Ben-Min Chang, Yun Zhang, and Markus Keller*
*Washington State University, 24106 N. Bunn Rd., Prosser, WA 99350 (mkeller@wsu.edu)

Grape berry growth after veraison is regulated by internal pressure and the rheological properties of the skin. Internal pressure originating from cell turgor is the driving force for berry expansion. Skin elasticity, the combination of cuticle and a few cell layers, works against that internal pressure to constrain expansion. Berries may split if the skin accumulates excessive tensile stress. In this study, the mechanical response to internal pressure in berries from different cultivars was recorded using a custom-built injection tester. Based on the pressure-strain curve, the splitting resistance was determined. Further, the minimum internal pressure for berry growth and skin elasticity was estimated by determining yield strength. Compressive strain as an indicator of softness was also measured using a skin fold caliper. Hard-green Merlot, Syrah, and Zinfandel berries behaved as a brittle material, while soft berries had properties of viscoelastic material. In contrast, Concord grapes behaved as viscoelastic material even when the berries were still hard and green. The splitting resistance and yield strength generally correlated negatively with berry total soluble solids, except there was no correlation with yield strength in Concord. Compressive strain increased abruptly in berries with less than 10 Brix and in ripe berries. Since splitting resistance and yield strength were relatively stable during ripening, the abrupt increase of compressive strain suggested that internal pressure decreased due to dehydration in overripe berries. This study suggests that the minimum turgor for berry growth decreased at the onset of ripening and the responses of splitting resistance and yield strength to internal pressure remained unchanged by late-season berry dehydration. Thus, grape berries are susceptible to splitting after the onset of softening and before being fully ripe.  

Funding Support: Chateau Ste. Michelle Distinguished Professorship, Washington State Grape and Wine Research Program, Washington State University Graduate School