Compound
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.
Also called Ethanal.
- Class
- Aldehydes
- Formula
- CH3CHO
- How often it matters
- Present in every cider
What it does in cider
- Occurs as the last intermediate before ethanol in fermentation, accumulating mid-ferment and then being taken back up by healthy yeast.
- Forms in finished cider when ethanol is oxidised, whether by acetic acid bacteria, by peroxide from coupled phenolic oxidation, or by film yeasts at an exposed surface.
- Binds sulphur dioxide tightly and almost irreversibly, making it the single largest consumer of added sulphite and the main reason a dose disappears overnight.
- Loses its own aroma when bound, so sulphiting an oxidised cider can mask the fault without removing the compound.
- Defines the oxidative character of styles that seek it, and the flat, bruised, sherry-like fault in styles that do not.
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.
- bruised apple
- sherry
- green nut
- cut grass at low level
- flat and papery in combination with oxidation
Free acetaldehyde is perceptible well below the total figure, because bound acetaldehyde is not smelled at all. Sensory thresholds quoted for wine sit in the region of a hundred milligrams per litre of total acetaldehyde, but in a cider carrying little free sulphur dioxide the same total is far more obvious.
Descriptors it is responsible for
Sensory records that name Acetaldehyde as a cause. Each states the perception and the mechanism behind it.
- Bruised apple — The brown, soft, faintly sweet note of enzymatic browning, met wherever apple tissue or juice has met air.
- Green apple — A harder, sharper apple note than fresh-cut apple, driven by (E)-2-hexenal and reinforced by high malic acid.
- Bread dough — A raw, yeasty, faintly sour note from live yeast in suspension, characteristic of unfiltered and bottle-conditioned cider.
- Drains — A heavy, stagnant, sewer-like note from mercaptans formed when hydrogen sulphide is left to react.
- Honey — A sweet, waxy floral note from phenylacetaldehyde, formed by mild oxidation of 2-phenylethanol as cider ages.
- Sherry — A nutty, oxidised, faintly volatile character from acetaldehyde and sotolon in long-air-exposed cider.
- Struck match — The sharp, mineral, matchhead note of free sulphur dioxide above its perception threshold.
- Walnut — A dry, slightly bitter nut note from advanced phenolic oxidation, characteristic of long-matured tannic cider.
- Wet cardboard — The flat, papery note of (E)-2-nonenal and related aldehydes, the sensory signature of oxidative staling.
- Apricot — A concentrated, faintly honeyed stone-fruit note from lactones at higher concentration, common in ice cider and sweet styles.
- Dried apple — Concentrated, leathery apple character from moderate oxidation, sitting between fresh fruit and full oxidative development.
- Dried fruit — Raisin and sultana character from long oxidative ageing, most often met in concentrated and fortified styles.
- Fresh yeast — The clean, faintly sulphidic smell of an active yeast population, met in young cider and during fermentation.
- Hazelnut — A lighter, sweeter nut note from early oxidative development and from lees contact.
- Maderised — Heat-and-air damage together: cooked, browned, flat character from cider stored warm in the presence of oxygen.
- Overripe apple — Heavy, faintly alcoholic fruit aroma from apples past the climacteric, where the fruit has begun fermenting on its own account.
- Raspberry — A raspberry aroma arising in unfruited cider from bacterial spoilage — the French *framboisé* — as well as from added fruit.
- Soured milk — A sharp, unclean dairy note from uncontrolled lactic activity, recorded as a fault in every style.
- Straw — A dry, dusty, cereal-like note from advanced degradation of green volatiles in aged cider.
- Windfall apple — Bruised, soil-touched ripe fruit character from apples collected off the orchard floor, sitting on the border between depth and taint.
- Yoghurt — A tangy fermented-dairy note from lactic acid with acetaldehyde, indicating vigorous bacterial activity.
- Bletted pear — Deep, faintly winey overripe pear character from fruit softened past ripeness, met in traditional perry and in medlar-like notes.
- Dried apricot — Concentrated stone fruit with an oxidative edge, from lactones developing alongside acetaldehyde and phenolic oxidation.
- Fermenting vat — The pungent, CO2-lifted smell of a ferment in full activity, met in very young cider straight from the vessel.
- Forest floor — Damp leaf, soil and fungal character in aged cider, from the combination of oxidised phenolics with low-level fungal volatiles.
The structure it moves
| Dimension | What it is |
|---|---|
| Oxidative character | Nutty, bruised-apple, sherry-like or cardboard notes from exposure to air. |
| Freshness | Whether the drink smells and tastes of live fruit or of time and air. |
| Fruit character | How strongly the drink smells of apple or pear, and of fruit generally. |
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.
Acetaldehyde2.0–3.0 mg/L
Villaviciosa, Asturias, Spain, 2001–2002 · 4 samples · Gas chromatography with flame ionisation detection · Suárez Valles, Pando Bedriñana, Fernández Tascón, Querol Simón and Rodríguez Madrera, Food Microbiology 24(1):25–31
Sound, fully fermented cider that had not been sulphited. Acetaldehyde binds sulphur dioxide, so a sulphited cider of the same age can carry considerably more of it while smelling of less; and an oxidising cider climbs well above this band. The figure describes healthy cider, not a limit.
The intermediate between sugar and ethanol, which is also what oxidation turns ethanol back into — so the same number can mean a young ferment or an old fault. Measured in milligrams per litre.
What forms it
Processes that put this compound into the drink, or increase how much of it is there.
Juice oxidation
The enzymic browning that begins the moment apple juice meets air, which some traditions deliberately allow and others suppress.
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.
Wild fermentation
Letting the microflora already present on the fruit and in the press house carry the ferment, in a succession rather than as a single organism.
Inoculated fermentation
Starting a ferment by pitching a chosen yeast culture so that one known strain, rather than the fruit’s resident population, does the work.
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.
What removes or limits it
Processes that reduce it, hold it below a threshold, or stop it forming in the first place.
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.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
Lees ageing
Deliberately holding a cider or perry on its fine yeast deposit so that autolysing cells release material that changes texture and foam behaviour.
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.
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.
Acetaldehyde excess
A bruised-apple, green-nut or sherry aroma from acetaldehyde, produced by oxidation, by film yeast, or left behind by a ferment that was interrupted.
Oxidation
The cumulative effect of oxygen on finished cider: fruit aroma flattens, colour deepens towards amber, and a bruised-apple or sherry-like character replaces the fresh one.
Flat and lifeless
A cider with nothing obviously wrong and nothing to say: aroma faded, acidity dulled, finish short — usually the cumulative result of over-processing or slow oxidation.
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.
Excess sulphur dioxide
A struck-match or burnt-match aroma and a prickle at the back of the nose from too much free sulphur dioxide, with a real health consideration for sulphite-sensitive people.
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.
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.
Framboise
A raspberry-and-rotten-fruit character with sulphurous overtones, produced by *Zymomonas mobilis* in sweet ciders that still contain sugar.
Metallic taint
A metallic, inky or blood-like taste from dissolved iron, copper or zinc picked up from equipment, usually accompanied by accelerated oxidation.
Organisms that produce it
Which organism is responsible usually decides whether the compound is a feature or a symptom.
Saccharomyces cerevisiae
The yeast that finishes essentially every cider, whether it arrives in a sachet or from the fruit, the press and the vessel.
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.
Pichia membranifaciens
A film-forming yeast that grows as a skin on the surface of cider left in contact with air, consuming ethanol and acid and leaving the cider thin.
Candida species
A large, historically artificial grouping of yeasts that appears throughout cider microbiology, containing organisms with little in common beyond the absence of a sexual stage.
Film yeasts
A functional grouping rather than a taxon: the oxidative yeasts that form a skin on cider exposed to air and consume its alcohol and acid.
About Acetaldehyde
Every molecule of ethanol in a cider passed through acetaldehyde on the way. Yeast decarboxylates pyruvate to acetaldehyde and then reduces acetaldehyde to ethanol, and in a healthy ferment the intermediate rises during the vigorous phase and is largely reclaimed as the ferment finishes. A ferment that is stressed, sulphited heavily at the start, or stopped abruptly leaves more of it behind, which is one reason an arrested fermentation can taste flatter than expected.
In a finished cider the compound arrives by a different route: oxidation. Acetic acid bacteria oxidise ethanol to acetaldehyde as their first step, film yeasts do the same at an exposed surface, and there is a purely chemical route as well, in which phenolic compounds oxidised by air generate hydrogen peroxide that oxidises ethanol directly. That last route matters because it needs no organism at all — a sound, sterile cider left in contact with air will still develop acetaldehyde, and the more phenolic it is the faster it does so.
The sulphite relationship is the practically important one. Acetaldehyde reacts with bisulphite to form a hydroxysulphonate, and the binding is strong enough to be effectively irreversible on cellar timescales. Bound sulphite has no antimicrobial activity whatever, so a cider carrying acetaldehyde consumes its sulphite additions without gaining protection. This is why an oxidised or bacterially spoiled batch seems to swallow sulphite: the added SO2 is being locked to the aldehyde rather than doing any work. It is also why measured total SO2 can be high while free SO2 — the only fraction that protects anything — is negligible.
The same reaction runs in the other direction as a treatment, and it deserves stating carefully. Binding acetaldehyde removes its smell, so a modest sulphite addition can make a lightly oxidised cider taste noticeably fresher. Nothing has been repaired: the aldehyde is still there, the sulphite has been consumed, and the cider is now less protected than the analysis of total SO2 suggests. Meanwhile, in styles built on air — traditional Asturian sidra, some barrel-matured English and Norman ciders — the same compound is not a fault at all but a recognised part of the character, judged against expectations the style established long before anybody could measure it.
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.
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.
Acids
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.
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
Ascorbic acid
An oxygen scavenger that protects colour and aroma while it lasts and can promote browning once it is exhausted, which is why it is never used without sulphur dioxide.
Enzymes
Polyphenol oxidase
The copper enzyme that turns cut apple brown within seconds, and the reason a cidermaker has to decide, at the press, whether to let the juice oxidise or to stop it.
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.
- Which yeast ferments cider — Saccharomyces cerevisiae finishes almost every cider fermentation, whether it was pitched or arrived from the press house. In a spontaneous ferment it is not the first organism present, only the one that survives the alcohol it makes.
- 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 is oxidation in cider
- How does cider ferment — Yeast consumes the sugar in apple juice and produces roughly equal masses of ethanol and carbon dioxide, along with the esters and higher alcohols that give cider much of its aroma. In a wild ferment several yeast species take turns before *Saccharomyces* finishes the job.
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.
Peer-reviewed literature on apple phenolics and cider sensory perception
Various journals · peer-reviewed literature · registered as competent for this subject
Registered as a class rather than as one paper, because the mechanisms CiderHQ describes — tannin chain length driving the split between bitterness and astringency, salivary protein precipitation, enzymatic browning — are established across many studies rather than resting on any single one. Individual papers are cited where a specific number is quoted.