Oxygen has a major effect on juice, must and wine, some of which are beneficial:

• It is essential for the development of yeasts at the start of fermentation
• It enables redox reactions, which allows the wine to develop or mature in tank or barrel

And some of which are negative:

• It affects colour due to the oxidation of coloured phenolic compounds
  • In white musts, this causes temporary browning
  • In white wines, this causes permanent browning
  • In red musts, colour can precipitate out, resulting lighter wine.
• It affects flavour by:
  • Oxidising primary aroma compounds and often neutralising them
  • Increasing the level of acetaldehyde (ethanal) in wine, giving it a flat ‘sherry-like’ flavour.
  • Creating bitter-tasting components from the oxidation of phenolics
  • Encouraging the development of acetic bacteria.

The positive effects are the result of a limited and controlled exposure to oxygen.
The negative effects are usually due to the exposure of must or wine to sudden, high levels of dissolved oxygen, absence of anti-oxidants and the activity of oxidative enzymes:
The following oxidative enzymes, catalyse and accelerate the oxidation process and can lead to the rapid degradation of wine or must.

• Tyrosinase, found in all healthy grapes. The activity of this enzyme can be controlled using sulphur dioxide.
• Laccase, found in fruit affected by high levels of grey rot. This enzyme is best controlled by pasteurisation.

Most winemakers choose to limit air contact with crushed grapes and white juice and use sulphur dioxide pre-fermentation in order to limit browning, preserve phenolics and aromas.

However, for white winemaking, the protection of must from oxidation is not universally accepted. In hyper-oxygenation, a non-sulphited juice exposed to oxygen can improve the stability of white wine without producing oxidation-type flaws. On the other hand, primary aromas are lost, which can render the wine more neutral.