Glossary · Equipment
Demijohn
Also written carboy. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
A large narrow-necked glass or plastic vessel, typically of one to five gallons, used for small-scale fermentation and maturation under an airlock.
- Subject
- Equipment
- Also written
- carboy
In more detail
The vessel that made home cider making possible, and the one that teaches the beginner’s hardest lesson about headspace. A demijohn is narrow-necked so that the surface of the liquid exposed to the air in it is small, which is exactly what a cider needs during maturation: acetic acid bacteria oxidise ethanol and both steps require oxygen, so a large surface under a large headspace is an invitation.
Glass and plastic are not equivalent for this. Glass is impermeable; food-grade PET is not, and a cider left in one for a year takes up measurably more oxygen than the same cider in glass. For a primary fermentation that is irrelevant, because carbon dioxide is streaming out and nothing is getting in. For a two-year maturation it is the difference between a cider that is developing and one that is oxidising.
A gallon is a convenient scale for another reason: it is roughly what a domestic press yields from a sack of apples, and small enough that a lot that goes wrong is a lot that can be poured away without regret.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
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
Vat
A large vessel for fermenting or storing cider, historically an open or loosely covered wooden tub, now more often a closed stainless steel tank.
Fermentation
Racking
Transferring cider off its sediment into a clean vessel, leaving the lees behind. It is the basic maintenance operation of the cellar and the traditional method of slowing a fermentation.
Equipment
Trial jar
A tall narrow cylinder in which a sample is held for a hydrometer reading, deep enough for the instrument to float clear of the base and narrow enough to need little juice.
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 New Cider Maker’s Handbook: A Comprehensive Guide for Craft Producers
Claude Jolicoeur, Chelsea Green Publishing, 2013. ISBN 9781603584739 · reference work · bibliographic record verified, not opened 2026-08-25
Bibliographic record verified on 2026-08-25 against the Open Library union catalogue: Chelsea Green Publishing, 2013, ISBN 9781603584739, one edition recorded. That establishes the citation points at a real book in a stated edition, which is what a citation needs and is all it establishes. No copy was opened and nothing is quoted from it. The book itself is in print and not digitised in any open collection; where CiderHQ needs a figure from this territory it uses an accessible research source instead and says so.
Cider: Making, Using and Enjoying Sweet and Hard Cider
Annie Proulx and Lew Nichols, Storey Communications, 1997; first published as Sweet & Hard Cider by Garden Way, 1980 · reference work · bibliographic record verified, not opened 2026-08-25
Bibliographic record verified on 2026-08-25 against Open Library, and the verification changed what CiderHQ knows about it. The work first appeared in 1980 as Sweet & Hard Cider: Making It, Using It, & Enjoying It (Garden Way, Charlotte, Vermont) and was retitled for the 1997 Storey edition the site had registered; nine editions are recorded in total. That matters for a source cited on North American practice, because a passage in the 1980 text and a passage in the 1997 one are not interchangeable and neither is the 2003 revision. Two scans exist in the Internet Archive — sweethardciderma0000prou for the 1980 edition and cidermakingusing0000prou for the 1997 — both access-restricted to the lending library and the print-disabled collection, so neither is openable by retrieval.