Factors Affecting Mechanical Properties of the Skin of Sweet Cherry Fruit

The skins of all fruit types are subject to sustained biaxial strain during the entire period of their growth. In sweet cherry (Prunus avium L.), failure of the skin greatly affects fruit quality. Mechanical properties were determined using a biaxial bulging test. The factors considered were the following: ripening, fruit water relations (including turgor, transpiration, and water uptake), and temperature. Excised discs of fruit skin were mounted in a custom elastometer and pressurized from their anatomically inner surfaces. This caused the skin disc to bulge outwards, stretching it biaxially, and increasing its surface area. Pressure (p) and biaxial strain (a) due to bulging were quantified and the modulus of elasticity [E (synonyms elastic modulus, Young's modulus)] was calculated. In a typical test, a increased linearly with p until the skin fractured at p(fracture) and epsilon(fracture). Stiffness of the skin decreased in, ripening late stage III fruit as indicated by a decrease in E. The value of p(fracture) also decreased, whereas that of epsilon(fracture) remained about constant. Destroying cell turgor decreased E and p(fracture) relative to the turgescent control. The E, value also decreased.with increasing transpiration, while p(fracture) and (especially) epsilon(fracture) increased. Water uptake, fracture-fracture had little effect on E, whereas epsilon(fracture) and p(fracture) decreased slightly. Increasing temperature decreased E and p(fracture), but had no effect on epsilon(fracture). Only the instantaneous elastic strain and the creep strain increased significantly at the highest temperatures. A decrease in E indicates decreasing skin stiffness that is probably the result of enzymatic softening of the cell walls of the skin in the ripening fruit, of relaxation of the cell walls on eliminating or decreasing turgor by transpiration and, possibly, of a decreasing viscosity of the pectin middle lamellae at higher temperatures. The effects are consistent with the conclusion that the epidermal and hypodermal cell layers represent the structural "backbone" of the sweet cherry fruit skin.