Simply put, yeast is what turns grape juice into wine. The single-cell microorganisms, which are naturally present on the skin of grapes, convert the grape's sugar into alcohol in a process called fermentation. Most winemakers, however, prefer to control fermentation by adding cultured (i.e., man-made) strains of yeast— referred to in the business as "pure culture" or "inoculated yeast"—which are easier to control and predict. Yeast also imparts a unique character to a wine.