Sulfur Dioxide

Sulfur Dioxide (SO₂) is commonly used during the winemaking process as a tool to prevent bacterial spoilage and oxidation.

Sulfur adjustments are typically made during key periods when the grapes or wine are at greatest risk for corruption. For red wine, SO₂ is first added after the destemming/crushing process. During this initial stage, concentration levels of between 20 to 50 mg/L are used to inhibit bacterial growth and prevent wild yeast strains from starting fermentation. Because white wines rarely go through the destemming and crushing process, they have SO₂ added immediately after the grapes are pressed.

Aromatic white varietals, such as Sauvignon Blanc or Viognier, require the antioxidant effect of SO₂ to maintain the complex bouquet associated with the grape varietals. However, the protective qualities of SO₂ may prevent any bitter and astringent phenolics from oxidizing out from the wine. Depending on the style of Chardonnay being made, different levels of SO₂ are sometimes used by winemakers. At higher levels, SO₂ is used to produce fresh, lighter styled Chardonnays. With lower levels of SO_2, fruit characteristics take a backseat to more complex aromatics and nutty tones. However, because the wine isn't protected from oxidation, the color takes on a brownish quality.

SO₂ is also added just after malolactic fermentation and is maintained at a level (e.g., 25 parts per million) throughout the life of the wine. SO₂ levels that are too high can increase bitterness while bleaching color from a wine. Without SO₂, wines would typically oxidize and/or be attacked by microbes. Note that organically labeled wines do not use SO₂ and tend to not age more than a year or two.