The timing of grape harvest is critical in winemaking, as grape maturity directly influences composition, sensory attributes, and overall quality of the final wine. The aim of this study was to test the effect of the grape maturation stage on the yeast microbiome, which plays a key role in spontaneous fermentation and wine typicity.

Grapes from different varieties and regions were harvested in three different maturation stages. The must mixtures were fermented spontaneously, and the microbial population was assessed during fermentation. Yeast and mould diversity were assessed at the species level via an RFLP PCR of the 5.8S-ITS region. The fermentation potential of the different S. cerevisiae strains and non-Saccharomyces species was determined. The results showed that the proportion of microorganisms in the grape varied significantly during the different maturation stages: while in the first stages, mould predominated in the grape population, during the optimum and late stages, fermentative yeasts were present at a higher proportion. Generally, in the middle of the fermentation process, non-Saccharomyces yeasts, such as Torulaspora delbrueckii and Hanseniaspora spp., predominated at around 50%; at the end of the fermentation process, strains of S. cerevisiae predominated (around 100%). This study allows us to conclude whether the maturity stage of the grape impacts the yeast population and diversity.