Glossary · Equipment
Bung
Definition
The stopper closing the filling hole of a cask or vat. During fermentation it is left loose or replaced by an airlock; afterwards it is driven home to exclude air.
- Subject
- Equipment
In more detail
The state of the bung is a routine cellar decision with real consequences. Bunged tight too early, a fermenting cask can build pressure; left loose too long, it admits the oxygen that acetic bacteria and film yeasts need.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
Equipment
Shive
The wooden or plastic plug driven into the bung hole on the belly of a cask, carrying a smaller central section through which a spile can be pushed to vent gas.
Fermentation
Airlock
A water-sealed valve fitted to a fermenting vessel that lets carbon dioxide escape while preventing air, insects and airborne organisms from entering.
Equipment
Cask
A wooden vessel of staves bound by hoops, bulging at the middle so it can be rolled and turned. Casks both store cider and shape it, admitting a slow ingress of oxygen through the wood.
Fermentation
Ullage
The empty space between the surface of the cider and the top of its container, and by extension the volume lost that creates it. Ullage in a maturing cask is where oxidation and acetification start.
Faults
Acetification
The oxidation of ethanol to acetic acid by acetic acid bacteria, turning cider progressively into vinegar. It requires oxygen and is the classic fault of a slack, part-empty cask.
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.
Hereford Museum of Cider collections and interpretation
Hereford Cider Museum Trust · museum · retrieved 2026-08-24