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

Postharvest dehydration is a widely employed technique in winemaking to enhance sugar concentration and secondary metabolites from grapes. Different grape varieties exhibit varying responses in terms of dehydration rate and the resulting chemical composition. Additionally, the harvest time, reflecting different ripeness levels, can significantly influence the process [1]. This study aimed to investigate the impact of ripening level on the composition of must and the resulting straw wine. Vitis vinifera L. cv ‘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). Grapes were subjected to controlled postharvest dehydration until achieving 20% grape weight loss. Musts were produced and sugar concentration, readily assimilable nitrogen, pH, titratable acidity were evaluated. Micro scale winemaking trials were performed with skin maceration, monitoring the inoculated fermentation by weight decrease. At the end of alcoholic fermentation, the wines were racked, stabilized and bottled. The sugar concentration in musts increased from the samplings at LR to MR, while decreased in grapes harvested at HR. This could be due to the glucose respiration occurring during the withering that seemed to be facilitated for more ripe grapes [2]. Readily assimilable nitrogen varied by variety: it increased in ‘Corvina’, but decreased in the others varieties. The pH rose of about 0.3 units from LR to HR in all varieties, while the titratable acidity remained stable in ‘Corvina’ and it reached its lowest level in the HR harvested sample for ‘Moscato di Scanzo’ and ‘Nebbiolo’. The musts obtained from grapes harvested at LR completed the alcoholic fermentation within 10–11 days for all the grape varieties. Similar fermentation rates were found for ‘Corvina’ musts from withered grapes harvested at MR and HR being complete within 12–13 days. In contrast, the fermentation was completed in 16 days for ‘Nebbiolo’ musts, while the musts from ‘Moscato di Scanzo’ withered grapes collected at MR and HR did complete the fermentation at day 20. From an applicative point of view, the grapes harvested at the higher ripening level resulted more susceptible to pathogen infection suggesting that the ripest grapes are less adapted to the withering even if this process took shorter time. This study expands the knowledge on the varietal effect in grape postharvest dehydration, covering red varieties majorly used in this production technique.