Compound
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.
Also called 4-EG.
- Class
- Phenolics
- Formula
- C9H12O2
- How often it matters
- Occasional
What it does in cider
- Is produced from ferulic acid by the same decarboxylase and reductase pair that produce 4-ethylphenol from p-coumaric acid.
- Accompanies 4-ethylphenol almost invariably, in a ratio set by the fruit’s balance of the two precursor acids.
- Shifts the character of a phenolic cider towards smoke, clove and bacon rather than horse and plaster.
- Lowers the concentration at which the pair together become perceptible, so a mixture is noticed sooner than either alone.
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.
- smoke
- clove
- bacon
- spice
- medicinal
Generally reported at a lower threshold than 4-ethylphenol, and the two are judged together in practice because they almost always occur together.
Descriptors it is responsible for
Sensory records that name 4-Ethylguaiacol 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.
- Horse blanket — An animal, stable-like note from 4-ethylphenol combined with branched short-chain fatty acids.
- Smoky — A wood-smoke note from 4-ethylguaiacol, the guaiacyl counterpart of the ethylphenol pathway.
- Toast — Toasted-bread and caramel-wood character from furfural and related furans generated when barrel staves are heated.
- Acrid smoke — Harsh, ashy smoke from atmospheric smoke absorbed by the fruit, chemically distinct from microbial smokiness.
- Bacon — A savoury smoked-meat note from 4-ethylguaiacol with residual sweetness, met in tannic ciders with Brett character.
- Cedar — A dry, resinous, pencil-shaving wood note from oak volatile phenols with tannin, distinct from sweet oak character.
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. |
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.
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.
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.
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.
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.
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.
About 4-Ethylguaiacol
Ferulic acid differs from p-coumaric acid by a single methoxyl group on the ring, and that one substituent changes the smell of the product entirely. Where 4-ethylphenol is horse and sticking plaster, 4-ethylguaiacol is smoke, clove and cured bacon — recognisably related, considerably more attractive to most tasters, and much closer to what people mean when they describe a phenolic cider as spicy rather than dirty.
The two are made by the same enzymes in the same organisms, so they arrive together, and the ratio between them depends mainly on how much of each precursor acid the fruit supplied. Apples generally carry more p-coumaric than ferulic acid, which is why cider phenolic character tends to sit at the medicinal end rather than the smoky one.
A third member of the family is worth naming here because it is specific to apple. Caffeic acid, released from chlorogenic acid, yields 4-ethylcatechol by the same route, and this compound is reported at higher concentrations in cider than in wine — one reason cider Brettanomyces character does not smell quite like wine Brettanomyces character even when the organism is the same.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
Phenolics
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.
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
Eugenol
The clove compound of oak and of actual cloves, arriving in cider from wood contact or from deliberate spicing rather than from the fruit.
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.
- 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.
- 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.
- What causes a farmyard or barnyard smell in cider — Usually 4-ethylphenol, made by Brettanomyces from hydroxycinnamic acids in the juice. Whether it is a fault depends on the cider: a trace adds complexity to a traditional cider and dominates a fresh fruit-forward one.
- Is brettanomyces anomalus different from brettanomyces bruxellensis
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.