Oxidation

Excessive oxygen exposure during winemaking or storage. Caused by poor topping of barrels, faulty seals, or storing opened wine too long. Acetaldehyde is the key compound. All wines oxidize slowly over time, but premature oxidation (premox) is a fault.

Whites turn deep gold or amber; reds turn brick or brown. Aromas shift from fresh fruit to bruised apple, sherry-like nuttiness, and stale character. Whites taste flat with diminished acidity. At advanced stages, vinegar (acetic acid) notes appear.

Inert gas (nitrogen, argon) blanketing during racking and transfer. Adequate SO2 levels. Proper closure integrity. Minimal headspace in containers. White wines are more vulnerable than reds (which have protective phenolics).