Analysing the vineyard soil as a natural reservoir for wine yeasts

The cultivable microorganisms isolated from grapes and soils of two contiguous vineyards were analysed. Two types of management system were tested in each vineyard: conventional tillage (CT), and no-tillage with natural green cover vegetation (NV), both under semi-arid rainfed conditions. The main objective was to gather evidence as to whether the yeasts present in ripe grapes before harvest and those found in spontaneous wine fermentations came from the vineyard soil which could then be regarded as a natural reservoir for these yeasts. Bacteria isolated from the grapes all corresponded to three genera (Pseudomonas, Stenotrophomonas, and Bacillus) that were very abundant in soil samples taken just before grape harvest, indicative of probable contamination of the grapes with soil microorganisms. The amounts of fermentative yeasts in vineyard soil increased significantly during the dates close to harvest. Some yeasts were isolated from soils and spontaneous fermentations (Saccharomyces and Lanchacea), while others were only isolated from fermentations (Hanseniaspora, Metschnikowia, and Pichia) or from soils (Torulaspora). Saccharomyces yeasts were isolated from vineyard soil only after grape harvest, to thereafter become undetectable. The analysis of sterile-must fermentations inoculated with soil samples showed that soil was not the origin of the most abundant fermentative yeasts in spontaneous grape fermentations (Saccharomyces and Hanseniaspora), regardless of the soil management system applied. In contrast, other fermentative wine yeasts such as Lanchacea and Torulaspora seemed to be permanently resident in the vineyard soil, especially in the NV vineyard. Hence, contamination of the grapes with vineyard soil may increase yeast biodiversity during spontaneous fermentation, which could affect the wine's organoleptic quality.