As each glucose molecule enters the yeast, it is broken down in a 10-step process called glycolysis . ATP supplies energy to the yeast and allows it to multiply. The fermentation process takes about two to four weeks. During this time, the winemaker samples the fermenting must and measures the pH or acid levels to determine that the fermentation process is proceeding as it should. Sulfites Sulfur-containing compounds called sulfites are naturally found on grapes to retard the growth of bacteria and mildew.
Most winemakers add sulfites to the wine to help stabilize it as it ages.
Once the fermentation process is completed, red wines are sent to the press to separate the skins from the wine. The red wines are then filtered to remove the yeast. White wines are allowed to settle and are filtered to remove the yeast. In some red wines, a second type of fermentation, called malolactic fermentation , is undertaken while in storage. Lactic acid is a milder acid than malic acid.