Organism
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.
Also called Brettanomyces bruxellensis, Brett.
- Kind
- Yeast
- Binomial
- Brettanomyces bruxellensis
- Role
- Secondary fermentation, Spoilage
What it does
- Decarboxylates hydroxycinnamic acids from the fruit — p-coumaric and ferulic acid — to vinylphenols, then reduces them with vinylphenol reductase to 4-ethylphenol and 4-ethylguaiacol, the compounds responsible for the character it is named for.
- Ferments residual sugars that Saccharomyces leaves, including cellobiose and other sources it can access with β-glucosidase, which is why it can grow in a cider that appears finished and stable.
- Grows extremely slowly at low nutrient levels over months, so it typically appears during maturation, in barrel or in bottle, rather than during the primary ferment.
- Produces tetrahydropyridines in some conditions, the compounds responsible for mousiness, so a Brett population can carry a second and much less welcome fault alongside the first.
- Generates acetic acid under aerobic conditions, adding to volatile acidity where oxygen ingress is present.
Conditions it works in
What the organism tolerates and what suppresses it. These are the levers a maker actually has: temperature, acidity, air, alcohol and sulphite.
| Condition | What is recorded |
|---|---|
| Temperature | Grows from roughly 10 °C to 32 °C; cold cellaring slows it substantially but does not stop it. |
| pH | Tolerates low pH and grows through the entire cider range. |
| Oxygen | Facultative, and notably active with small amounts of oxygen: slow ingress through a barrel or a closure is close to ideal for it. |
| Alcohol tolerance | High, generally reported to around 13–15% ABV, well above cider strength. |
| Sulphite tolerance | Variable and a subject of active research. Molecular SO2 suppresses it, and control depends sharply on pH because pH governs how much of the added sulphite is in the molecular form; populations differing in tolerance are documented, and CiderHQ does not make strain-level claims about which. |
What it produces
Compounds this organism makes. Which organism made a compound usually decides whether it reads as a feature or as a symptom.
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.
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.
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.
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.
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.
Faults it causes
Faults this organism is implicated in. Several are faults only against a particular expectation — the same activity is a signature elsewhere.
Brettanomyces character
Farmyard, horse-blanket, smoky and sticking-plaster aromas from *Brettanomyces* yeast converting hydroxycinnamic acids into volatile phenols.
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.
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.
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.
Where in the process it appears
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.
Tank maturation
Resting cider in stainless steel or a lined vessel, where the point of the container is that it contributes nothing and admits almost no oxygen.
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.
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.
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.
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.
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.
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.
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.
Microbial succession
The ordered handover of a spontaneous ferment from apiculate yeasts to *Saccharomyces* to lactic acid bacteria, and the spoilage organisms waiting at the end of it.
Pasteurisation
Heating cider enough to inactivate the organisms that would spoil it, either in the sealed package or in-line before filling, at a measurable cost in fresh aroma.
Fermentation vessels
The container a cider ferments in — wood, stainless, plastic, glass or concrete — and how its permeability, thermal mass and resident microflora shape the result.
Sulphite stabilisation
Managing free sulphur dioxide through maturation and at packaging so that a useful antimicrobial and antioxidant fraction survives into the bottle.
About Dekkera bruxellensis
Two things about this organism have to be said together or neither is true. Brettanomyces produces 4-ethylphenol and 4-ethylguaiacol, which at low concentration read as leather, spice, smoke and farmyard, and at high concentration as sticking plaster, medicinal phenol and horse. And in a great deal of traditional cider — West Country farmhouse cider matured in wood, some Basque and Asturian sidra, some French cidre fermier — that character is not an accident that got past quality control. It is what the drink has always tasted like, what its drinkers expect, and in some cases what distinguishes it from an industrial equivalent.
CiderHQ therefore does not classify Brettanomyces as a spoilage organism outright, and does not classify it as a virtue either. The same compounds, in the same cider, are a defining character to one set of drinkers and a fault to another, and the honest position is that the judgement is made by the tradition and by the maker, not by the microbiology. What the microbiology can say is that concentration matters enormously, that the compounds do not go away, and that a population left unchecked moves a cider in one direction only.
The mechanism is a two-step conversion of the fruit’s own hydroxycinnamic acids: decarboxylation to a vinylphenol, then reduction to an ethylphenol. Apples carry these precursors, which is why cider is susceptible at all; the second enzyme, vinylphenol reductase, is what Saccharomyces generally lacks and Brettanomyces has. Wood is its favoured habitat because barrel staves are porous, hard to sanitise, and admit exactly the trace oxygen it prefers.
The nomenclature deserves its own note. Dekkera is the teleomorph genus, erected for the ascospore-forming state; Brettanomyces is the anamorph genus, named first and named from the brewing industry. They are the same organisms. With the abolition of dual naming for fungi, usage has moved decisively towards Brettanomyces, and Brettanomyces bruxellensis is the name in general current use — but Dekkera bruxellensis remains widespread in the literature and in laboratory reports, and this record is filed under the slug the platform’s canonical list assigns while stating the currently preferred binomial.
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.
- 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.
- 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.
- 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.
- Why is sulphite added to cider — Sulphur dioxide suppresses spoilage bacteria and wild yeast before fermentation and protects the finished cider from oxidation. How much of it is active depends strongly on pH, which is why low-acid juice is harder to protect.
Where to go next
- All organisms — Grouped by what each does in the ferment, and by what kind of organism it is.
- Compounds — The chemistry this microbiology produces.
- Troubleshooting — Work from the symptom in the glass back to the organism.
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.
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.