Abstract
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 cryo‑scanning 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.
Highlights
The production of specific wine types is based on the application of protocols that are purposefully aimed at allowing harvested grapes to lose water
In a recently published review, Lufu et al (2020) report that the factors which determine the difference in water loss among species and among cultivars of the same species are fruit surface-areato-volume ratio, the surface structure of the fruit, and cuticle thickness and composition
Our results indicate that these factors are likely involved in the modulation of wine grape berry postharvest dehydration, along with the observed differences in epicuticular wax particle morphology, epidermis, and hypodermis thickness, as has already been observed for table grapes and raisins (Riva and Peri, 1986; Crisosto et al, 2001; Zhang et al, 2001; Ramming, 2009)
Summary
The production of specific wine types (sweet, reinforced) is based on the application of protocols that are purposefully aimed at allowing harvested grapes to lose water. Besides the intensity of the dehydration, weight loss (WL) rate plays an important role, as clearly demonstrated by Bonghi et al (2012), who showed that flavonoid metabolism and related gene expression are variably affected by these two factors in cv ‘Raboso Piave’ berry skins. Cuticle resistance to water loss seems to be mainly due to the presence and nature of waxes, which occur on the epidermis, but can be deposited in cells (Esau, 1977), giving the fruit a nice luster. This property appears to be primarily determined by the particular mixture of intra-cuticular and epicuticular waxes and their arrangement, rather than their amount. The aim of the present paper was to identify the different morphological and histological features of the outer pericarp layers of four wine grape varieties at commercial harvest, as well as the changes occurring in these fruit tissues after partial postharvest dehydration
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