Glossary · Chemistry
pH
Definition
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.
- Subject
- Chemistry
In more detail
pH governs almost everything protective. It determines what fraction of added sulphite exists in the antimicrobial molecular form, it determines how comfortable lactic and acetic bacteria are, and it determines whether malolactic conversion proceeds cleanly or turns into ropiness and mousiness. Above about pH 3.8 a cider is materially harder to protect by any means short of filtration or pasteurisation.
Because the scale is logarithmic, a difference that looks small is not: pH 3.4 has roughly three times the hydrogen ion activity of pH 3.9. Bittersweet-dominated juice is often high-pH, which is why the sharp fruit in a blend does structural work as well as sensory work.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
Production
Acid adjustment
Correcting the acidity of low-acid juice before fermentation, usually with malic acid, and the difference between the pH question and the titratable acidity question.
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.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
Chemistry
Titratable acidity
The total concentration of titratable acid in a juice or cider, measured by neutralising a sample with alkali to an end point and expressed as an equivalent of malic acid in grams per litre.
Chemistry
Buffering capacity
The resistance of a juice or cider to a change in pH when acid or alkali is added, arising from its weak acids and their salts — chiefly malate and potassium.
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.
Fruit
Sharp
In the Long Ashton classification, a cider apple high in acid and low in tannin. Sharps supply freshness and lower the pH of a blend into a range where spoilage organisms are less comfortable.
Fermentation
Malolactic fermentation
The bacterial conversion of malic acid into lactic acid and carbon dioxide, which lowers total acidity, raises pH slightly and softens the drink. It is a bacterial process, not a yeast fermentation.
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.
Peer-reviewed literature on cider fermentation microbiology
Various journals · peer-reviewed literature · registered as competent for this subject
Covers the microbial succession of spontaneous cider fermentation, the role of non-Saccharomyces yeasts in the early stages, malolactic conversion by Oenococcus and Lactobacillus species, and the organisms behind the principal spoilage faults.
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.