Mechanical properties and microstructure of apple peels during storage

The apple peel (i.e., outermost tissue) plays a crucial role in protecting and preserving apple life, as well as in reducing fruit injury. Apple peels of the Danxia and Starkrimson varieties were compared for their tensile properties, shear properties, and tensile-property variations after 0, 14, and 28 days of storage at room temperature. Apple peel microstructures were observed and imaged using scanning electron microscopy. Cubic polynomials proved superior for quantifying the stress–strain non-linear relationship of peels. With increased storage period, tensile strength and fracture strain gradually decreased but elastic modulus increased. The mechanical indices of Starkrimson peels were greater than those of Danxia peels after the same storage period. Significant differences between cultivars were observed in terms of tensile strength, elastic modulus, and shear strength; significant storage differences also existed for both cultivars. Mechanical properties were closely related to peel microstructures. Strengths and weaknesses mainly depended on the number and width of microcracks on the cuticle, cuticle form, cell aspect radio, and space between cells.