ac·et·al·de·hyde []
noun
colorless liquid with distinctive smell: a colorless volatile liquid with a pungent smell. Use: manufacture of acetic acid, acetic anhydride, and butanol.
Formula: C2H4O
In Beer: Acetaldehyde is perceived in both aroma and flavor as green apples, and in an oxidized state as acetic-cider. In the natural anaerobic fermentation process, Acetaldehyde is a precursor to ethanol:
Glucose → pyruvic acid → acetaldehyde → ethanol
Underdeveloped or young beer exhibits acetaldehyde wherein the yeast cannot reabsorb or finish the conversion process. The other version manifests from oxidation of ethanol or bacterial contamination, the cycle being:
ethanol → acetaldehyde → acetic acid
Brewers Control (how to change)
Cold storage for short durations promotes acetaldehyde in the final product, whereas longer cold storage would ultimately reduce acetaldehyde into ethanol — hence the cure.
Sources of...
Acetaldehyde is typically inappropriate in any style, though Budweiser has integrated it within their flavor profile with obvious success intentionally. Furthermore, Salvator & EKU-28 also display acetaldehyde, though in lower amounts.