Ultrastructural analysis of berry skin from four grapes varieties at harvest and in relation to postharvest dehydration

In several production areas, dessert (sweet) and reinforced wines are made after a more or less intense dehydration of harvested grapes. The dehydration process depends on several factors, including the size, morphology and anatomy of the berries, all genetically defined traits that can be affected by vineyard management and microclimate conditions. At harvest, berry outer surface and skin ultrastructural features of cvs Aleatico, Sagrantino, Sangiovese and Trebbiano berries were investigated in a frozen-hydrated state using cryoscanning electron microscopy (Cryo‑SEM). The berries were subjected to postharvest dehydration at 23 °C for twelve days and the differences in weight loss were determined. In terms of weight loss rate, Aleatico and Trebbiano were the fastest and the slowest respectively. Therefore, the ultrastructural changes of berry skin of these two varieties were also investigated at the end of the dehydration process. At harvest, the structure of the epicuticular waxes on the skin outer surface differed between berries of different cultivars. The thickness of the cuticle, epidermis and hypodermis was also found to be significantly different, with Trebbiano berries having the thickest skin. At the end of the dehydration process all the measured parameters decreased, in particular Aleatico epicuticular wax, hypodermis and cuticle thickness, as well as the hypodermis cell wall and the mesocarp parenchyma cell area. The high weight loss rate recorded for Aleatico can be partly explained by the thickness of the berry skin at harvest, which was significantly thinner than that of Trebbiano, as well as by other skin-related morphological and histological factors possibly affecting permeability.