Fault
Mousiness
A retronasal taint of mouse cage, stale popcorn or crackers that appears only after swallowing — and that a substantial fraction of people cannot detect at all.
Also called mousy taint, mouse cage character.
- Severity
- Serious — the batch may not be salvageable
- Where it starts
- Maturation, Blending, Storage
- Can it be fixed?
- Yes, if caught in time
- When you notice it
- Fermentation, Maturation, Storage
- Signs
- 5 recorded
What it is
Mousiness is a taint produced by certain lactic acid bacteria and by Brettanomyces yeasts, caused by a group of nitrogen heterocycles known collectively as the tetrahydropyridines. Its defining feature is perceptual rather than chemical: the compounds are not volatile at cider pH, so the drink smells clean in the glass, and the taint appears only in the mouth and after swallowing, when saliva raises the pH and releases them. It is described as mouse cage, stale popcorn, cracker or dirty dishcloth, and it does not go away.
What you notice
Grouped by sense, because that is how the fault presents itself rather than how it works.
On the nose
- Nothing wrong on the nose at all — A clean glass is characteristic, not reassuring. The compounds are not volatile at the pH of cider.
On the palate
- A stale popcorn, cracker or corn-chip flavour arriving several seconds after swallowing
- A dirty dishcloth or caged-rodent impression on the back palate
- A long, dragging aftertaste that builds rather than fades across repeated sips
How the batch behaves
- Two people disagreeing completely about whether a cider is faulty — The most diagnostic sign in practice, for the reason set out below.
The words for it: Mousy. Each links to what produces it.
When it appears. Can be present for months before anyone identifies it, because it is essentially undetectable in the aroma of the glass. It is usually found by a taster rather than by a cellar check.
The tetrahydropyridines responsible are non-volatile at cider pH and become volatile only when raised towards neutrality — which happens in the mouth, in saliva, after swallowing. That is the mechanism behind the delay, and behind the wide variation between people in whether they detect it at all.
Various journals, Andrew Lea
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.
- Brett character — Brett is smelled; mousiness is not. If the aroma is clean and the finish is wrong some seconds after swallowing, it is not Brett.
- Lactic off-flavours — Both come from lactic acid bacteria and can share a cellar. Lactic off-flavours are on the palate immediately; mousiness arrives only retronasally and only after swallowing.
- A taster who cannot detect it — Detection depends on the taster’s own saliva pH raising the compounds into their volatile form. A person who reports nothing may be reporting accurately about themselves, which is why a second opinion is worth more here than anywhere else on this list.
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.
The compounds responsible are substituted tetrahydropyridines — principally 2-acetyltetrahydropyridine and 2-ethyltetrahydropyridine — together with 2-acetylpyrroline. They are formed from the amino acid lysine, or from ornithine, in the presence of ethanol, by heterofermentative lactic acid bacteria including Lactiplantibacillus plantarum and Lactobacillus brevis, and by Dekkera bruxellensis and Brettanomyces anomalus.
These molecules are bases. At the pH of cider they are protonated, charged and effectively non-volatile, so they stay in solution and cannot reach the olfactory epithelium through the nose. Saliva, at around neutral pH, deprotonates them in the mouth; they become volatile and are carried retronasally. This is the entire explanation for the delayed, after-swallowing character of the fault, and it is why smelling a suspect cider tells you nothing.
Detection varies enormously between people, and this is the single most important practical fact about the fault. A substantial fraction of tasters cannot perceive mousiness at all, and this appears to be related to individual differences in saliva pH and buffering as well as to differences in sensitivity to the compounds themselves. Two competent judges can therefore reach opposite conclusions about the same sample in good faith.
The organisms responsible are favoured by the same conditions that favour other spoilage: high pH, low or absent free sulphur dioxide, residual sugar or malic acid, warmth and time. Oxygen exposure increases production by Brettanomyces.
How it happens
| Cause | Stage | How often |
|---|---|---|
| Uncontrolled lactic acid bacteria activity in a low-acid, high-pH cider | Maturation | common |
| A Brettanomyces population in a cellar or in wood | Maturation | common |
| Little or no free sulphur dioxide during a long maturation | Maturation | common |
| A blend built entirely from low-acid bittersweet fruit, leaving the pH high | Blending | occasional |
| Oxygen ingress during maturation, which raises production by Brettanomyces | Maturation | occasional |
| Warm storage over an extended period | 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 |
|---|---|---|
| Treat the diagnosis as information about the cellar | Reliable | Mousiness in one vessel usually means the conditions for it exist across the cellar. Checking pH, sulphite and hygiene across every batch is the useful action. |
| Stabilise the batch to prevent it worsening | Partial | Filtration or pasteurisation stops further production. Only worth doing on a batch caught very early, where the level is genuinely marginal. |
| Blend into a larger clean volume | Unlikely to work | Sometimes attempted; rarely satisfactory, because the perception accumulates across a tasting rather than being judged sip by sip, and because it carries the organisms into the clean cider. |
| Remove tetrahydropyridines from affected cider | Not possible | No treatment available to a home or small commercial producer removes them. This is a fault to be prevented, not managed. |
| Mask the taint with sweetening or carbonation | Not possible | The taint is retronasal and post-swallow, and it builds with repeated sips. Nothing added to the cider covers it. |
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
- Build blends that reach a genuinely low pH; the acid fruit in a traditional blend is doing microbiological work as well as sensory work.
- Measure pH rather than judging acidity by taste, because a cider can taste sharp and still sit at a pH that offers little protection.
- Maintain free sulphur dioxide appropriate to that measured pH, following the supplier’s guidance and the applicable legal maximum.
- Keep vessels full and oxygen out during maturation.
- Take cleaning of wood, hoses and fittings seriously, since the organisms are cellar residents.
- Taste maturing cider by swallowing a small amount and waiting, rather than by nose alone — and have someone else taste it too.
Mousiness is the fault most often missed, and the reason is worth stating clearly rather than in passing: a substantial proportion of people simply cannot detect it. Anosmia to the tetrahydropyridines is common enough that a maker can taste their own cider carefully, find nothing, and hand out bottles that other people find undrinkable. It is the one fault where an inability to perceive it is not evidence that it is absent.
The second reason it is missed is that it hides from the nose. Every habit a taster has — swirl, sniff, assess — is defeated by a compound that is non-volatile until it meets saliva. The only reliable method is to take a small amount, swallow, wait ten or fifteen seconds, and pay attention to what arrives afterwards. Some tasters improve their chances by holding a little cider in the mouth for longer, or by breathing out through the nose after swallowing.
Because of both of those, the practical advice is social rather than technical. Get more than one person to taste, and take seriously the person who says something is wrong even when nobody else can find it. Disagreement about a cider is a signal, not a stalemate.
It should not be described as dangerous. There is no evidence that the compounds involved present a health risk at the concentrations found in fermented drinks; the drink is unpleasant, sometimes profoundly so, and that is a different claim. The reason to treat mousiness as serious is that it cannot be fixed and it renders a batch unusable.
The compounds involved
Tetrahydropyridines
The compounds behind mousiness, which cannot be smelled in the glass because they are not volatile at cider pH and appear only as an aftertaste once saliva has raised the pH in the mouth.
Lactic acid
The softer acid that replaces malic when malolactic fermentation runs, halving the acid a cider carries and changing its texture as much as its sharpness.
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.
Amino acids
The largest usable nitrogen fraction in apple juice, and the raw material from which yeast builds both its own protein and most of the aroma compounds a cider carries.
The organisms involved
Dekkera bruxellensis
The yeast behind 4-ethylphenol, and the organism that a cider tradition may regard as its signature or its ruin depending on where and how it is made.
Brettanomyces anomalus
The second Brettanomyces species regularly recovered from cider and beer, less studied than *B. bruxellensis* but capable of the same phenolic chemistry.
Lactiplantibacillus plantarum
A versatile lactic acid bacterium, renamed out of *Lactobacillus* in 2020, capable of malolactic conversion and of a range of faults depending on conditions.
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.
Lactobacillus collinoides
A lactic acid bacterium first described from cider, and the organism most closely associated with acrolein bitterness through its conversion of glycerol.
Where in the process it arises
Malolactic fermentation
A bacterial conversion of sharp malic acid into softer lactic acid, which lowers acidity, changes aroma, and in most traditional cider happens whether it was planned or not.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
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.
Acid adjustment
Correcting the acidity of low-acid juice before fermentation, usually with malic acid, and the difference between the pH question and the titratable acidity question.
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 confused with
These present similarly. What separates them is set out on each page.
Brettanomyces character
Farmyard, horse-blanket, smoky and sticking-plaster aromas from *Brettanomyces* yeast converting hydroxycinnamic acids into volatile phenols.
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.
Ropiness
Cider that pours thick and pulls into an oily thread, caused by lactic acid bacteria secreting long-chain glucan polymers into the drink.
Low acidity
A cider without enough acid to give it definition, tasting soft, heavy and dull — and sitting at a pH that leaves it exposed to spoilage organisms.
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.
Diagnosing a batch that has gone wrong
Something is wrong with my cider. How do I work out what?
How cider is made, start to finish
How is cider actually made?
How to taste cider and perry
How do I taste cider properly?
Making perry
How do I make perry, and what is different about it?
Your first batch of cider
I have apples and no equipment. What do I actually do?
From sound cider to good cider
I can make cider without faults. How do I make it better?
Structured sensory assessment
How is cider assessed when the answer has to hold up?
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 malolactic fermentation — Malolactic fermentation is a bacterial conversion of sharp malic acid into softer lactic acid, releasing carbon dioxide. It lowers total acidity and raises pH, and in cider it is often the source of a farmyard or buttery note as well.
- What is brettanomyces — *Brettanomyces*, correctly *Dekkera* in its spore-forming form, is a slow yeast that ferments sugars other strains leave behind and produces volatile phenols smelling of farmyard, leather or sticking plaster. In cider it is common and not always unwanted.
- Why does cider contain lactic acid — Because malolactic bacteria converted the malic acid into it. Malic acid has two acid groups and lactic acid has one, so roughly half the titratable acidity disappears and the cider tastes softer.
- Is brett a fault in cider — It depends on the tradition and the level. A trace of *Brettanomyces* character is expected in much West Country and Spanish cider; at high concentration it flattens fruit and dominates everything else, and few would defend it then.
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.
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.
Washington State University cider research programme
Washington State University Northwestern Washington Research and Extension Center · university · passage verified 2026-08-24
The single most useful open cider dataset CiderHQ has found. The programme’s cultivar performance database gives juice chemistry, orchard behaviour, bloom and harvest timing and cider-maker tasting notes for 73 cultivars grown at one maritime site over fifteen years, and — unusually — publishes its classification thresholds alongside its figures, so the classification can be checked rather than taken on trust. It also states outright that its results diverge from the English ones. Read in full on 2026-08-24 and transcribed into `data/trials/wsu-mount-vernon.ts`; four published figures were found to be impossible and are withheld there with their reasons.