To aid fermentation, yeast is added to convert the sugars to alcohol at different stages. Fermentation takes place in large vats, from which air is excluded to prevent oxidation and discourage the growth of vinegar-forming bacteria. Fermentation takes from ten to 30 days. The best temperature for yeast growth is about 25 degrees Celsius. After fermentation, the wine is drawn off to separate it from the sediment of largely dead yeast cells.
Some wines deposit their sediment quickly, but other wines remain cloudy for long periods.
The suspended particles must be removed by clarification. Wine is usually aged in wooden containers made of oak or redwood. The process allows oxygen to enter and water and alcohol to escape. Acidity decreases, additional clarification takes place, and the components of the wine form compounds that enhance flavor and aroma. The wood from the containers also contributes flavor.