Glossary · Faults
Oxidation
Definition
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.
- Subject
- Faults
In more detail
Oxidation in cider proceeds largely through phenolics, which react with oxygen and generate hydrogen peroxide that in turn oxidises ethanol to acetaldehyde. A tannic cider therefore has a built-in buffer, consuming oxygen in its phenolic fraction, while a light low-phenolic cider oxidises quickly and shows it.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
Fault
Oxidation
The cumulative effect of oxygen on finished cider: fruit aroma flattens, colour deepens towards amber, and a bruised-apple or sherry-like character replaces the fresh one.
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.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
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.
Chemistry
Acetaldehyde
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.
Chemistry
Dissolved oxygen
Oxygen carried in solution. Useful to yeast at the start of fermentation, and damaging afterwards, when it drives oxidation, acetification and the loss of aroma.
Fermentation
Ullage
The empty space between the surface of the cider and the top of its container, and by extension the volume lost that creates it. Ullage in a maturing cask is where oxidation and acetification start.
Fermentation
Sulphiting
Adding sulphur dioxide, usually as potassium or sodium metabisulphite, to juice or cider to suppress spoilage organisms, inhibit oxidation and enzymatic browning, and give a selected yeast a clear field.
Chemistry
Polyphenol
The broad class of plant compounds carrying multiple phenol rings. In apples this includes procyanidins, hydroxycinnamic acids such as chlorogenic acid, flavonols, dihydrochalcones such as phloridzin, and anthocyanins in red-fleshed fruit.
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.