What flavors in wine come from yeast?

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Dear Dr. Vinny,

What flavors in wine come from yeast?

—Ana, Oslo, Norway

Dear Ana,

Yeast is essential to the production of wine—it's responsible for converting sugar to alcohol during fermentation. Fermentation unlocks all those flavors and aromas inside the grapes, which is what makes wine so magical. It’s also why beer, cheese, chocolate and pickles taste so amazing.

Some winemakers don’t add any yeast, rather letting the ambient or native yeasts in the vineyard and winery take over. Others choose to inoculate with cultured yeast strains, which can act a little more predictably. The yeast itself doesn’t have a flavor that will remain in the wine, but some yeasts are known to ferment quickly or slowly, and might enhance floral, fruit or mineral notes. Again, these aren’t flavors the yeasts are adding, but rather what they're revealing from the grapes.

However, yeast can serve another purpose in winemaking, long after fermentation is complete and the yeast cells have died. Those dead yeast cells, along with other sediment, form what's known as "lees," and they can contribute to a wine's complexity as well. If the wine is left to age on the lees (a winemaking process known as sur lie aging), the dead yeast cells leftover from fermentation can enhance an aging wine with added richness, flavor and aroma complexity, and can also bind with excess tannins. The degree to which the lees impact an aging wine can also be magnified by stirring them up, a process known as "bâtonnage."

Now you might be wondering, what about all those deliciously "yeasty" Champagnes? Where does that wonderful bread dough aroma come from? Champagnes and other sparkling wines made in the traditional method go through a very extended sur lie aging process that can result in autolysis, a chemical reaction between the wine and the lees by which enzymes break down the dead yeast cells, producing amino acids and releasing proteins and carbohydrates into the wine. It takes about 18 months of sur lie aging for the effects of autolysis to impact a wine's flavor profile, and the process can continue for years beyond that. These "autolytic" characteristics include increased richness and creaminess and Champagne's distinct "yeasty" aromas of bread dough, toast or brioche.

—Dr. Vinny

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