Factors Affecting Mechanical Properties of the Skin of Sweet Cherry Fruit

Abstract

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 (e) due to bulging were quantified and the modulus of elasticity [E (synonyms elastic modulus, Young’s modulus)] was calculated. In a typical test, e increased linearly with p until the skin fractured at pfracture and efracture. Stiffness of the skin decreased in ripening late stage III fruit as indicated by a decrease inE. The value of pfracture also decreased, whereas that of efracture remained about constant. Destroying cell turgor decreased E and pfracture relative to the turgescent control. The E value also decreased with increasing transpiration, while pfracture and (especially) efracture increased. Water uptake had little effect on E, whereas efracture and pfracture decreased slightly. Increasing temperature decreased E and pfracture, but had no effect on efracture. Only the instantaneous elastic strain and the creep strain increased significantly at the highest temperatures. A decrease inE 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. Rain cracking is a limitation in sweet cherry production almost wherever this high-value crop is grown (Christensen, 1996). Fruit cracking in many species, including grape (Vitis vinifera L.) and sweet cherry, is thought to be related to water uptake by the fruit. Thus, any net water uptake necessarily increases fruit volume and subjects the skin to additional strain (Considine and Brown, 1981). 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 characteristics— these 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., 2002, 2005; Beyer and Knoche, 2002; Weichert 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). Investigating the mechanical properties of the fruit skin is challenging. First, the sweet cherry is about spherical and hence, any strain of its skin during growth and water uptake is biaxial. Thus, a biaxial laboratory test is essential, if the in vivo strain in the skin is to be mimicked with any degree of accuracy. Second, uniaxial tests of skin strips result in marked narrowing of the sample as tension is applied. This is indexed by a high Poisson ratio and a gross overestimation of fracture strain and extensibility, and a gross underestimation of the E (Br€uggenwirth et al., 2014). Third, the sweet cherry skin is already markedly strained in vivo and these elastic and viscoelastic strains are reversible (Grimm et al., 2012). Thus, on excision the skin relaxes almost instantaneously and special care has to be taken to prevent this relaxation (Br€uggenwirth et al., 2014). The first biaxial tensile test for sweet cherry fruit skin was described by Bargel et al. (2004). Excised fruit skin segments were pressurized from the inner side and the extent of bulging for each pressure increment was quantified. Br€uggenwirth et al. (2014) modified the test to prevent relaxation of the fruit skin after excision, thereby maintaining the in vivo strain in the fruit surface. Furthermore, the skin segments were pressurized using silicone oil rather than water thereby eliminating any uncontrolled water uptake and bursting of cells on the inner side (Simon, 1977). This test protocol offers a standardized test for identifying mechanical properties of sweet cherry fruit skins. Among the factors known to affect mechanical properties of fruit is the degree of ripening, especially during the later stages of development (Brummell, 2006; Christensen, 1996). Also, the fruit’s turgescence is expected to be affected by the balance between transpiration and water uptake (Considine and Brown, 1981). Furthermore, many fleshy fruits, including sweet cherries, are soft to the touch when warm, but firmer at low temperatures (M. Br€uggenwirth and M. Knoche, unpublished Received for publication 17 Sept. 2015. Accepted for publication 27 Oct. 2015. This researchwas funded in part by a grant from theDeutsche Forschungsgemeinschaft. We thank Dieter Reese and Christoph Knake for constructing, engineering, and programming the elastometer, Friederike Schroeder andSimonSitzenstock for technical support, Bishnu P. Khanal and Sandy Lang for helpful discussion and useful comments on an earlier version of this manuscript. Corresponding author. E-mail: moritz.knoche@obst.uni-hannover.de. J. AMER. SOC. HORT. SCI. 141(1):45–53. 2016. 45 data). Also, increasing temperature markedly increased cracking (for review, see Christensen, 1996). As with all materials, one would expect the mechanical properties of a fruit skin to be affected by temperature. The objectives of this study were to quantify the effects of 1) ripeness stage; 2) turgor, transpiration, and water uptake; and 3) temperature on the mechanical properties of sweet cherry fruit skin using a biaxial tensile test. Materials and Methods PLANT MATERIAL. Fruit of sweet cherry cultivars Adriana, Kordia, Sam, and Sweetheart were harvested at commercial maturity from field-grown or glasshouse-grown trees at the Horticultural Research Station of the Leibniz University in Ruthe, Germany (lat. 52 14#N, long. 9 49#E) in the 2012, 2013, and 2015 growing seasons. Trees were grafted on ‘Gisela 5’ rootstocks (Prunus cerasus L. · Prunus canescens Bois). Fruit were selected for freedom from visual defects and for uniformity of development based on size and color. In a preliminary experiment, the effect of storing sweet cherry at 2 C and at water vapor concentrations close to saturation for up to 30 d on mechanical properties of the fruit skin was investigated. During the first 4 d of storage, stiffness of the skin increased as indexed by a 81% increase in the E. The pressure at fracture slightly increased (+10%) and the strain at fracture decreased [–26% (M. Br€uggenwirth, unpublished data)]. There was no further change in mechanical properties between 4 and 30 d of storage. To avoid potential artifacts resulting from storage, all fruit were harvested in the mornings, covered by a damp paper towel, brought to the laboratory, and used in experiments within 2 h. GENERAL EXPERIMENTAL PROCEDURE. Biaxial tensile tests were performed using the elastometer described in detail by Br€uggenwirth et al. (2014). Briefly, a brass washer (12 mm i.d.) was mounted on one of the two shoulders of a sweet cherry fruit using cyanoacrylate adhesive (Loctite 406; Henkel/Loctite Deutschland, Munich, Germany). Next, an exocarp segment [ES (synonym fruit skin segment)] comprising the exocarp and some adhering mesocarp [thickness (mean ± SE) 2.5 ± 0.0 mm] was excised by cutting horizontally beneath the washer using a razor blade. This procedure preserved the in vivo strain of the skin after excision (Br€uggenwirth et al., 2014; Grimm et al., 2012; Knoche and Peschel, 2006). The brass washer with the ES was mounted in the elastometer, whose chamber was then filled with silicone oil (AK10; Wacker Chemie, Munich, Germany) and any visible air bubbles were carefully removed. The chamber was pressurized by driving a motorized piston into the oil. This displaced the oil, reduced the chamber volume, and caused the ES to bulge outward. Pressure in the chamber was recorded using a pressure transducer (Typ 40PC100G; Honeywell International, Morristown, NJ) and the deflection of the bulging ES was recorded by the displacement transducer (KAP-S/5N; AST Angewandte System Technik, Wolnzach, Germany) of a universal material testing machine (BXCFR2.5TN; Zwick Roell, Ulm, Germany). Unless specified otherwise, pressure was increased continually until the ES fractured. The pressure at the moment of failure is referred to as the fracture pressure [pfracture (kilopascals)] and the area (i.e., biaxial) strain at failure as the fracture strain [efracture (square millimeters per square millimeter)]. The strain (e) was calculated as the fractional increase in surface area; i.e., the increase in area [DA (square millimeters)] due to the bulging of the ES, relative to the initial area [A0 (square millimeters)] before bulging: