Impact of grape ripening and post-harvest withering on must composition and fermentation kinetics

Postharvest dehydration is a widely adopted winemaking technique aimed at increasing sugar concentration and secondary metabolites in grapes. Different grape varieties respond differently in terms of dehydration rate and resulting chemical composition. Moreover, harvest timing, which reflects varying ripeness levels, can significantly influence this process [1]. This study investigated how ripening level affects the composition of must and the resulting straw wine. Vitis vinifera L. cultivars ‘Corvina,’ ‘Moscato di Scanzo,’ and ‘Nebbiolo’ were harvested at three ripeness levels: low (LR; 18°Bx), medium (MR; 21°Bx), and high (HR; 23°Bx). The grapes underwent controlled postharvest dehydration until a 20% weight loss was achieved. Musts were then produced and analyzed for sugar concentration, readily assimilable nitrogen, pH, and titratable acidity. Micro-scale winemaking trials were conducted with skin maceration, and the inoculated fermentation was monitored by weight loss. At the end of alcoholic fermentation, wines were racked, stabilized, and bottled. Sugar concentration in the musts increased from LR to MR but decreased in grapes harvested at HR. This reduction may be attributed to glucose respiration during withering, which appeared more pronounced in riper grapes [2]. Readily assimilable nitrogen showed varietal differences: it increased in ‘Corvina’ but decreased in ‘Moscato di Scanzo’ and ‘Nebbiolo.’ The pH increased by approximately 0.3 units from LR to HR across all varieties, while titratable acidity remained stable in ‘Corvina’ and reached its lowest level in HR samples of ‘Moscato di Scanzo’ and ‘Nebbiolo.’ Musts from grapes harvested at LR completed alcoholic fermentation within 10–11 days for all varieties. Similar fermentation rates were observed in ‘Corvina’ musts from withered grapes harvested at MR and HR, with completion within 12–13 days. In contrast, ‘Nebbiolo’ musts finished fermentation in 16 days, while those from ‘Moscato di Scanzo’ grapes harvested at MR and HR completed fermentation in 20 days. From a practical perspective, grapes harvested at higher ripening levels were more susceptible to pathogen infections, suggesting that riper grapes are less suitable for the withering process despite the shorter dehydration time required. This study enhances the understanding of varietal effects during grape postharvest dehydration, focusing on red grape varieties commonly used in this production technique.