Q: Is yeast added to all wine?
—Greg, Woolwich Township, N.J.
A: Yeast is essential to the winemaking process: It converts the sugar in grapes to alcohol during fermentation. But you asked if yeast is added to all wine, and the answer is no.
But some winemakers prefer to use native yeasts (sometimes called "wild" or "indigenous"), which means they don't add any commercial yeasts. Rather, they let ambient strains of yeast that are already naturally present in the vineyard or winery go to work. Some winemakers think these unique yeasts are a truer expression of terroir, or make their wine more distinct. It can be risky, however, as wild yeast fermentations can be unpredictable.