Wine Analysis

Grape juice oxidation

Causes

Consequences

Minimisation
















Causes


• Oxidation of grape juice involves the bonding of oxygen molecules to oxidisable compounds present in the juice.
  
  
• Oxidation of grape juice occurs quite rapidly once it is exposed to oxygen, contained in the air.
  
  
• Oxidation of juice only occurs in the presence of organic catalysts called enzymes because of the unreactivity of oxygen.
  
  
• Grape juice is exposed to air when the grape berry skin looses its integrity through -
  
  
  • bird damage
  • disease
  • splitting
  • physical damage -
    • transport
    • processing.






Oxidation of juice results in -


• The production of brown compounds increasing the browning of juice.
  
  
• The destruction of grape derived flavour compounds.
  
  
• The production of new undesirable flavour compounds, which unpleasant in themselves, can mask desirable grape derived and potential flavour compounds produced during fermentation and wine aging.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Grape juice oxidation minimisation (also see minimisation of SO₂ use).


• Inhibition/denaturation of oxidative enzymes through the additions of sulphur dioxide (SO₂).
  
  
• Reduction in -
  
  
  • juice temperature - lower temperatures reduce the rate of enzyme activity and hence oxidation
    
    
  • time before addition of SO₂ (sulphur dioxide) = early protection
    
    
  • time before onset of fermentation - yeast uptake of oxygen will reduce oxidation and the production of carbon dioxide gas by yeast will purge the juice/must/wine of oxygen and provide a inert gas cover.
  
  
  
• Early settling and removal of juice from grape particulate matter containing oxidative enzymes
  
  
• Minimising contact between grape juice and air (containing oxygen) by -
  
  
  • Providing inert gas cover (CO₂/N₂) (mainly once in tank)
    
    
  • Reducing excessive handling (eg, rackings)
  
  
  
• The overriding element that determines the degree of grape juice oxidation is enzyme activity. The amount of SO₂ used will be the overriding element determining enzyme activity.