The oxidation of wine is perhaps the most common of wine faults, as the presence of oxygen and a catalyst are the only requirements for the process to occur. It is also known as maderized wine, from Madeira wine, which is intentionally oxidized.
Oxidation can occur throughout the winemaking process (hence exposure to oxygen in the winery is carefully controlled, although not completely avoided), and even after the wine has been bottled. Anthocyanins and other phenols present in wine are those most easily oxidised, which leads to a loss of colour, flavour and aroma - sometimes referred to as 'flattening'. In most cases compounds such as sulfur dioxide is added to wine by winemakers, which protect the wine from oxidation and also bind with some of the oxidation products to reduce their organoletptic effect.
When a wine becomes oxidized it will turn towards brown - just as a cut apple left on a bench top. White wines will start to show an amber tint and red wines will start to develop a brown edge when viewed in a glass that is tilted. In extreme cases where there is excessive air exposure over longer periods of time, the wine can develop a nutty to caramel aroma, and may also develop slight off-flavours that resemble raisins or cough syrup. It is also important to note that white wines are affected more easily by oxidation than red wines. This is mainly because red wines have more colour pigmentation than white wines. This extra colour pigmentation acts as an anti-oxidant, preserving the wine's colour and flavour.
The biggest factor in oxidation is the amount of wine surface contact with air following opening and pouring. Therefore, wine preservation products that are most effective at reducing air contact with the wine surface will best reduce oxidation.
Once a bottle of wine has been opened for some time, or if oxygen has seeped past a faulty cork or a damaged screw-cap, the wine will oxidise, so simply check all wines before serving.