Glossary · Fermentation
Airlock
Also written fermentation lock, bubbler. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
A water-sealed valve fitted to a fermenting vessel that lets carbon dioxide escape while preventing air, insects and airborne organisms from entering.
- Subject
- Fermentation
- Also written
- fermentation lock, bubbler
In more detail
An airlock is a sign of activity, not a measurement of it: gas can come from temperature change or from carbon dioxide coming out of solution, and a slow ferment can be working with no visible bubbling. Gravity readings, not bubble counts, tell whether a fermentation is progressing.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
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.
Fermentation
Fermentation
The conversion of sugar to ethanol and carbon dioxide by yeast, together with the many secondary reactions that occur alongside it and which supply most of a cider’s aroma.
Chemistry
Specific gravity
The density of a liquid relative to water at the same temperature. In cider it is the routine measure of dissolved sugar before fermentation and of progress during it.
Microbiology
Vinegar fly
The small fly, chiefly *Drosophila melanogaster*, that breeds on fermenting fruit and carries acetic acid bacteria and spoilage yeasts on its body from pomace heap to open vessel.
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.