Impact of Withering on Phenolic Composition in Red Grape Varieties

Passito wines are a particular category of special wines obtained from the vinification of grapes subjected to withering. This process leads to an increase in sugar content and variations in the secondary metabolites of the grapes [1]. Post-harvest withering is influenced by various factors, such as temperature (T) and relative humidity (RH), and its duration determines the amount of water loss from the grapes and, consequently, the weight loss (WL). Grape varieties may respond differently to withering, in terms of both rate and WL, and of the physicochemical changes that occur in the berries. Withering also has a relevant effect on the secondary metabolites of the grape, significantly impacting phenolic compounds responsible for the color, astringency, and bitterness of red wine. These metabolites undergo variations during withering, due both to a concentration effect and degradation phenomena [2]. The present study aims to deepen the varietal impact on post-harvest withering and to investigate the phenolic composition during withering. Five Italian red grape varieties ('Aleatico', 'Corvina', 'Moscato di Scanzo', 'Nebbiolo', and 'Sagrantino') used for the production of Italian passito wines with designation of origin were considered. The grapes were subjected to post-harvest withering under controlled conditions (T 14-15 °C, RH 40-50%). The withering kinetics were monitored, and 4 samplings were conducted (harvest, 10, 20, and 30% WL). For each point, the total contents of flavonoids (FLVs) and anthocyanins (ACNs), and proanthocyanidins were determined. A different withering kinetics was observed depending on the grape variety: 'Moscato di Scanzo' and 'Aleatico' grape showed a higher withering rate (approximately 1% WL/day), while slower withering was observed for 'Corvina' grape (less than 0.4% WL/day). The desired WL, equal to 30%, was achieved in approximately 30, 50, 60, and 90 days respectively for the 'Moscato di Scanzo' and 'Aleatico' pair, then 'Sagrantino', 'Nebbiolo', and 'Corvina'. As expected, significant differences were highlighted in the phenolic concentration of the investigated varieties: 'Sagrantino' grape were the richest in FLVs (6.14±0.77 g/kg of grape at harvest), while 'Moscato di Scanzo' grape had the highest concentration of ACNs (1.75±0.12 g/kg of grape at harvest). During withering, the total FLV content, expressed in g of catechin per kg of grapes, increased with increasing weight loss for all varieties. However, a decrease in the concentration of total FLVs per berry was observed except for 'Corvina' grapes, where total FLVs remained stable during withering. For this cultivar, the level of total ACNs already decreased at 10% WL, while only slight fluctuations were observed for the other varieties. This study contributes to the optimization of passito wine production by enhancing knowledge of the role played by withering in relation to grape variety, thus supporting the wine industry.