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Acetaldehyde

A colorless, volatile, and water-soluble compound found naturally in grapes and wines in trace amounts and produced both by fermentation and oxidation. It has a pungent, fruitlike odor and is desirable in small amounts in good table wines and in high amounts in oxidized wines such as Sherry or Madeira. During fermentation, it is produced by yeast in the fourth of five stages of enzymatic action culminating in the production of ethyl alcohol. The enzyme carboxylase forms acetaldehyde and carbon dioxide from pyruvic acid. At the next (final) stage, most of the acetaldehyde is reduced to ethyl alcohol, but a trace remains and adds to the flavor and complexity of the wine. If too much remains, it taints the wine with a strong, oxidized off-taste.

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