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Wine oxidation and the role of sulphur dioxide

• SO₂'s role in the reduction of oxidation in wine is distinctly different from its role in juice oxidation.
  
  
• Oxidation in wine, as in grape juice, occurs in the presence of oxygen and catalysts. However, unlike grape juice oxidation, the catalysts involved in wine oxidation are not enzymes but metal ions such as copper and iron ions (Cu⁺⁺, Fe⁺⁺⁺).
  
  
• The exception to this is when wine is made from botrytised infected grapes, when the enzyme laccase can play a role in the oxidation of wine.
  
  
• The oxidation process, involving metal ion catalysts, occurs much more slowly than the enzymatic oxidation of grape juice and hence is more insidious. That is by the time it is detected, the damage is done.
  
  
• The main oxidisable compounds in wine are phenolics, which include wine pigments, especially those found in in red wine (see red wine oxidation).
  
  
• When a phenolic is oxidised, hydrogen peroxide (H₂O₂) is produced as a side product. Since H₂O₂ (hydrogen peroxide) is a strong oxidising agent, the presence of free sulphur dioxide is required to bond with the hydrogen peroxide to form sulphuric acid, a stable compound.( see equation)
  
  
• Note that sulphur dioxide does not prevent oxidation from occuring.
  
  SO₂ prevents further, more destructive oxidative reactions from occuring once oxidation has occurred.
  
  Therefore all measures, including SO₂ use, to reduce oxidation in wine should still be applied.
  
• If a wine is to be put through malolactic fermentation (MLF) no SO₂ should be added until after the MLF (malolactic fermentation) has been completed. It pays to have the wine analysed to ensure that all the malic acid has been converted to the more microbiologicaly stable lactic acid during MLF before any SO₂ is added.