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Grape Juice Oxidation and the role of sulphur dioxide

• SO₂'s role in the reduction of oxidation in grape juice is distinctly different to its role in wine oxidation.
  
  
• Sulphur dioxide (SO₂) does not act as an antioxidant by reacting with oxygen (see SO2 and ascorbic acid)
  
  
• Sulphur dioxide instead denatures and inhibits the oxidative enzymes involved in the oxidation of grape juice.
  
  
• These grape derived enzymes, (tyrosinase), are quite unstable at grape juice pH values, and their activity deteriorate with time and are totally inactive once the juice has been converted to wine.
  
  
• Oxidation of juice made from botrytised infected grapes are more difficult to control with SO₂, due to the presence of the enzyme laccase.