Glossary · Cider
Cider vinegar
Also written apple cider vinegar, ACV. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
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.
- Subject
- Cider
- Also written
- apple cider vinegar, ACV
In more detail
Deliberate vinegar production supplies oxygen and warmth to a bacterial culture, often a Komagataeibacter or Acetobacter film known as mother of vinegar. Accidental acetification in a maturing cask is the same chemistry, driven by ullage and a loose bung.
Health claims attached to cider vinegar in retail marketing are not supported by evidence CiderHQ would cite, and none is made here.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
Style
Cider vinegar
Not a cider but its next stage: cider whose ethanol has been oxidised to acetic acid by bacteria, made deliberately as a condiment and arrived at accidentally as a fault.
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.
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
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.
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
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.
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.
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.
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.
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.