Glossary · Fermentation
Ullage
Also written headspace. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
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.
- Subject
- Fermentation
- Also written
- headspace
In more detail
Wooden casks lose liquid steadily by evaporation through the staves, so ullage appears without anything leaking. Air in that space supports acetic acid bacteria and film yeasts, both of which need oxygen at a surface. The traditional answer is to top the cask up regularly with like cider; the modern alternatives are an inert gas blanket or a full, sealed tank.
The word is also used in the trade for the sediment-and-lees layer left in an emptied cask, which is a related but distinct sense.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
Production
Topping up
Refilling a maturing vessel as evaporation and racking losses lower the level, so that no significant surface of cider is ever left in contact with air.
Fault
Acetification
The active conversion of a cider’s ethanol into acetic acid by acetic acid bacteria at an air interface — the process, running in the vessel, that produces volatile acidity.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
Fermentation
Topping up
Refilling a maturing vessel with similar cider to eliminate the headspace, so that no air surface remains for oxidative and acetic spoilage.
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.
Faults
Film yeast
Aerobic yeasts, chiefly *Pichia* and *Candida* species, that grow as a dull white or grey film on the surface of cider in a part-empty vessel, metabolising ethanol and acids.
Faults
Oxidation
The chemical damage done by oxygen to a finished cider: loss of fresh fruit aroma, browning, and the arrival of acetaldehyde, nutty and sherry-like notes.
Equipment
Bung
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.
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.
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.
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.