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SO₂ decolourisation of red wine.

• Sulphur dioxide (SO₂) will bind to red wine pigments extracted from grape skins.
  
  
• The new pigment/ SO₂ compound formed will not exhibit the red colour that the pigment alone showed.
  
  
• When SO₂ is added in excess to red wines, unacceptable bleaching occurs.
  
  
• SO₂ added to red wine musts can show this de-colourisation but is rapidly reversed on the onset of fermentation due to the production of acetaldehyde which more strongly binds SO₂.