Glossary · Chemistry
Bound sulphur dioxide
Also written bound SO2, fixed sulphite. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
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.
- Subject
- Chemistry
- Also written
- bound SO2, fixed sulphite
In more detail
Binding is why a juice from mouldy or oxidised fruit consumes a large dose without becoming protected: the acetaldehyde and quinones already present take the sulphite out of play. It is also why repeated small additions are more efficient than one large one.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
Chemistry
Free sulphur dioxide
The portion of added sulphite still in equilibrium in solution as molecular SO₂, bisulphite and sulphite ions, and therefore still available to protect the cider.
Chemistry
Sulphur dioxide
The antimicrobial and antioxidant additive of cider and wine-making, added as a metabisulphite salt or as gas. In solution it exists as molecular SO₂, bisulphite and sulphite, in proportions set by pH.
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.
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.
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.