Post-fermentation oxidation is one of the chief causes of loss of quality in white wines. When oxidised, the colour of wines change, becoming brown and cloudy. The aroma and flavour also change; the wine loses fruit, but increases in bitterness and develops a sherry-like aroma.

The extent to which wines oxidise depends on:

• The presence of an oxidative enzyme (laccase) derived from Botrytis
• The presence of yeasts and acetic bacteria
• The level of phenolics in the wine
• The level of oxygen dissolved in the wine
• The concentration of free sulphur dioxide in the wine
• Levels of metals iron and copper
• Temperature
• Time

To test if a wine is susceptible to oxidation, it can be exposed to air for 24 hours in a glass at room temperature. If the wine clouds or changes colour, aroma or flavour, it is susceptible to oxidation. If the browning is likely to be a result of laccase action the wine should be tested for laccase. The only really effective treatment for laccase activity is pasteurisation.

In other cases, the prevention of browning is usually achieved by storing the wine at a low temperature, the addition of sulphur dioxide, and the exclusion of air with inert gas.

After the alcoholic fermentation, it is really important to protect all white wines from oxidation by ensuring that they contain at least 20 mg/l of free sulphur dioxide and that they are protected from contact with air by keeping the tanks topped up (or blanketing them with inert gas) and by ensuring that there are no leaks in hoses or fittings when transferring wine.

The loss of aroma and flavour in oxidised wine cannot be replaced, but the taints produced by oxidation can be treated by fining with agents such as PVPP and casein.