Sensory descriptor
Vinegar
Acetic acid from acetic acid bacteria oxidising ethanol in the presence of air — the classic cider spoilage.
Also called Acetic, Sour, Volatile acidity.
- Aroma family
- Volatile and fault-related
- Does it indicate a fault?
- Usually, yes
What causes it
Acetobacter and Gluconobacter species oxidise ethanol to acetic acid, and they require oxygen to do it. Any cider standing in contact with air — an ullaged barrel, a partly emptied tank, a bag-in-box past its integrity, an open vessel — is a substrate. The bacteria also esterify some of the acetic acid with ethanol to give ethyl acetate, which is why vinegar and nail varnish so reliably arrive together and why the combination is more damaging than either alone. Vinegar flies carry the organisms between vessels, which is why they are treated as a hygiene issue rather than a nuisance.
A low level of acetic acid is present in every cider and contributes to the lift and definition of the aroma; traditional styles carry more of it than modern ones and the Asturian and West Country traditions accept a level that would fail a modern specification. Above that it is the dominant impression, it rises continuously while oxygen is available, and the process does not reverse.
The compounds responsible
Where the molecule is established. Some descriptors in this lexicon name a compound the compound register does not yet hold a record for; those are named in the cause above instead.
Acetic acid
The vinegar acid, made by bacteria oxidising ethanol whenever air reaches a cider, and the one fault in cider that no later processing can undo.
Ethyl acetate
The most abundant ester in cider, giving lift and pear-drop at low concentration and nail varnish at high, and the earliest audible warning that acetic bacteria are at work.
Ethanol
The alcohol yeast makes from fruit sugar, which converts a perishable juice into a keepable drink and carries most of its aroma to the nose.
Dissolved oxygen
Essential to a healthy yeast population at the start of fermentation and the principal enemy of a cider from the moment fermentation ends.
The organisms that make it
Where a specific organism is implicated rather than the fruit. Naming one answers a different question from naming a molecule: what is in the cellar, rather than what is in the glass.
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.
Acetobacter pasteurianus
The film-forming acetic acid bacterium of traditional vinegar production, and a common cause of surface growth and volatile acidity in cider.
Gluconobacter oxydans
A sugar-preferring acetic acid bacterium abundant on damaged fruit and in fresh juice, which oxidises glucose to gluconic acid before fermentation begins.
Komagataeibacter species
Highly acetic-acid-tolerant bacteria that build the cellulose pellicle known as mother of vinegar, and the organisms of industrial vinegar fermentation.
What raises it
Processes that increase this character, or preserve it where it would otherwise be lost.
Topping up
Refilling a maturing vessel as evaporation and racking losses lower the level, so that no significant surface of cider is ever left in contact with air.
Racking
Moving cider off the sediment it has thrown, which both cleans the liquid and — by taking yeast and nitrogen away with the deposit — slows what is left of the ferment.
Sanitation
The cleaning and sanitising discipline that underlies every other process, and why removing soil must come before any attempt to kill organisms.
Sulphiting
Adding sulphur dioxide to juice to suppress spoilage organisms and oxidative browning, at a level that only means anything once the juice pH is known.
Bag-in-box
A collapsing laminate bladder in a card outer, the standard farmgate and draught format, which keeps air out during dispense but lets oxygen through the film over weeks.
The fault it points at
Meeting this descriptor above a certain intensity is how the fault below is usually noticed first.
Fault
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.
About Vinegar
Acetification is the oldest and commonest way to lose a batch of cider, and it is entirely a question of oxygen exclusion. Keeping vessels full, racking cleanly and maintaining free sulphur dioxide are the whole of the prevention.
It is worth separating the two components when recording it. Acetic acid alone is sharp and sour; ethyl acetate alone is solvent and lifted; the two together are what most people mean by vinegary, and their ratio indicates how far the process has gone.
Others in the volatile and fault-related family
Vinegar, glue, mousiness, sourness out of place.
Glue
Ethyl acetate at fault concentration, past the solvent stage and into adhesive.
Mousy
A retronasal-only taint of mouse cage and stale grain, from tetrahydropyridines produced by lactic bacteria and Brettanomyces.
Flat and lifeless
The absence of aroma and lift where both were intended — a fault defined by what is missing rather than present.
Metallic
A blood-and-tin taste from dissolved iron or copper, which also catalyses oxidation.
Peppery bitterness
A harsh, burning bitterness from acrolein formed when lactic bacteria degrade glycerol.
Ropy
An oily, viscous, pouring-in-a-thread texture from bacterial exopolysaccharide, with little aroma of its own.
Acetone
A harsher, more chemical solvent note than ethyl acetate alone, from ketones alongside high volatile esters.
What people ask next
Questions readers ask about the things this page mentions. Each one goes to the section that answers it rather than to a page written to receive the question.
- How many calories are in cider — Roughly 40 to 60 kcal per 100 ml for most ciders, so a UK pint falls somewhere around 200 to 250 kcal. Alcohol contributes about 7 kcal per gram and residual sugar about 4, so both strength and sweetness matter.
- How do i sterilise cider bottles — Wash them clean first, then sanitise with a no-rinse sanitiser or a sulphite solution, and fill while still wet with it. Bottles that look clean but have dried deposits inside are the usual source of bottle spoilage.
- What is racking in cider making — Racking is siphoning cider off the sediment it has thrown into a clean vessel, leaving the lees behind. It clarifies the cider and, in traditional practice, is also used to slow a ferment by removing yeast with the lees.
- Why is sulphite added to cider — Sulphur dioxide suppresses spoilage bacteria and wild yeast before fermentation and protects the finished cider from oxidation. How much of it is active depends strongly on pH, which is why low-acid juice is harder to protect.
- How much acetic acid is normal in cider — A small amount is present in every cider and contributes lift. Sound Asturian sidra natural has been measured at 0.2 to 0.3 g/L, and a clean modern cider much above about 0.7 g/L is generally showing a fault rather than a style.
- What turns cider into vinegar — Acetic acid bacteria oxidising ethanol, in two steps that both consume oxygen. This is why acetification is an air problem: a full sealed vessel does not acetify however many bacteria it contains.
Where to go next
- The sensory model — The thirteen structural dimensions and the twenty aroma families, in full.
- Compounds — The chemistry behind the words.
- Troubleshooting — When a perception turns out to be a problem, this is where it is diagnosed.
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.
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.