Oxidation

Both. Mainly the phenolics and (to a lesser extent) the alcohol, but also some other flavour compounds.

Oxygen is involved in many chemical and biological reactions in wine. On the positive side, O2 is involved in the polymerization reactions of tannins and anthocyanins facilitating the softening of astringency and stabilization of color, respectively.

On the negative side, O2 is known to promote the formation of acetaldehyde (ethanal) in wine, giving rise to a rotten apple smell. Acetaldehyde is formed through a two step process. First step, O2 reacts with a vicinal phenol, such as caftaric acid, to produce a quinone and H2O2. In the second step, H2O2 reacts with ethanol, the most abundant alcohol in wine, yielding acetaldehyde and water. The quinone product of step one is also electrophilic and is believed to oxidize other components in the wine. The presence of oxygen is necessary for obligate aerobes such as Acetobacter to grow and potentially spoil wines by producing large amounts of acetic acid or other off-flavors and aromas. It goes downhill from there, Acetic acid and ethanol can form ethyl acetate which has a sharp, nail-polish-remover-like odor. O2 also contributes to browning as it is required for production of polyphenol oxidase (PPO)- and laccase-catalyzed quinones which undergo further chemical reactions to produce brown pigments.

You can find more specific info in: Boulton, R. et al. 1996. Principles and Practices of Winemaking. Chapman and Hall, New York.

RD

The first materials oxidised include the phenolics to quinone these further react to produce other products. But possibly even before these is the oxidation of sulphur dioxide. When all the free SO2 is oxidised further oxidation of SO2 releases acetaldehyde from the bound SO2. This acetaldehyde reacts with phenolics producing brown coloured resinous materials. The resins tend to precipitate and some also have a bitter flavour.

Bob M