Yeast

Yeast is the catalyst for fermentation in wine production. A member of the fungi kingdom, this single-celled organism feeds off sugars present in grapes and produces alcohol and CO₂ as by-products of this process. In addition to creating alcohol, yeast strains can influence the aroma and flavor of wine.

There are many strains of wild or “native” yeast present on the grapes as they come from the vineyard. Some are good, some are dangerous to the wine. The winemaker can choose to ferment with these native yeasts for more natural winemaking. Proponents believe that the resulting wines are more complex but their impact on the finished wine is not predictable. Native yeast often creates more volatile acidity and stuck fermentations are not uncommon.

Alternatively, the winemaker can choose to use commercial cultured yeast that has been proven to have specific results in the finished wine. Proponents of cultured yeast strains point to a cleaner fermentation, control over the finished product and lower probability of stuck fermentations as advantages.

There are middle ground practitioners that start with native yeast and then add a large amount of cultured yeast near the end of the fermentation to ensure that it completes. Very little real-world research has been done to compare these techniques.