Comparative Analysis of Yeast Population Dynamics in Spontaneous Fermentation of Barbera Must: Amphorae vs. Steel Tanks

The use of traditional clay containers for winemaking has garnered significant interest in oenology. A notable example is Kakhetian-style wines from Georgia, fermented in Qvevri, which are recognized as premium products worldwide. Although some studies have explored in-amphora wines from an oenological perspective, the impact of earthenware vessels on yeast populations remains under-researched. Spontaneous fermentations of Barbera must (Monferrato, Piedmont, Italy) were carried out in amphorae and stainless steel vats during the 2022 vintage without the addition of SO2. Yeast populations were analyzed using both culture-dependent and a metagenomics approach. On average, fermentations in steel tanks lasted 72 hours longer than those in clay jars. A higher biodiversity, particularly linked to the presence of oxidative yeasts within 72-96 hours from the start of the process, was observed in amphorae compared to steel vessels. The analyses revealed the dominance of four main yeast groups: Metschnikowia pulcherrima, Hanseniaspora uvarum, Hanseniaspora opuntiae, and Saccharomyces cerevisiae. Other minor populations were present but not identified. Non-Saccharomyces yeast species were detected until the end of fermentation in the earthenware vats (approximately 2 g/L residual sugars), whereas S. cerevisiae quickly dominated the fermentation in steel tanks. Despite the persistence of potential high-acetic acid-producing species in amphorae, similar values of volatile acidity (<0.6 g/L acetic acid) were found in both types of wines. In conclusion, the results suggest that earthenware material promotes the formation of a balanced yeast consortium where Saccharomyces and non-Saccharomyces species coexist, completing the fermentation. The environmental conditions in traditional vessels may favour yeast metabolic interactions, thus allowing the production of more complex wines.