Rain cracking limits the production of sweet cherries in all areas of the world when rainfall occurs during the harvest season (Christensen, 1996). Fruit cracking is thought to be related to water uptake into the fruit. When the volume of the fruit increases, the skin is strained, and this must eventually lead to mechanical failure. On the basis of this concept, water uptake into the fruit and the mechanical properties of the skin are the most critical factors in cracking.
Only a few studies have addressed the mechanical properties of the skin (Bargel et al., 2004; Brüggenwirth and Knoche 2016a, 2016b; Brüggenwirth et al., 2014). Because of the high Poisson’s ratio of the sweet cherry fruit skin, realistic experiments require biaxial tensile tests such as were first described by Bargel et al. (2004). In this test, an excised segment of the fruit surface is pressurized from its inner side. As a result, the segment bulges and its surface area increases thereby mimicking the increase in area associated with water uptake into the fruit (Brüggenwirth et al., 2014). Compared with the original protocol, the test procedure has been refined by maintaining the in vivo strain of the fruit skin and by pressurizing the exocarp segments (ES; synonym fruit skin segments) with silicone oil (AK10; Wacker Chemie, Munich, Germany) rather than with water (Brüggenwirth and Knoche, 2016a, 2016b; Brüggenwirth et al., 2014). Using this protocol, the pfracture were of similar magnitude to the turgor pressures reported for individual cells or those determined at the whole-fruit level (Knoche et al., 2014; Schumann et al., 2014). However, the εfracture (range 0.17 to 0.22 mm2·mm−2) was consistently higher in the biaxial tensile tests than in immersion assays where fruit was immersed in water to determine its cracking susceptibility (Brüggenwirth and Knoche, 2016a). In these assays, a strain may be calculated from the water uptake required for 50% of the immersed fruit to crack. Typical (area) strain values in such immersion assays are in the order of 0.003 to 0.01 mm2·mm−2 (Brüggenwirth and Knoche, 2016a). The mechanistic basis for this difference in strain between the biaxial tensile tests and the immersions assays is unknown. One possible explanation would be the difference in the tfracture. In the immersions assays, fruit required on average at least 3 h to fracture, whereas in a biaxial tensile test, the ES fractured within ≈2 min. To our knowledge, the effect of tfracture on the mechanical properties of fruit skins is unknown.
The objective of this study was to quantify the effect of tfracture on the measured mechanical properties of excised sweet cherry fruit skins. We varied the tfracture by changing the prate in the biaxial tensile tests.
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