Failure Tests of Intact Grape Berry Skins and What They Tell Us About Berry Cracking

We developed a “Berry Balloon System” (BBS) in order to perform tensile failure tests on intact grape berry skins in their original 3D conformation.  The mode (cell fracture) and position (stylar end in Flame Seedless) of skin failure in these lab tests were the same as those found in the field.  Pressure, stress, and overall strain at failure of Flame Seedless berries progressively decreased over berry development, consistent with the observation that cracking susceptibility in this variety increases over time.  Over three field seasons, excessive irrigation (%ETc) caused a clear increase in vine water status (midday stem water potential, SWP) compared to grower irrigation (%ETc), but did not cause increased fruit cracking.   Irrigation cutoff prior to veraison caused a clear decrease in SWP and significantly more cracking than any other irrigation treatment.  Under this treatment, vines experienced higher daily fluctuation in SWP, and hence there may have been higher daily fluctuations in berry size.  Strain hardening of the berry skin was demonstrated using repeated BBS tests on the same skin, and may explain why deficit irrigation, which we hypothesized should have reduced stress and strain on the berry skin, actually caused higher berry cracking.  Ethephon sprays, used to increase berry pigmentation, increased berry cracking compared to water-sprayed controls, but skin mechanical properties were not affected.  However, exposing the berry skin to an ethephon solution (1.5 mM) during BBS testing caused significantly lower skin stress and strain at failure. The failure to detect differences in skin properties using field sampled berries may have been due to the fact that only sound (non-cracked) berries could be used for BBS tests, and may not have been typical of the berries that did crack in the field. The BBS was also used to compare skin mechanical properties of different grape genotypes. In the intact state, the skins of berries from all genotypes were found to be under significant “preload” strain caused by internal pressure of the flesh. Genotypes ranged from 3.7% to 13.1% in preload strain and also varied substantially in the relation between stress and strain.  Among all mechanical properties measured, the increase in skin strain from the intact condition to the cracking threshold was best correlated (R²=0.65) with cracking susceptibility as measured in a soaking test.