Firmness‐Dependent Mechanical and Viscoelastic Properties of “ Huaniu ” Apple Peel and Flesh: Experimental Evaluation and Constitutive Modeling

ABSTRACT

This study investigated the firmness‐dependent mechanical and viscoelastic properties of “Huaniu” apple peel and flesh to support engineering design for postharvest handling. Fruits were categorized into low (5.33 N), medium (7.31 N), and high (10.30 N) firmness groups. Quasi‐static properties were evaluated via peel tensile and flesh compression tests in both axial and radial orientations, while time‐dependent behaviors were characterized using stress relaxation and creep experiments. Constitutive modeling was performed using generalized Maxwell and Kelvin models. Results indicated no statistically detectable axial–radial differences within firmness groups ( p  > 0.05); however, strength and apparent elastic modulus increased significantly with firmness ( p  < 0.05). Linear regressions calibrated on individual fruit measurements between firmness and mechanical indices yielded high coefficients of determination ( R ²  = 0.921–0.973), with 95% confidence and prediction intervals reported to reflect calibration and prediction uncertainty. A five‐element generalized Maxwell model ( R ²  = 0.9967–0.9971) and a six‐element generalized Kelvin model ( R ²  > 0.9995) effectively described the stress relaxation and creep data, respectively. Furthermore, higher firmness tissues exhibited larger equilibrium moduli and reduced axial creep strain within the 60‐s loading window. These findings establish a systematic laboratory‐scale relationship between fruit firmness and tissue mechanical resistance. The derived constitutive parameters are only valid under the controlled short‐duration loading conditions of this work, and further validation under real long‐term postharvest stacking and full supply‐chain environments is required before these parameters can be reliably adopted for apple mechanical simulation in industrial logistics operations.