Yeast: the organisms doing the work

Yeast are on grape skins, in vineyard soil, on equipment and throughout a winery building. A great deal of the resident population in an old cellar lives on the walls and the presses rather than arriving with the fruit.

The choice between inoculating with a cultured strain and letting the resident population work is one of the most argued-about decisions in winemaking, and one where the evidence is more equivocal than either side usually allows.

Grape must carries a diverse microbial population: Hanseniaspora, Candida, Metschnikowia, Pichia and others, alongside relatively small numbers of Saccharomyces.

In a spontaneous fermentation these non-Saccharomyces species dominate the first days, contributing their own metabolites, and are then progressively inhibited as ethanol rises. Most tolerate only a few percent alcohol.

Saccharomyces cerevisiae tolerates 15% and above and therefore finishes essentially every dry fermentation, which is why the succession converges even in wineries that never inoculate.

Cultured strains are selected for specific properties: reliable completion, alcohol tolerance, low hydrogen sulfide production, cold tolerance, foam behaviour, and in some cases the release of particular aroma compounds — the thiol-releasing strains used for Sauvignon Blanc being the best-known example.

Nitrogen is the limiting nutrient in most fermentations. Yeast assimilable nitrogen below roughly 150 mg/L is associated with sluggish fermentation and with hydrogen sulfide production, which is why nutrient additions are so routine.

Temperature bounds the whole process: too cold and fermentation stalls, too warm and yeast stress and die, typically above the mid-30s Celsius.

It is the mechanism behind stuck fermentation, behind reductive sulfur faults, and behind a great deal of the aromatic difference between two wines from the same fruit.