Glossary · Fermentation
Sulphiting
Also written sulfiting, SO2 addition. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
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.
- Subject
- Fermentation
- Also written
- sulfiting, SO2 addition
In more detail
Effectiveness depends on pH, not on dose alone, because only the small molecular fraction of the free sulphite is antimicrobial. At pH 3.2 a modest addition is effective; at pH 3.8 the same addition does very little, and above about pH 3.8 sulphiting is generally regarded as futile and the answer is to correct the acidity instead.
Sulphite is also bound irreversibly by acetaldehyde, sugars and other carbonyls, so a proportion of any addition is lost to binding and only the free fraction protects. Labelling law requires a declaration above 10 mg/L, and a minority of people are genuinely sensitive to it.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
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.
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.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
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
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.
Chemistry
pH
A logarithmic measure of hydrogen ion activity, and in cider the single most important number for microbial safety. Apple juice usually falls between pH 3.2 and 3.9.
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.
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.
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.