Compound
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.
Also called DO, Oxygen.
- Class
- Gases
- Formula
- O2
- How often it matters
- Central to how cider behaves
What it does in cider
- Enables yeast to synthesise the sterols and unsaturated fatty acids it needs for robust membranes, which is why a completely anaerobic ferment is more likely to stick.
- Feeds acetic acid bacteria and film yeasts, neither of which can do any damage without it.
- Drives chemical oxidation of phenolics, producing quinones and hydrogen peroxide, which in turn oxidises ethanol to acetaldehyde.
- Is consumed rapidly by juice at pressing, which is why juice browns in minutes and finished cider takes weeks to show the same damage.
- Is admitted deliberately and in small quantities during barrel maturation, where slow ingress polymerises tannin and softens a cider.
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
- cardboard
- flat
- nutty depth in controlled exposure
Not perceived as itself. What is perceived is what oxygen produces — acetaldehyde, oxidised phenolics, acetic acid — and the perception lags the exposure by days or weeks, which is why oxygen damage is usually detected too late to prevent.
Descriptors it is responsible for
Sensory records that name Dissolved oxygen 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.
- Fresh-cut apple — The smell of a cut apple surface, produced within seconds of the fruit tissue being broken by six-carbon aldehydes released from membrane fats.
- Vinegar — Acetic acid from acetic acid bacteria oxidising ethanol in the presence of air — the classic cider spoilage.
- Hay — A dry, sweet-grassy note from coumarin and degraded green-leaf volatiles, characteristic of traditional farmhouse cider.
- 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.
- 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.
- Flinty — A sub-threshold reductive note read as struck flint rather than as sulphur, the mildest state of reduction.
- Maderised — Heat-and-air damage together: cooked, browned, flat character from cider stored warm in the presence of oxygen.
- Metallic — A blood-and-tin taste from dissolved iron or copper, which also catalyses oxidation.
- Mushroom — A distinctly fungal note from 1-octen-3-ol, produced by moulds on the fruit or by film yeast on the surface.
- Rubber — A burnt-rubber note from disulphides and higher mercaptans, generally a late stage of reductive fault.
- Straw — A dry, dusty, cereal-like note from advanced degradation of green volatiles in aged cider.
- Dried apricot — Concentrated stone fruit with an oxidative edge, from lactones developing alongside acetaldehyde and phenolic oxidation.
- Forest floor — Damp leaf, soil and fungal character in aged cider, from the combination of oxidised phenolics with low-level fungal volatiles.
- Hawthorn — A heavy, faintly fishy hedgerow-blossom note from trimethylamine and phenylacetaldehyde in ageing cider.
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. |
What forms it
Processes that put this compound into the drink, or increase how much of it is there.
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.
Bottling
Transferring finished cider into glass, where the dominant variable is how much oxygen the liquid picks up in the few seconds it takes to fill and close each bottle.
Juice oxidation
The enzymic browning that begins the moment apple juice meets air, which some traditions deliberately allow and others suppress.
Oxygen management in fermentation
Giving the yeast the oxygen it needs early to build viable membranes, then excluding it once fermentation is under way and especially once it slows.
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.
What removes or limits it
Processes that reduce it, hold it below a threshold, or stop it forming in the first place.
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.
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.
Bottle conditioning
Carbonating cider by letting a second fermentation finish inside the sealed bottle, so the gas is generated where it is going to stay.
Closure selection
Choosing between cork, crown, screwcap, cage and swing-top, a decision that fixes how much oxygen reaches the cider, whether it can hold pressure, and which faults it is exposed to.
Sterile filtration
Membrane filtration at a rating that retains viable yeast and bacteria, which is what allows a sweet cider to be shelf-stable without heat or preservative — provided the filling that follows is aseptic.
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.
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.
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.
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.
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.
Enzymatic browning
The rapid darkening of milled fruit and fresh juice as polyphenol oxidase converts phenolics to quinones, taking colour and some tannin structure with it.
Colour loss
A cider left noticeably paler than it should be, usually because fining, filtration or sulphite has removed the phenolic material that gave it colour.
Reduction
A closed, stale, slightly sulphurous character in cider held under strongly oxygen-free conditions, sometimes clearing with air and sometimes not.
Atypical ageing
A cider that loses its fruit unusually early and develops a flat, faintly acrid or naphthalene-like character — a syndrome described in white wine and less firmly established in cider.
Metal haze
A haze or dark cast caused by dissolved iron or copper picked up from equipment, forming insoluble complexes with phosphate or with the cider’s phenolics.
Metallic taint
A metallic, inky or blood-like taste from dissolved iron, copper or zinc picked up from equipment, usually accompanied by accelerated oxidation.
Pinking
An unexpected pink or salmon tint developing in a pale cider or perry, associated with oxidation of colourless phenolic precursors.
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.
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.
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 Dissolved oxygen
Oxygen is not simply bad for cider. At the start of fermentation it is necessary: yeast cannot make sterols or unsaturated fatty acids without it, and a population built without them has fragile membranes, poor alcohol tolerance and a marked tendency to stop before the sugar does. This is why a juice that has been handled with obsessive oxygen exclusion can ferment worse than one splashed into the tank, and why a controlled aeration early in fermentation is a legitimate technique rather than a mistake.
Once the yeast has what it needs, the balance reverses completely. Oxygen dissolved in a fermenting or finished cider feeds acetic acid bacteria and film yeasts, neither of which can grow without it; it oxidises phenolics to quinones, which brown the cider and strip aroma; and the hydrogen peroxide generated in that reaction oxidises ethanol to acetaldehyde, giving the bruised-apple, flat character of an oxidised cider even where no organism is involved.
The rate depends on what else is in the liquid. Fresh juice, with active polyphenol oxidase, consumes oxygen within minutes. Finished cider, where the enzyme is long gone, oxidises chemically and slowly, over weeks. Sulphur dioxide intercepts the peroxide and reduces the quinones, which is why free SO2 and oxygen exposure have to be considered together rather than separately.
Traditional practice has always used oxygen as well as excluded it. A wooden barrel admits a small, steady amount through the staves, and that slow ingress polymerises tannin, softens astringency and builds the oxidative depth that characterises long-matured cider. The distinction between this and spoilage is entirely one of rate and of what else is present: slow ingress into a full, sulphited, tannic cider is maturation, and the same total quantity arriving quickly into a half-empty vessel is a fault.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
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.
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.
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.
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.
Phenolics
Total phenolics
The single number used to summarise everything phenolic in a juice, useful for comparing fruit and misleading whenever it is used to predict how a cider will taste.
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
Carbon dioxide
The other product of fermentation, which protects a cider from air while it is being made and, dissolved in the finished drink, changes both its texture and its perceived acidity.
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.
- 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.
- 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
- Where does the fizz in cider come from — Either from fermentation trapped in a sealed container, or from carbon dioxide dissolved into the cider under pressure before filling. The French cider appellations permit only the first, and require at least 1.0 to 1.5 bar at 20 °C depending on the name.
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.
Hochschule Geisenheim University — beverage technology
Hochschule Geisenheim · university · retrieved 2026-08-24
German beverage-technology research covering apple wine and fruit juice processing, including the enzymology of clarification.
The New Cider Maker’s Handbook: A Comprehensive Guide for Craft Producers
Claude Jolicoeur, Chelsea Green Publishing, 2013. ISBN 9781603584739 · reference work · bibliographic record verified, not opened 2026-08-25
Bibliographic record verified on 2026-08-25 against the Open Library union catalogue: Chelsea Green Publishing, 2013, ISBN 9781603584739, one edition recorded. That establishes the citation points at a real book in a stated edition, which is what a citation needs and is all it establishes. No copy was opened and nothing is quoted from it. The book itself is in print and not digitised in any open collection; where CiderHQ needs a figure from this territory it uses an accessible research source instead and says so.