🍷 Wild yeast

Wild yeasts (native, indigenous or ambient yeasts) are the yeasts that live on grapes, in the vineyard and in the cellar, and start fermentation on their own when the winemaker has not added a bought culture. Making wine this way is called spontaneous fermentation. It is rarely the work of one species. More often it is a relay.

Researchers think the first yeasts come from the soil, the surface of the vine and the winery itself. In one study every species found in the cellar was also found in the vineyard and then in the fermenting must. Damaged or unhealthy berries can carry more species.

The early runners are not Saccharomyces: Hanseniaspora, Metschnikowia and Candida. They tolerate little alcohol, so they drop out as it rises. In steel tanks one team counted 7 to 12 species at the start. In the middle only 1 to 5 were left, and at the end six. The finish usually belongs to Saccharomyces cerevisiae, which copes better with high alcohol and low oxygen.

The risks are real. In a study from organic vineyards no wild isolate finished a dry wine on its own; sugar stayed above 28 g/L. In another cellar the pause before fermentation took off was about 5 days in 2010 and 11 days in 2011. That later ferment was still not dry on day 14 and carried around 0.65 g/L of acetic acid. The authors concluded that the longer the lag, the likelier a failure.

Cultured yeast gives a predictable result, which is why most wineries use it. People turn to wild yeast when they want a taste of place and a cheaper start. There is a middle road too: mixed starters that pair a wild species with Saccharomyces. For the wider family, see yeast.