Glossary · Chemistry
Sulphur dioxide
Also written SO2, sulphite, sulfur dioxide. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
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.
- Subject
- Chemistry
- Also written
- SO2, sulphite, sulfur dioxide
In more detail
Sulphur dioxide does three jobs: it inhibits bacteria and wild yeasts, it inactivates polyphenol oxidase and so prevents browning, and it scavenges oxygen and binds acetaldehyde. Only the molecular fraction does the antimicrobial work, and that fraction shrinks rapidly as pH rises.
Total sulphite is the sum of free and bound; only the free fraction is active, and only part of the free fraction is molecular. Legal maxima are set on total sulphur dioxide, and any drink above 10 mg/L must declare it on the label.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
Chemistry
Sulphur dioxide
The antimicrobial and antioxidant on which most modern cidermaking depends, and whose effectiveness collapses as pH rises — which makes every sulphiting decision a pH decision first.
Production
Sulphiting
Adding sulphur dioxide to juice to suppress spoilage organisms and oxidative browning, at a level that only means anything once the juice pH is known.
Fault
Excess sulphur dioxide
A struck-match or burnt-match aroma and a prickle at the back of the nose from too much free sulphur dioxide, with a real health consideration for sulphite-sensitive people.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
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
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
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.
Chemistry
Molecular sulphur dioxide
The undissociated fraction of free sulphur dioxide, and the only form that is antimicrobial. Its share of the free sulphite is governed by pH and falls sharply as pH rises.
Chemistry
Campden tablet
A pressed tablet of sodium or potassium metabisulphite supplying a standard small dose of sulphur dioxide, sized for domestic and small-scale batches.
Law and labelling
Contains sulphites
The mandatory allergen declaration required on a drink containing more than 10 mg/L of sulphur dioxide, expressed as total SO₂.
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.
Regulation (EU) No 1169/2011 on the provision of food information to consumers
European Union · legislation · passage verified 2026-08-24
The instrument behind the "contains sulphites" declaration and the alcoholic-strength labelling rule. Read on 2026-08-24 in the assimilated text maintained on legislation.gov.uk, which is the version in force for Great Britain and keeps the EU numbering. Two provisions matter to cider and both are widely misquoted: the sulphite threshold is 10 mg/L rather than a percentage, and a labelled cider strength carries a tolerance of a full percentage point either way — twice what still grape wine is allowed.
European Food Safety Authority scientific opinions
EFSA · regulator · retrieved 2026-08-24
Registered for the food-safety questions cider genuinely raises: patulin in juice from rotten fruit, sulphite sensitivity, and the toxicology behind additive limits.