Influence of Sequential Harvest on Chemical Composition of Merlot Wines

The influence of grape maturity over three consecutive years (2020–2022) on Merlot (Vitis vinifera L.) juice and wine chemical composition was investigated. Grapes were harvested at three time points (H1, H2, and H3) in weekly intervals. Despite the fact that vintage (environmental conditions) had a predominant effect on juice and wine chemical composition, clear separation of samples according to the harvest date was observed in all three vintages. Compounds with the highest contribution towards harvest date separation were common maturity-related juice and wine variables (titratable acidity, pH) as well as some volatiles, whereas differences in total soluble solids between dates were minor and often insignificant. In particular, concentrations of 3-isobutyl-2-methoxypyrazine (IBMP), (Z)-3-hexenol, and 1-hexenol in wines decreased with delayed harvest. All the more, concentrations of 3-mercaptohexanol (3MH) were the lowest in wines from H3 in all three years, whereas concentrations of 3-mercaptohexyl acetate (3MHA) and 4-mercapto-4-methylpentan-2-ol (4MMP) were not influenced by harvest date. Other compounds, such as esters and higher alcohols, with the exception of 1-propanol, did not exhibit a common trend related to the harvest date across three vintages. These results indicate that, during late ripening, harvest-related shifts in juice and wine composition occur even when differences in berry sugar concentration (TSS) at harvest are minor.