Oxidation: the chemistry, the fault and the style

This is the clearest case in wine of a process being judged by intention rather than by chemistry, and it is worth being explicit about it.

A Sauvignon Blanc showing oxidative character has failed. An Oloroso showing the same family of aromas has succeeded.

Oxygen does not react with ethanol directly at any useful rate. The pathway runs through phenolic compounds, which are oxidised to quinones, generating hydrogen peroxide, which then oxidises ethanol to acetaldehyde.

That pathway is why phenolic content matters so much: a wine rich in phenolics has more oxygen-consuming capacity and is generally more robust, which is part of why reds outlast whites once opened.

Acetaldehyde is the compound behind the bruised-apple, sherry-like note. Free sulfur dioxide binds it, which both removes the aroma and explains why a wine running low on free SO₂ becomes vulnerable.

Further reactions produce sotolon, associated with curry-leaf, fenugreek and walnut characters, which is prominent in Vin Jaune and in aged fortified wines.

Enzymatic oxidation is a separate and much faster route that operates on juice and must before fermentation, driven by polyphenol oxidase enzymes from the grape. It is why must handling and pressing conditions matter, and why botrytised fruit — which carries laccase, a far more aggressive enzyme — is handled with such care.

It underpins the whole practical model for opened bottles: how long a wine lasts, why fortified oxidative styles last for months, and why a half-full bottle deteriorates faster than a nearly full one.

It also explains why the same descriptors — nuts, dried fruit, caramel — appear both in fault vocabulary and in the tasting notes of celebrated wines.