Glossary · Faults
Acetification
Also written going over, vinegary. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
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.
- Subject
- Faults
- Also written
- going over, vinegary
In more detail
The sequence is predictable: ullage develops, a film establishes at the surface, volatile acidity and ethyl acetate rise, and the cider first smells sharp and solvent-like before it tastes sour. Warmth accelerates everything.
Nothing removes acetic acid from a cider at cellar scale. Mildly affected batches can sometimes be blended away; badly affected ones are vinegar, which is a product but not the intended one.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
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.
Microbiology
Acetobacter aceti
An acetic acid bacterium that oxidises ethanol to acetic acid wherever cider meets air, and the organism behind most volatile acidity in cider.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
Chemistry
Volatile acidity
The fraction of a cider’s acidity that can be distilled off — overwhelmingly acetic acid — measured separately from fixed acidity and used as an index of bacterial spoilage.
Chemistry
Acetic acid
The acid of vinegar, produced in cider by acetic acid bacteria oxidising ethanol and, in small amounts, by yeast during fermentation. It is the main constituent of volatile acidity.
Chemistry
Ethyl acetate
The ester of ethanol and acetic acid. At low concentration it reads as fruity and solvent-like; at higher concentration it is the nail-varnish note that accompanies acetic spoilage.
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
Mother of vinegar
The gelatinous cellulose mat produced by acetic acid bacteria at a liquid surface. In a vinegar barrel it is the working culture; in a cider cask it is proof that the batch is being lost.
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.
Cider
Cider vinegar
Cider whose ethanol has been oxidised to acetic acid by acetic acid bacteria, producing a vinegar of about 5 per cent acidity. In a cider house the same process, unwanted, is a fault.
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 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.
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.