Glossary · Faults
Unwanted refermentation
Also written bottle refermentation. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
Fermentation restarting in a sealed container that was not intended to be conditioned, producing gas, haze, sediment and, in bottles, dangerous pressure.
- Subject
- Faults
- Also written
- bottle refermentation
In more detail
The cause is always the same combination: fermentable sugar plus viable yeast plus insufficient stabilisation. Sweetened ciders packaged without sterile filtration, pasteurisation or an effective sorbate-and-sulphite regime are the usual victims, and sulphite-tolerant Zygosaccharomyces is a frequent culprit.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
Fault
Unwanted refermentation
Fermentation restarting in a sealed container, generating carbon dioxide that was not planned for and pressure that a bottle may not withstand.
Fault
Gushing
Cider that erupts from the bottle on opening, either because it is over-pressurised or because something in it is nucleating the dissolved gas violently.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
Fermentation
Back-sweetening
Adding sugar, juice or concentrate to a fully fermented cider to reach a target sweetness. Because the added sugar is fermentable, the cider must then be stabilised or kept cold.
Fermentation
Sterile filtration
Filtration through a membrane fine enough — conventionally 0.45 micron — to remove yeast and most bacteria, allowing a sweetened cider to be packaged without refermenting.
Fermentation
Pasteurisation
Heating cider to destroy yeast and bacteria, either by flash pasteurisation before filling or by tunnel pasteurisation in the sealed container. It stabilises a sweetened product against refermentation.
Chemistry
Sorbate
A preservative that prevents yeast from multiplying, used with sulphite to stabilise a back-sweetened cider against refermentation. It does not kill an active population.
Microbiology
Zygosaccharomyces
A genus of osmotolerant, highly sulphite-resistant yeasts, chiefly *Zygosaccharomyces bailii*, which can referment sweetened cider and concentrate despite preservative doses that stop other yeasts.
Faults
Gushing
The violent foaming of a cider on opening, so that a substantial part of the contents is lost. It follows over-carbonation, refermentation in pack, or nucleating material in suspension.
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.
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.
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.