Yeast

Second to grapes, the most important live component of winemaking is the yeast, a type of fungus. Without yeast, the sugar will not turn to alcohol and the juice will not turn into wine. Grapes often have wild strains of yeast on the skin surface when harvested, and these will eventually begin to ferment if left uncontrolled. These yeasts may or may not be desirable, so winemakers add sulfur dioxide to control wild yeast and later add a certain dosage of wine yeast to ferment the juice.

Active dry wine yeast typically comes in a 5 gram packet which is enough to ferment 5 gallons of juice. Yeast can be purchased in larger packages as well. Store yeast in the refrigerator and be aware of the expiration date stamped on the package. Wine yeast is available in wine and beer supply stores and catalogs.