Glossary · Microbiology
Acetic acid bacteria
Also written AAB, Acetobacter. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
Aerobic bacteria — Acetobacter, Gluconobacter and Komagataeibacter — that oxidise ethanol to acetic acid. They require oxygen, which is why they colonise surfaces, ullage and slack fittings.
- Subject
- Microbiology
- Also written
- AAB, Acetobacter
In more detail
They are carried by fruit flies, live in pomace heaps and spent lees, and survive in hoses and pump seals. Because they need an air interface, the controls against them are physical before they are chemical: full vessels, sound bungs, prompt topping up and clean equipment.
Where this is covered properly
The glossary defines; these pages explain. Each is described in its own words.
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.
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.
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.
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.
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.
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.
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.
Milling and pressing
Sanitation
The routine cleaning and disinfection of everything juice touches. In a cider house it is the single most effective control on spoilage, and it is separated into cleaning, which removes soil, and sanitising, which reduces the organisms left behind.
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.