Biaxial tensile tests identify epidermis and hypodermis as the main structural elements of sweet cherry skin

Abstract

The skin of developing soft and fleshy fruit is subjected to considerable growth stress, and failure of the skin is associated with impaired barrier properties in water transport and pathogen defence. The objectives were to establish a standardized, biaxial tensile test of the skin of soft and fleshy fruit and to use it to characterize and quantify mechanical properties of the sweet cherry (Prunus avium) fruit skin as a model. A segment of the exocarp (ES) comprising cuticle, epidermis, hypodermis and adhering flesh was mounted in the elastometer such that the in vivo strain was maintained. The ES was pressurized from the inner surface and the pressure and extent of associated bulging were recorded. Pressure : strain responses were almost linear up to the point of fracture, indicating that the modulus of elasticity was nearly constant. Abrading the cuticle decreased the fracture strain but had no effect on the fracture pressure. When pressure was held constant, bulging of the ES continued to increase. Strain relaxation upon releasing the pressure was complete and depended on time. Strains in longitudinal and latitudinal directions on the bulging ES did not differ significantly. Exocarp segments that released their in vivo strain before the test had higher fracture strains and lower moduli of elasticity. The results demonstrate that the cherry skin is isotropic in the tangential plane and exhibits elastic and viscoelastic behaviour. The epidermis and hypodermis, but not the cuticle, represent the structural 'backbone' in a cherry skin. This test is useful in quantifying the mechanical properties of soft and fleshy fruit of a range of species under standardized conditions.

Keywords: Biomechanics; Prunus avium; fracture; mechanical properties; rheology; skin; stiffness.; strain.

Figure 1.

(A) Schematic drawing of the elastometer used for biaxial tensile testing. A hydrostatic pressure is generated by displacing silicone oil using a piston. An increase in pressure causes the ESs of sweet cherries to bulge outwards. The system pressure and height of bulging are measured using electronic pressure and displacement transducers, respectively. (B) The contour of the bulged ES was determined from a cross-section of an imprint and digitized. A range of geometrical models (spheroid, paraboloid and ellipsoid) were fitted to the contour.

Figure 2.

Uniaxial tensile test of a biconcave, dumbbell-shaped, exocarp strip carved from sweet cherry skin. (A) Representative time course of force and strain (ɛ; inset) until fracture. (B) Relationship between force and the strains in the directions of the applied force (axial strain; ) and perpendicular to the applied force (transverse strain; ). Note that the negative results from the marked narrowing of the strip when subjected to a uniaxial load. Data represent means ± SEM (n = 10).

Figure 3.

Representative time courses of the change in pressure (p; A), in strain (ɛ; B), in height of the bulging ES of sweet cherry fruit (B, inset), and in the modulus of elasticity (E; C) during a biaxial tensile test. The last six data points represent points recorded after fracture. (D) Pressure/strain diagram of a representative ES until fracture.

Figure 4.

Frequency distributions of the modulus of elasticity (E; A), the pressure at fracture (p_fracture; B) and the strain at fracture (ɛ_fracture; C) of ESs of sweet cherry fruit in 2012. Insets: probability plots of cumulative frequency distributions of E, p_fracture and ɛ_fracture of the same cultivar in 2012 (n = 84) and 2013 (n = 115).

Figure 5.

Relationship between the modulus of elasticity (E; A), the pressures at fracture (p_fracture; B) and the strain at fracture (ɛ_fracture; C) of pairs of ESs excised from opposite shoulders of the same sweet cherry fruit. Data represent means ± SEM (n = 24).

Figure 6.

Relationship between pressure (p) and longitudinal or latitudinal strains (ɛ) of ESs excised from sweet cherry. The longitudinal strain is that in the direction of the stylar scar/pedicel axis, and the latitudinal strain is perpendicular to it. The two strains were measured using a square pattern of dots applied to the ES. Inset: relationship between the biaxial strain calculated from the height of the bulging ES and the biaxial strain measured using the dot pattern. The regression line has a slope of 2.16 ± 0.06 (r² = 0.99***). Data represent means ± SEM (n = 10).

Figure 7.

Effect of diameter of ESs excised from sweet cherry fruit on the modulus of elasticity (E; A), pressure at fracture (p_fracture; B) and the strain at fracture (ɛ_fracture; C). Data represent means ± SEM (n = 20).

Figure 8.

Effect of thickness of ESs excised from sweet cherry skin on the modulus of elasticity (E; A), the pressure at fracture (p_fracture; B) and the strain at fracture (ɛ_fracture; C). Data represent means ± SEM (n = 20).

Figure 9.

Representative time course of strain (ɛ; A) and pressure (p; A, inset) of an ES excised from sweet cherry skin during a biaxial, creep-relaxation test. The pressure was increased during the initial loading phase, held constant during the holding phase and decreased during the subsequent unloading phase. (B) Strain during the holding phase is redrawn on a log-transformed timescale.