Impact of Grape Harvest Time on Wild Yeast Biodiversity and Its Influence on Wine Fermentation

Grape harvest time influences the berry composition, which impacts the organoleptic wine characteristics. Winemakers monitor technological, phenolic, and aromatic grape maturity to decide the harvest date. Little is known about the evolution of yeast ecology of grapes during the maturation period. The microbiota involved in the fermentation impacts the wine composition and characteristics; therefore, changes in grape biodiversity could have an impact in fermentation kinetics and aroma compound formation. In this study, the yeast biodiversity of Grenache Noir and Carignan grapes from Montsant DO (Denomination of Origin) were analyzed at different ripening stages to assess how harvest time influences microbiota. The fermentation performance of the yeasts obtained was studied at both laboratory and pilot scales to evaluate the impact of these yeasts, both in pure and mixed cultures, on the wine's chemical and aromatic composition as well as its sensory impact. The results indicated that early harvest favored a higher diversity of non-<i>Saccharomyces</i> species, whereas in more mature grapes, <i>Saccharomyces cerevisiae</i> species was dominant. The isolated strains of <i>Saccharomyces</i> differed in their fermentation performances, as well as ethanol content and acidity of wine. In general, they produced higher concentration of fermentative volatile compounds than a commercial strain. The non-<i>Saccharomyces</i> yeasts in mixed fermentations with the <i>S. cerevisiae</i> strains also impacted wine composition and characteristics, leading lower ethanol content or enhancing aromatic balance and sensory equilibrium. The results highlight that grape harvest timing influences microbial diversity and fermentative performance and thus should be considered to better determine the optimum harvest date and ensure consistent wine characteristics.