Glossary · Chemistry
Acetaldehyde
Also written ethanal. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
A volatile carbonyl formed as an intermediate in fermentation and by the oxidation of ethanol. In small amounts it lifts fruit character; in excess it smells of bruised apple, sherry and cheap paint.
- Subject
- Chemistry
- Also written
- ethanal
In more detail
Acetaldehyde binds sulphite avidly, so a cider that has been oxidised carries a high bound-sulphite figure and a low free one. Yeast reabsorbs much of the acetaldehyde it produces during a healthy ferment, which is why the compound is more often an ageing and handling problem than a fermentation one.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
Chemistry
Acetaldehyde
The compound sitting one step short of ethanol, which smells of bruised apple and sherry, binds most of the sulphite added to a cider, and is the chemical signature of oxidation.
Fault
Acetaldehyde excess
A bruised-apple, green-nut or sherry aroma from acetaldehyde, produced by oxidation, by film yeast, or left behind by a ferment that was interrupted.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
Faults
Oxidation
The chemical damage done by oxygen to a finished cider: loss of fresh fruit aroma, browning, and the arrival of acetaldehyde, nutty and sherry-like notes.
Chemistry
Bound sulphur dioxide
Sulphite that has combined with acetaldehyde, sugars, quinones and other carbonyl compounds. It counts towards the legal total but has no antimicrobial or antioxidant effect.
Sensory
Bruised apple
A descriptor for the soft, slightly oxidised apple note characteristic of acetaldehyde and of mild oxidation. Distinct from fresh-cut apple, which is bright and green.
Sensory
Oxidative
Showing the character that oxygen exposure brings — bruised apple, nuts, dried fruit, sherry and toffee — with fresh fruit aromas diminished. It may be a fault or a deliberate style.
Where to go next
- Cider glossary — The whole vocabulary, by subject and A–Z.
- Comparisons — Terms that are only really clear when set against the thing they are not.
- Search — Finds terms by their aliases and by their spellings in other languages.
Sources
What this page rests on. Where a source is marked as registered rather than read, CiderHQ is recording that the body is authoritative on the subject without claiming to have worked through the document itself. See our evidence policy for what each state means.
The Science of Cidermaking and associated technical writing
Andrew Lea · reference work · passage verified 2026-08-24
Written by a food chemist who worked at Long Ashton on apple phenolics. Unusual among specialist cider writing in that it is primary-research-adjacent: the author is describing work he did, and cites the literature. This is why it is registered at tier 1 for chemistry while a general cider book is not.
Cider and perry production technical training material
Peter Mitchell / Cider and Perry Academy · reference work · registered as competent for this subject
Searched again on 2026-08-25 and it remains the one printed source here that a bibliographic check cannot fix, because it is not a published work with an edition. It is course material issued to participants of a training programme, so there is no catalogue record to verify, no ISBN, and no lawful public copy. Its state stays `registered` for that reason rather than through inattention.