Fault
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.
Also called bruised apple character, sherry note.
- Severity
- Quality — the drink is worse for it
- Where it starts
- Maturation, Fermentation, Storage
- Can it be fixed?
- Partly — it can be reduced, not removed
- When you notice it
- Fermentation, Maturation, Packaging
- Signs
- 5 recorded
What it is
Acetaldehyde is the intermediate between ethanol and acetic acid, and it appears in cider from several directions at once. Yeast produces it during fermentation and normally reduces most of it back down; oxygen generates it from ethanol in a finished cider; and film-forming yeasts make large amounts of it at a vessel surface. Above its threshold it smells of bruised apple, green walnut and, higher still, of dry sherry. It is one of the few faults with a genuine partial remedy, because sulphur dioxide binds it chemically.
What you notice
Grouped by sense, because that is how the fault presents itself rather than how it works.
On the nose
- A bruised or over-ripe apple note that dominates the nose
- A green-nut or cut-walnut smell
- A dry sherry character in a cider that has no business having one
On the palate
- A hard, slightly hollow finish with the fruit flavour cut short
How the batch behaves
- A cider that improves noticeably in the glass over several minutes as the compound volatilises
The words for it: Bruised apple. Each links to what produces it.
When it appears. Normal and transient during active fermentation, when it is an intermediate the yeast will reabsorb. It becomes a fault when the ferment ends before it is taken back up, or when oxygen generates more after fermentation is over.
Detectable at low concentration and one of the few cider faults that can genuinely diminish with time, because a live yeast population will take acetaldehyde back up if given the opportunity. That makes early detection worth more here than for most faults on this list.
What it is mistaken for, and how to tell
One observation per rival, chosen because it separates them rather than because it describes either. Several of these are not faults at all — a style feature, a normal outcome, or the fruit behaving as it does.
- Oxidation — Acetaldehyde is green, sharp and bruised-apple, with a slight sting; advanced oxidation is nutty and sherried. The first can still resolve, the second cannot.
- Sulphur dioxide excess — These interact rather than merely resembling each other: sulphur dioxide binds acetaldehyde. A cider showing both is often one where the sulphite has been consumed binding it, leaving neither doing what was intended.
- A ferment that has not finished — Take a gravity reading. Acetaldehyde in a still-active ferment is ordinary and will be reabsorbed.
What is actually happening
The chemistry or microbiology behind it. Understanding the mechanism is what makes the prevention make sense rather than being a list of rules.
During fermentation, Saccharomyces cerevisiae forms acetaldehyde by decarboxylating pyruvate, then reduces it to ethanol using alcohol dehydrogenase. Acetaldehyde peaks early in the ferment and falls as the yeast reclaims it. Anything that interrupts the ferment while the peak is high — cold shock, an arrested fermentation, racking the yeast away, or an early sulphite addition — leaves the intermediate stranded in the cider.
After fermentation the compound is generated chemically. Metal-catalysed reduction of oxygen produces hydrogen peroxide, which oxidises ethanol to acetaldehyde. This is the same sequence that drives general oxidation, and it is why an oxidised cider and an acetaldehyde-affected cider so often smell like the same thing.
Film yeasts are the third route and the most dramatic. Pichia membranifaciens and Candida species growing as an aerobic film on a standing vessel oxidise ethanol to acetaldehyde directly, and a vessel with a well-established film can develop a strong sherry character in weeks.
Sulphur dioxide binds acetaldehyde as a hydroxysulphonate adduct, which is non-volatile and cannot be smelled. This is genuinely useful and also explains a common confusion: a cider with a large bound-sulphite fraction has usually had a large acetaldehyde problem at some point, and the sulphite is not free to protect against anything else.
How it happens
| Cause | Stage | How often |
|---|---|---|
| Oxygen contact during racking, storage or bottling | Maturation | common |
| A film of aerobic yeast on the surface of a part-empty vessel | Maturation | common |
| Fermentation stopped or arrested while acetaldehyde was still high | Fermentation | occasional |
| Sulphite added during active fermentation, binding acetaldehyde in place before the yeast could reclaim it | Fermentation | occasional |
| Yeast stressed by temperature swings or nutrient shortage | Fermentation | occasional |
| Long storage in a warm place with no free sulphur dioxide remaining | Storage | occasional |
Can it be put right?
Most cider faults cannot be reversed. Where that is the answer it is given plainly — an honest 'this batch is what it is' is more useful than a procedure that will not work.
| Option | Effectiveness | What it involves |
|---|---|---|
| Add sulphite to bind free acetaldehyde | Partial | The one fault where this genuinely works: the adduct is odourless and the bruised-apple note drops away. It consumes sulphite that will then not be available as an antioxidant, and the addition must be worked out from the supplier’s guidance and kept within the legal limit that applies where the cider is made. |
| Rack off a film yeast and seal the vessel | Partial | Removes the source. Does not remove the acetaldehyde already produced, and the film will return if headspace remains. |
| Air the cider in the glass and serve it slightly warmer | Partial | Acetaldehyde is highly volatile and a mildly affected cider does clear in the glass. A presentation adjustment rather than a repair. |
| Blend into a larger clean volume | Partial | Works against a threshold, so it can succeed where the fault is mild. |
| Restart a healthy fermentation so yeast metabolises the acetaldehyde | Unlikely to work | Yeast can reduce acetaldehyde, but a finished dry cider gives it nothing to work with. This is only worth considering where sugar remains and the ferment genuinely stalled, and never in a sealed container. |
CiderHQ gives no additive dosage figures. An addition is a food-safety decision that depends on legal limits, on the juice in front of you and on what you are protecting against, and the right figure comes from your supplier’s or regulator’s own guidance rather than from a general reference.
Preventing it
- Let the primary fermentation run to its natural end before racking or treating, so the yeast reduces its own acetaldehyde.
- Do not sulphite a fermenting cider; wait until fermentation is genuinely complete.
- Keep vessels full so no aerobic film can establish.
- Maintain a steady fermentation temperature rather than letting a cellar swing day to night.
- Handle finished cider without splashing, and consider inert gas for headspace if it is available.
Acetaldehyde is a good compound to understand early, because it sits at the junction of three different things that go wrong. It is the intermediate on the road from ethanol to vinegar, it is the marker of oxygen having reached the cider, and it is the signature of a yeast film. A bruised-apple nose therefore tells a maker to look at the vessel, not at the recipe.
It is also the compound behind one of the most misunderstood aspects of sulphiting. Free sulphur dioxide is what protects a cider; bound sulphur dioxide protects nothing. Because acetaldehyde binds sulphite avidly, a cider that has been oxidised will consume an addition almost immediately and appear to need more, and adding more to chase a number is how a maker ends up with a cider that smells of struck match as well as bruised apple.
A modest amount is normal and even desirable in some contexts. It is part of the aroma of a young, freshly bottled cider and it falls during early bottle maturation. The fault is a matter of level and of persistence, not of presence.
Where it comes from a deliberate oxidative style — a flor-influenced or long-aired cider — the same molecule is not a fault at all. As with much of this file, the question is whether it belongs in the drink that was intended.
The compounds involved
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.
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.
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.
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 involved
Saccharomyces cerevisiae
The yeast that finishes essentially every cider, whether it arrives in a sachet or from the fruit, the press and the vessel.
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.
Where in the process it arises
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.
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.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
Fermentation monitoring
Watching a running ferment through gravity, temperature, smell and surface behaviour, and reading the rate of change rather than any single figure.
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.
Faults it is confused with
These present similarly. What separates them is set out on each page.
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.
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.
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.
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.
Where this comes up in a guide
The link lands on the step where the fault arises rather than at the top of the pathway, because that is where the decision that causes it is taken.
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 long does cider take to ferment — A warm ferment with cultured yeast can finish in one to two weeks; a cool wild ferment in a cellar may take three months or more. Slow is not the same as stuck — the test is whether gravity is still falling.
- 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.
- What is oxidation in cider
- 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.
Where to go next
- The fault finder — Describe what you can smell and see, and narrow it down from the signs.
- All faults — Grouped by where they come from and how serious they are.
- Cider science — The chemistry and microbiology these faults come out of.
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.
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.
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.