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

Abstract

Sweet cherry and other fleshy fruit crack when the surface is exposed to water. Osmotic water uptake is believed to increase fruit volume and hence surface area, thereby subjecting the skin to stress and strain. The objectives were to (1) establish a standardized biaxial tensile test that simulates the in vivo strain of the skin and (2) characterize its mechanical properties. A bulging device was used to pressurize skin segments. Pressure and extent of bulging were monitored. The data demonstrate that (1) epidermis and hypodermis form the structural backbone of the fruit skin and (2) deformation is reversible.

Malic Acid Promotes Cracking of Sweet Cherry Fruit

“…Compared with the control treatment, fruit incubated in glucose solution required more water to crack which would be interpreted as mechanically more robust, less susceptible fruit. Direct effects of glucose on the mechanical properties of the fruit skin that forms the structural backbone of the sweet cherry (Brüggenwirth et al, 2014), are unlikely, particularly within the short time period of the experiment. Any indirect effects for example on strain of the fruit skin would be essentially osmotic and, therefore, equally expected for fructose.…”

Factors Affecting Mechanical Properties of the Skin of Sweet Cherry Fruit

“…If the limit of extensibility of the fruit skin is exceeded, the fruit cracks. On the basis of this concept, the following two categories of factors will affect cracking susceptibility: 1) the fruit's water uptake/loss characteristicsthese determine the rate and total amount of water accumulated by the fruit, and 2) the fruit's mechanical characteristics, particularly those of the principle load-bearing layer of the fruit-its skin (Br€ uggenwirth et al, 2014). Water uptake by sweet cherries has been investigated in some detail, including that through the fruit surface (Beyer et al, , 2005Weichert and Knoche, 2006) and that through the pedicel vasculature (Hovland and Sekse, 2004;Measham et al, 2010).…”

“…Water uptake by sweet cherries has been investigated in some detail, including that through the fruit surface (Beyer et al, , 2005Weichert and Knoche, 2006) and that through the pedicel vasculature (Hovland and Sekse, 2004;Measham et al, 2010). In contrast, there have been only a few studies that have addressed the mechanical properties of the sweet cherry skin (Bargel et al, 2004;Br€ uggenwirth et al, 2014;Knoche and Peschel, 2006).…”

Water Uptake Through the Surface of Fleshy Soft Fruit: Barriers, Mechanism, Factors, and Potential Role in Cracking

“…This aspect is technically challenging and has received little attention till now. Experimental approaches used to address the mechanical properties include fruit pressure probe techniques (Bernstein and Lustig 1985;Lang and Düring 1990) and biaxial tensile tests (Bargel et al 2004;Brüggenwirth et al 2014). Using the fruit pressure probe technique, bursting pressures, strains, and volumetric moduli of elasticity were quantified in different grape berry cultivars (Bernstein and Lustig 1985;Lang and Düring 1990).…”

“…Using the fruit pressure probe technique, bursting pressures, strains, and volumetric moduli of elasticity were quantified in different grape berry cultivars (Bernstein and Lustig 1985;Lang and Düring 1990). Biaxial tensile tests were employed by Bargel et al (2004) and Brüggenwirth et al (2014) to quantify the modulus of elasticity, fracture strains, and fractures pressures of excised sweet cherry fruit skin. No attempts to relate a fruit skin's mechanical characteristics to its anatomical and physiological one seem to have been published.…”