Compound
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.
Also called 4-EP, Ethylphenol.
- Class
- Phenolics
- Formula
- C8H10O
- How often it matters
- Central to how cider behaves
What it does in cider
- Is made in two steps from p-coumaric acid: a decarboxylase removes the acid group to give 4-vinylphenol, and a reductase converts that to 4-ethylphenol.
- Requires an organism carrying both enzymes, which in practice means Dekkera bruxellensis and its relatives, and to a lesser extent some lactic acid bacteria.
- Accumulates slowly, because the rate is usually limited by the release of free p-coumaric acid from its bound ester rather than by the microbiology.
- Cannot be removed by anything short of adsorptive treatment, so control means preventing formation rather than correcting it.
- Persists in wood, so an infected barrel reinfects every subsequent fill.
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.
- horse
- stable
- sticking plaster
- leather
- smoke
- medicinal
- barnyard
Reported in wine at a few hundred micrograms per litre, and lower when combined with 4-ethylguaiacol, which almost always accompanies it. In cider the concentration at which it is judged a fault is not a fixed figure: several traditional styles carry it as an expected part of their character, and the same level that would condemn a fresh modern cider is unremarkable in a farmhouse one.
Descriptors it is responsible for
Sensory records that name 4-Ethylphenol as a cause. Each states the perception and the mechanism behind it.
- Farmyard — The composite animal-and-straw note of volatile phenols with short-chain fatty acids, central to traditional farmhouse character.
- Sticking plaster — The sharp antiseptic-and-adhesive note of 4-ethylphenol, the principal volatile phenol of *Brettanomyces* activity in cider.
- Horse blanket — An animal, stable-like note from 4-ethylphenol combined with branched short-chain fatty acids.
- Medicinal — A hospital-antiseptic note from volatile phenols, or from chlorophenols formed by chlorine contacting phenolic material.
- Leather — A dry, tanned-hide note from low levels of volatile phenols over well-polymerised tannin.
The structure it moves
| Dimension | What it is |
|---|---|
| Phenolic character | Smoky, spicy, leathery or medicinal aromas that sit apart from fruit. |
| Fermentation character | Aromas made by the ferment rather than carried in from the fruit. |
| Freshness | Whether the drink smells and tastes of live fruit or of time and air. |
What forms it
Processes that put this compound into the drink, or increase how much of it is there.
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.
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.
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.
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.
What removes or limits it
Processes that reduce it, hold it below a threshold, or stop it forming in the first place.
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.
Sanitation
The cleaning and sanitising discipline that underlies every other process, and why removing soil must come before any attempt to kill organisms.
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.
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.
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.
Organisms that produce it
Which organism is responsible usually decides whether the compound is a feature or a symptom.
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.
Lactobacillus collinoides
A lactic acid bacterium first described from cider, and the organism most closely associated with acrolein bitterness through its conversion of glycerol.
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.
About 4-Ethylphenol
The chemistry is short and specific. Apple juice contains p-coumaric acid, mostly bound as p-coumaroylquinic acid. Free it, and an organism carrying a hydroxycinnamate decarboxylase converts it to 4-vinylphenol; an organism carrying a vinylphenol reductase as well converts that to 4-ethylphenol. Plenty of yeasts and bacteria have the first enzyme. Very few have both, and in cider the organism that matters is Dekkera bruxellensis, the same yeast that produces the equivalent character in wine and in certain Belgian beers.
Two features of the pathway explain how the character behaves in a cellar. The first is that the supply of substrate is usually the bottleneck. Free p-coumaric acid in fresh cider is scarce, and it is released slowly by hydrolysis over months, so phenolic character builds gradually through long maturation rather than appearing when the organism arrives. A cider bottled at six months and the same cider bottled at three years, both carrying Dekkera, will differ substantially. The second is that Dekkera is exceptionally patient. It tolerates alcohol, tolerates low nutrient levels, works slowly at cellar temperature and survives sulphite better than most spoilage yeasts, so it can persist in a barrel for years and transfer to every cider put into it.
Whether the result is a fault is not a chemical question. Traditional West Country farmhouse cider, much Norman cider, and a good deal of Asturian sidra carry recognisable ethylphenol character, and in those contexts it is part of what the style is — the farmyard and leather notes that experienced drinkers of those ciders expect and that competition style guidelines describe rather than penalise. In a clean, aromatic, fruit-driven modern cider the same compound at the same concentration is a spoilage marker, and one that indicates a hygiene problem likely to recur.
Control is entirely preventive, because nothing removes it once formed. The levers are the ordinary ones: adequate free sulphur dioxide at a pH where sulphur dioxide works, prompt racking off lees, filling vessels full, sterile filtration before packaging where the product demands it, and — decisively — attention to wood. Barrels are porous and are effectively impossible to sterilise by rinsing, so a producer who wants no ethylphenol and also wants oak has a genuinely difficult problem, and a producer who wants both character and control has to accept managing an organism rather than excluding it.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
Phenolics
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.
Phenolics
4-Vinylguaiacol
The clove compound produced by any yeast carrying a ferulic acid decarboxylase, which gives a spicy character without any Brettanomyces being involved.
Phenolics
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.
Phenolics
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.
Phenolics
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.
Nitrogen compounds
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.
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.
- 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.
- 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.
- 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.
- 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.
- 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
- 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.
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.
Institut national de recherche pour l’agriculture, l’alimentation et l’environnement (INRAE)
INRAE · research institute · retrieved 2026-08-24
French national agricultural research. Its Angers programme produced much of the published work on apple procyanidin chain length and on the relationship between polymer size, bitterness and astringency.
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.