Compound
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.
Also called Ethanoic acid, Volatile acidity.
- Class
- Acids
- Formula
- CH3COOH
- How often it matters
- Present in every cider
What it does in cider
- Is produced by acetic acid bacteria oxidising ethanol, in two steps and with oxygen consumed at both — so it is a fault of air, not of yeast.
- Contributes a hard, sour, volatile sharpness distinct from malic acidity, and lowers pH while doing it.
- Travels with ethyl acetate, its ester, which is far more perceptible and usually announces the problem before the acid itself does.
- Cannot be removed. Blending dilutes it and nothing else touches it, which is why prevention is the whole of the management.
- Is present at low concentration in every cider, so the question is always how much rather than whether.
How it is perceived
What the compound registers as, and at roughly what concentration. Perception is not a property of the molecule alone: sugar, tannin and carbonation all change where a threshold falls.
- vinegar
- sharp
- nail varnish when accompanied by its ester
- sour finish
- nose prickle
A small quantity is normal and contributes lift. Sensory recognition in cider is generally reported from somewhere around 0.7 to 1.2 g/L, but it depends heavily on what else is in the glass: sugar masks it, and the accompanying ethyl acetate is detected far below the acid’s own threshold.
Descriptors it is responsible for
Sensory records that name Acetic acid as a cause. Each states the perception and the mechanism behind it.
- Nail varnish — The sharp solvent lift of ethyl acetate above threshold, the commonest volatile fault-adjacent note in cider.
- Vinegar — Acetic acid from acetic acid bacteria oxidising ethanol in the presence of air — the classic cider spoilage.
- Glue — Ethyl acetate at fault concentration, past the solvent stage and into adhesive.
- Cider shop — The characteristic smell of a working cider house — ferment, wood, fruit and a trace of acetic lift together.
- Acetone — A harsher, more chemical solvent note than ethyl acetate alone, from ketones alongside high volatile esters.
The structure it moves
| Dimension | What it is |
|---|---|
| Acidity | The sharp, mouth-watering quality that makes a cider taste fresh rather than flat. |
| Freshness | Whether the drink smells and tastes of live fruit or of time and air. |
| Fermentation character | Aromas made by the ferment rather than carried in from the fruit. |
Measured figures
Shown as they were measured, with the context each was taken in. They are not averaged: a concentration recorded in one country's fruit in one decade is not a constant.
Volatile acidity0.2–0.6 g/L
context not recorded · Andrew Lea
The band a sound, well-kept cider normally occupies, expressed as acetic acid. It is not a legal limit and not a style rule: traditional Asturian sidra natural and some barrel-matured English ciders sit above it by intention and are judged in that context, while a fresh, clean modern cider above it is showing a fault.
Mostly acetic acid — the vinegar fault, measured. Measured in grams per litre.
Acetic acid0.2–0.3 g/L
Villaviciosa, Asturias, Spain, 2001–2002 · 4 samples · HPLC, measured as acetic acid rather than titrated as volatile acidity · Suárez Valles, Pando Bedriñana, Fernández Tascón, Querol Simón and Rodríguez Madrera, Food Microbiology 24(1):25–31
The acid itself, not the volatile-acidity aggregate. Worth setting beside the expert-reference band above, which is a titration figure: these are sound Asturian ciders fermented spontaneously without sulphite, and they sit at the bottom of the range rather than above it, which is not what the style’s reputation would predict.
The vinegar acid, measured as itself rather than as the volatile-acidity aggregate it usually hides inside. Measured in grams per litre.
What forms it
Processes that put this compound into the drink, or increase how much of it is there.
Oxidative maturation
Ageing a cider with a deliberate, measured oxygen supply so that tannins polymerise and soften and an aldehydic character develops on purpose, which is not the same thing as the oxidation fault.
Barrel ageing
Holding cider in wood — which may mean an old neutral vat used simply as a vessel, or fresh or spirit-seasoned oak used as a source of flavour, and the two are not the same operation.
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.
What removes or limits it
Processes that reduce it, hold it below a threshold, or stop it forming in the first place.
Blending
Combining separate lots of cider or perry into one, which in cider is the historically normal way of making the drink rather than a remedy applied when single lots disappoint.
Sanitation
The cleaning and sanitising discipline that underlies every other process, and why removing soil must come before any attempt to kill organisms.
Faults it is implicated in
Being implicated is not the same as being a fault. Several of the compounds on this site are ordinary constituents of a sound cider and define a named fault only above a concentration.
Volatile acidity
The measurable fraction of acidity that can be steam-distilled off, dominated by acetic acid and read as vinegar sharpness in the nose and a hard, hot finish.
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.
Ethyl acetate taint
A solvent, nail-varnish or pear-drop aroma from ethyl acetate, the most abundant ester in cider and the one that turns from lift to fault over a narrow range.
Film yeast growth
A pale, powdery or wrinkled film of aerobic yeast growing on the surface of a standing cider, consuming alcohol and acid and producing acetaldehyde.
Vinegar fly contamination
Small flies carrying acetic acid bacteria and spoilage yeasts directly into juice and cider, and depositing them wherever they land.
Microbial haze
Cloudiness caused by a growing population of spoilage organisms, and therefore a symptom of something worse rather than a clarity problem in itself.
Lactic off-flavours
Sauerkraut, sour milk, silage or cheesy notes from lactic acid bacteria working on sugars and other substrates rather than on malic acid alone.
Organisms that produce it
Which organism is responsible usually decides whether the compound is a feature or a symptom.
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.
Lactobacillus brevis
An obligately heterofermentative lactic acid bacterium, renamed *Levilactobacillus* in 2020, associated with volatile acidity, biogenic amines and mousiness rather than with clean malolactic conversion.
About Acetic acid
Acetic acid bacteria do to cider exactly what they are named for. They oxidise ethanol to acetaldehyde and then acetaldehyde to acetic acid, and both steps require molecular oxygen. This single fact organises everything about the fault: acetification is an air problem. A full, sealed vessel does not acetify no matter how many bacteria it contains, and a half-empty one acetifies even if it was scrupulously clean, because the bacteria that matter are already present on the fruit and in the cellar and are waiting only for a surface to work at.
The genera divide their labour. Gluconobacter works readily in sugary juice and is the organism most active before and during fermentation; Acetobacter and Komagataeibacter prefer alcohol and take over afterwards, growing as a film at the liquid surface and building the cellulose mat that makes a vinegar mother. Vinegar flies carry all of them, which is why an open airlock in late summer is a genuine risk and not merely untidy.
What a taster usually notices first is not the acid. Acetic acid bacteria and stressed yeast both make ethyl acetate, the ester of acetic acid and ethanol, and it is perceptible at a small fraction of the acid’s concentration — solvent, pear-drop, nail varnish. A cider whose volatile acidity is climbing normally smells of glue before it tastes of vinegar, which gives a maker a warning worth acting on, because the process only runs in one direction.
Nothing reverses it. Sulphiting kills the bacteria but leaves the acid; filtration removes the cells and leaves the acid; the acid is not volatile enough to be driven off by any treatment a cider will survive. The remedies are all preventive — keep vessels topped up, keep closures sound, keep sulphite adequate for the pH, control vinegar flies, and clean the equipment that touches finished cider — and the only corrective action available afterwards is to blend the affected cider away into a much larger volume, or to accept it and make vinegar deliberately.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
Alcohols
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.
Esters
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.
Aldehydes
Acetaldehyde
The compound sitting one step short of ethanol, which smells of bruised apple and sherry, binds most of the sulphite added to a cider, and is the chemical signature of oxidation.
Gases
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.
Additives and processing aids
Sulphur dioxide
The antimicrobial and antioxidant on which most modern cidermaking depends, and whose effectiveness collapses as pH rises — which makes every sulphiting decision a pH decision first.
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.
- Does cider contain sulphites — Most does. Sulphur dioxide is added to protect juice and finished cider, and fermentation itself produces a small amount even when none is added. European labels must declare it above 10 mg per litre.
- 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.
- Why does my cider taste like vinegar — Acetic acid bacteria have reached the cider and, given air, are converting its alcohol into acetic acid. The cause is almost always oxygen — an unfilled vessel, a leaking bung, or a slow transfer.
- Can you fix a cider that has gone vinegary — Not really. Volatile acidity cannot be removed at small scale, and blending it away only dilutes it. The realistic options are to accept it, to blend a lightly affected batch into a much larger sound one, or to let it finish becoming vinegar.
- Why does my cider smell like nail varnish — That is ethyl acetate, formed when acetic acid combines with ethanol. In small amounts it reads as pear drops; above threshold it smells of solvent or nail varnish remover and usually accompanies rising volatile acidity.
- Why does my cider smell of nail varnish or solvent — Almost always ethyl acetate, the ester of acetic acid and ethanol. It is made by stressed yeast and by acetic acid bacteria, and it is perceptible at a small fraction of the concentration the acid itself needs, so it usually announces a problem before the vinegar does.
Where to go next
- All compounds — Grouped by what the fruit brings and what the ferment makes of it.
- Sensory — Every descriptor, with the compound or process that causes it.
- Microbiology — The organisms whose metabolism most of this chemistry belongs to.
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.
Denominación de Origen Protegida Sidra de Asturias
Consejo Regulador de la DOP Sidra de Asturias · designation authority · passage verified 2026-08-24
The regulating council for Asturian cider. Read on 2026-08-24, and it turned out to carry far more than a permitted-variety list: an agronomic and technological characterisation of each admitted variety, with total acidity, total polyphenols, vigour, flowering, ripening, cropping and susceptibility to fungal disease. The characterisation is credited to SERIDA’s fruit-research programme. Two things it settles are corrections to what CiderHQ previously held: Asturias sorts cider fruit into nine technological blocks rather than five, and it publishes acidity as sulphuric acid rather than malic. The figures are transcribed into `data/trials/serida-asturias.ts` and cite this source, because this is where they were read.