Oxidation (wine fault)
This term is used to describe wine faults caused by molecular compounds which require the presence of oxygen to develop. Technically speaking, oxidation is a chemical reaction, and in wine, it refers to the process whereby electrons are transferred from oxidisable molecular compounds to any oxygen which may be present, and the subsequent bonding of the oxygen molecules to those molecular compounds. Overall, the result of this process is a depletion of positive aroma compounds and an augmentation of negative ones.
In the early stages of winemaking, oxygen is helpful, and indeed necessary for fermentation to proceed. It also helps to oxidise the harsh polyphenols in red wine, making tough red wines softer and less tannic. But its overall impact on wine is negative if exposure to it is not carefully controlled. Once wine is exposed to a significant amount of oxygen – for example, if a bottle of wine is opened – its chemical components will gradually start to become oxidised. When oxidised, the colour of a wine changes, becoming brown and dull. Desirable aromas and flavours are destroyed, often being replaced by sherry-like aromas, and bitterness usually increases. In short, oxidation reduces the fruit character of a wine and slowly turns it into vinegar.
Unless it is inherent to a traditional style of wine like sherry or Tokaj, significant exposure to oxygen will most likely be unintentional, whether during the winemaking process, or after bottling. Poor bottling practices, excessive racking, wine transfers, leaks, partially filled vessels and careless filtering can all contribute to oxidation during the winemaking process. After bottling, the most common causes of oxidation are heat damage, temperature and humidity fluctuations and upright bottle storage, all of which lead to increased oxygen intake due to pressure differentials between the inside and the outside of the bottle or to drying out of the cork.
As with reduction, it is not the oxidation itself which causes the wine to have unpleasant organoleptic characteristics — it is the molecular compounds which develop in the presence of oxygen. The three most important oxidative wine faults are acetic acid (often referred to as volatile acidity), acetaldehyde, and acetic acid.
- WSET Level 4 Diploma in Wines and Spirits Unit 2 Study Guide, Issue 5, 2010.
- Understanding Wine Technology, The Science of Wine Explained, D. Bird, DBQA Publishing 2010.