Fault
Brettanomyces character
Farmyard, horse-blanket, smoky and sticking-plaster aromas from Brettanomyces yeast converting hydroxycinnamic acids into volatile phenols.
Also called brett, farmyard character, phenolic taint.
- Severity
- Quality — the drink is worse for it
- Where it starts
- Maturation, Fermentation, Packaging, Stabilisation
- Can it be fixed?
- Partly — it can be reduced, not removed
- When you notice it
- Maturation, Storage
- Signs
- 6 recorded
What it is
Brettanomyces character is the group of aromas produced by Dekkera bruxellensis and its relatives, yeasts that survive where Saccharomyces has finished, feed on sugars and acids that other organisms leave behind, and convert phenolic acids in the cider into volatile phenols. The result runs from a faint smoky, leathery lift at low concentration to farmyard, horse blanket, wet animal and sticking plaster at high. Whether it is a fault is genuinely contested, and depends on the style and on the level.
What you notice
Grouped by sense, because that is how the fault presents itself rather than how it works.
On the nose
- A farmyard or stable-floor smell — warm animal rather than manure
- A sticking-plaster or antiseptic note — This is 4-ethylphenol at higher concentration, and it is the point at which most tasters stop calling it complexity.
- Smoke, leather or clove sitting over the fruit
On the palate
- A dry, slightly metallic finish with the fruit thinned out
How the batch behaves
- A cider that develops the character progressively in bottle over a year or more
To look at
- A slow, fine haze and a light gas prickle in a cider that was thought to be stable — The same organism ferments residual sugar slowly, which is why brett and unexpected carbonation often appear together.
The words for it: Farmyard, Sticking plaster, Horse blanket, Medicinal, Smoky, Leather, Bacon. Each links to what produces it.
When it appears. Slowly, over months, and it continues in the bottle. A cider that carried a trace at bottling can be dominated by it a year later, because the organism is still present and still working.
Individual sensitivity to 4-ethylphenol varies substantially, so a split panel on a Brett-affected cider is an expected result rather than evidence of a badly run tasting. That variation is a large part of why the fault-or-feature argument is so persistent.
Various journals, Washington State University Northwestern Washington Research and Extension Center
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.
- Traditional farmyard character — Intensity and whether the fruit survives underneath. The same compounds are a defining feature of several traditional ciders and a fault when they have taken the fruit with them.
- Mould taint — Brett is barnyard, leather and sticking plaster; mould taint is damp cellar, mushroom and musty. Brett is a live organism in the cider, mould taint arrived on the fruit or the cork.
- Mousiness — Both can come from the same cellar conditions and mousiness is undetectable by smell. If a cider has Brett character, check the finish for the mousy note as well rather than assuming one explains everything.
- Smoke taint — Smoky is a Brett descriptor and also a taint descriptor. A Brett cider carries the rest of the family — leather, barnyard — with it; smoke taint arrives alone.
This is the clearest threshold case on the site. Low concentrations add complexity and are part of what a reader recognises in a traditional West Country or Norman cider; high concentrations flatten everything else. The record is written as a threshold question rather than a presence question, and the comparison page sets the two readings side by side.
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.
Apple juice carries hydroxycinnamic acids, chiefly p-coumaroylquinic acid and chlorogenic acid. Dekkera bruxellensis possesses two enzymes in sequence: a cinnamate decarboxylase that converts the free acid to a vinyl phenol, and a vinylphenol reductase that converts the vinyl phenol to the corresponding ethyl phenol. The pathway is the reason the yeast produces aromas that Saccharomyces does not.
The two principal products are 4-ethylphenol, from p-coumaric acid, and 4-ethylguaiacol, from ferulic acid. 4-Ethylphenol carries the barnyard, horse and plaster character; 4-ethylguaiacol is smokier and more clove-like. Their ratio differs between strains and between drinks, and the intermediate 4-vinylguaiacol contributes a separate clove note in its own right.
What makes the organism persistent is its metabolic range. It ferments sugars that Saccharomyces leaves, including some that survive in a nominally dry cider, tolerates ethanol and low pH, tolerates sulphur dioxide better than most yeasts, and can grow extremely slowly over months. A population too small to see can be large enough to change a cider over a year in bottle.
It also colonises equipment. Wood is the classic refuge, because the yeast penetrates the grain beyond the reach of surface cleaning, but hoses, valves, gaskets and filler heads all harbour it. A cellar that has brett tends to keep having it.
How it happens
| Cause | Stage | How often |
|---|---|---|
| Wooden vessels carrying an established population from previous fills | Maturation | common |
| Residual fermentable sugar in a cider believed to be dry | Maturation | common |
| Spontaneous fermentation of unsulphited juice, which admits whatever the orchard and cellar carry | Fermentation | common |
| Long, warm maturation with little or no free sulphur dioxide | Maturation | common |
| Cross-contamination through hoses, pumps and bottling equipment | Packaging | occasional |
| Cider at a pH high enough that any sulphite present is largely ineffective | Stabilisation | 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 |
|---|---|---|
| Stop the population and prevent further development | Partial | Sterile filtration or pasteurisation before packaging halts progression, so the cider stays as it is rather than getting worse in bottle. Neither undoes what has happened, and both require the appropriate equipment. |
| Blend into a larger clean volume | Partial | Works where the level is moderate, because the character is threshold-dependent. It also moves live organisms into the clean batch unless the blend is subsequently stabilised. |
| Accept the character as part of the cider’s identity | Partial | A legitimate answer at low levels, and one many traditional ciders have effectively taken. It is not available once the sticking-plaster note has arrived. |
| Recover a contaminated wooden vessel by cleaning | Unlikely to work | Surface treatment does not reach the depth the yeast occupies. Steaming and shaving help commercially; at home the realistic options are to keep the barrel for cider where the character is acceptable, or to stop using it. |
| Remove volatile phenols from cider once they are formed | Not possible | The compounds are stable and there is no domestic route to removing them. Fining with certain adsorbents lowers them slightly at considerable cost to everything else. |
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
- Decide explicitly whether the cellar is one that permits brett or one that excludes it, because the two require different equipment and different routines.
- Ferment to genuine dryness and confirm it by measurement rather than by assumption.
- Maintain free sulphur dioxide at a level appropriate to the measured pH, working from the supplier’s guidance and the applicable legal limit.
- Keep dedicated hoses and fittings for wooden vessels if wood is used at all.
- Clean and sanitise everything the cider touches on the way to the bottle, since the filler is a classic point of transfer.
- Where a cider is to be sweetened and bottled, take the microbial stability of the product seriously; brett will ferment sugar that other organisms cannot.
Brettanomyces is the fault entry that most needs its contested status stated plainly. In a Belgian lambic the organism is the point. In several traditional English and French ciders a low level of the same character is part of what people recognise and value. In a modern fresh-fruited cider it is a defect that the maker would be embarrassed by. The chemistry is identical in all three cases.
Where a line can be drawn with some confidence is at 4-ethylphenol, and specifically at the point where the aroma stops reading as leather or smoke and starts reading as sticking plaster or antiseptic. Very few tasters defend a cider past that point, and it is a useful practical marker for a maker deciding whether a batch has a character or a problem.
The behaviour that causes the most trouble is slowness. A population that is invisible at bottling can work through the residual sugar of a nominally dry cider over a year, producing volatile phenols and carbon dioxide as it goes. A cider that changes markedly in bottle, gains a faint prickle and throws a fine haze is describing an active organism, and the sensible response is to check whether the bottles are gaining pressure.
Almost everything about controlling it is about equipment rather than recipe. The yeast lives in the cellar, not in the fruit, and a maker who has decided against brett character has to make that decision about their wood, their hoses and their filler as much as about their sulphiting.
The compounds involved
4-Ethylphenol
The horse-and-sticking-plaster compound, made by *Dekkera* from a phenolic acid the fruit supplied, and the clearest case in cider of one molecule being a tradition in one glass and a fault in another.
4-Ethylguaiacol
The smoky, clove-like partner of 4-ethylphenol, made by the same organisms from ferulic acid, and the compound that pushes phenolic character from medicinal towards spicy.
4-Vinylguaiacol
The clove compound produced by any yeast carrying a ferulic acid decarboxylase, which gives a spicy character without any Brettanomyces being involved.
Hydroxycinnamic acids
The family of small phenolic acids that browns a juice, and whose release from their esters supplies the raw material for every volatile phenol a cider can develop.
p-Coumaroylquinic acid
The second most abundant hydroxycinnamic ester in apple, and the reservoir from which the p-coumaric acid behind 4-ethylphenol is slowly released.
Chlorogenic acid
The most abundant single phenolic in apple juice, the preferred substrate of the enzyme that browns it, and the precursor of one of the volatile phenols behind farmyard character.
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.
Where in the process it arises
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.
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.
Sanitation
The cleaning and sanitising discipline that underlies every other process, and why removing soil must come before any attempt to kill organisms.
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.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
Bottle maturation
What happens to a cider after it is sealed in glass — which for most ciders is slow decline rather than improvement, and saying so is more useful than implying otherwise.
Faults it is confused with
These present similarly. What separates them is set out on each page.
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.
Unwanted refermentation
Fermentation restarting in a sealed container, generating carbon dioxide that was not planned for and pressure that a bottle may not withstand.
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.
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.
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 to taste cider and perry
How do I taste cider properly?
Your first batch of cider
I have apples and no equipment. What do I actually do?
Writing a tasting note that is worth reading later
How do I write a tasting note that is actually useful?
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 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.
- 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 mousiness in cider — Mousiness is a fault caused by tetrahydropyridines produced by *Brettanomyces* and some lactic acid bacteria. It tastes of stale grain or a mouse cage and appears in the aftertaste rather than in the aroma.
- Why did my sweetened cider start fermenting again — Live yeast met the sugar that was added back. Sweetening is only stable if the yeast has been removed by sterile filtration, killed by pasteurisation, or held in check by sorbate together with sufficient sulphite.
- Does cider get better with age — Some does. Tannic, dry, bottle-conditioned ciders and ice ciders can gain for several years. Light, fruit-driven and commercially filtered ciders lose their aroma and do not gain anything in exchange.
- 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.
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.