests

Sunflower seeds are a major global oil crop, and their mechanical behavior critically affects processing efficiency. However, moisture-dependent anisotropic responses of different seed tissues remain poorly characterized. This study investigated effects of moisture content (0%-12%) on pericarp, kernel, and whole seed via puncture, shear, and compression tests, quantifying maximum breaking force, brittleness, stiffness, and energy-related parameters. Results showed moisture, tissue type, and loading direction all significantly influenced mechanical properties and that increasing moisture generally reduced strength and stiffness while enhancing deformability. Under shear, kernel shear modulus decreased from 4.28 MPa at 0% moisture to 1.12 MPa at 12.31% moisture, with most parameters decreasing by 12–70% versus dry conditions. Whole-seed elastic modulus reached a minimum of 0.79 MPa at 8.23% moisture, > 13-fold lower than the dry-state value (11.71 MPa). Significant mechanical anisotropy was detected in all sunflower seed tissues: kernel shear strength parameters followed the order length > width > height (with maximum strain showing the reverse trend), pericarp exhibited higher shear strength along the length direction than along the width direction, and whole seeds showed the greatest shear strength and stiffness in the height direction. At 12.31% moisture content, kernel maximum strain in the length direction increased by 76.55% compared with the dry state, demonstrating markedly enhanced deformability upon hydration. These results clarify how moisture, tissue architecture, and loading direction jointly govern sunflower seed mechanics, supporting moisture-conditioned dehulling and mechanics-based equipment design.