Compound
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.
Also called Cinnamic acids, p-coumaric, caffeic and ferulic acids.
- Class
- Phenolics
- Formula
- Not a single molecule — see below
- How often it matters
- Regularly encountered
What it does in cider
- Occurs in apple mostly bound as esters of quinic acid rather than free, and is liberated slowly by esterase activity during fermentation and maturation.
- Provides the substrate for polyphenol oxidase, driving enzymatic browning immediately after milling.
- Supplies the precursors of the volatile phenols: p-coumaric acid gives 4-ethylphenol, ferulic acid gives 4-ethylguaiacol, caffeic acid gives 4-ethylcatechol.
- Contributes mild bitterness and astringency itself, well below the level of the procyanidins.
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.
- mild bitterness
- slight astringency
Weak direct contributors. Their sensory significance lies almost entirely in what microorganisms and oxygen make of them.
Descriptors it is responsible for
Sensory records that name Hydroxycinnamic acids 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.
- Clove — The clove note of 4-vinylguaiacol, formed when yeast decarboxylates ferulic acid from the fruit’s own phenolics.
- Smoky — A wood-smoke note from 4-ethylguaiacol, the guaiacyl counterpart of the ethylphenol pathway.
- Apple pie — Cooked apple carrying a spice layer — clove, cinnamon or nutmeg — from added spice, from wood, or from yeast-derived vinylguaiacol.
- Crab apple — A small-fruit character combining very high acidity, hard green aldehydes and a marked phenolic grip.
- Pear skin — A dry, gritty, faintly tannic pear note from the fruit’s skin and stone cells, prominent in traditional perry pears.
The structure it moves
| Dimension | What it is |
|---|---|
| Bitterness | A taste sensed at the back of the tongue, distinct from the drying grip of astringency. |
| Phenolic character | Smoky, spicy, leathery or medicinal aromas that sit apart from fruit. |
| Oxidative character | Nutty, bruised-apple, sherry-like or cardboard notes from exposure to air. |
What forms it
Processes that put this compound into the drink, or increase how much of it is there.
Maceration
Holding milled pomace before pressing so that phenolics, aroma precursors and pectin have time to move out of the solid tissue and into the juice.
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.
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.
What removes or limits it
Processes that reduce it, hold it below a threshold, or stop it forming in the first place.
Juice oxidation
The enzymic browning that begins the moment apple juice meets air, which some traditions deliberately allow and others suppress.
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.
Fining
Adding a reactive agent that binds a target colloid and carries it to the bottom, chosen according to whether the problem is tannin, protein or a phenolic taste fault.
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.
Enzymatic browning
The rapid darkening of milled fruit and fresh juice as polyphenol oxidase converts phenolics to quinones, taking colour and some tannin structure with it.
Oxidation
The cumulative effect of oxygen on finished cider: fruit aroma flattens, colour deepens towards amber, and a bruised-apple or sherry-like character replaces the fresh one.
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 Hydroxycinnamic acids
The hydroxycinnamic acids are small, simple phenolic acids built on a three-carbon side chain: p-coumaric acid with one ring hydroxyl, caffeic acid with two, ferulic acid with one hydroxyl and one methoxyl. In apple they are present mostly not as free acids but esterified to quinic acid, chiefly as chlorogenic acid and p-coumaroylquinic acid, and the free pool in fresh juice is small.
That matters because only the free acids are usable by the organisms that turn them into volatile phenols. Dekkera bruxellensis and certain lactic acid bacteria carry a decarboxylase that removes the acid group to give a vinyl phenol, and Dekkera additionally carries a reductase that converts the vinyl compound to the corresponding ethyl phenol. The specific product depends on the specific acid: p-coumaric acid becomes 4-vinylphenol and then 4-ethylphenol, ferulic acid becomes 4-vinylguaiacol and then 4-ethylguaiacol, caffeic acid becomes 4-ethylcatechol.
The rate-limiting step in a cider is therefore usually the hydrolysis of the esters, not the microbial conversion. This is why phenolic character in cider tends to appear gradually over months of barrel maturation rather than at the moment an organism arrives, and why a cider that has spent two years in wood carries more of it than the same cider bottled at six months, given the same microbiology.
Before any of that, the same compounds are what a fresh juice browns with. Their catechol and guaiacyl rings are what polyphenol oxidase recognises, and the quinones they become drive the coupled oxidation that recruits the procyanidins into the reaction.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
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.
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.
Acids
Quinic acid
The second acid of apple juice, better known as the part of chlorogenic acid that is not caffeic acid, and a small but real contributor to the astringent grip of cider fruit.
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-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.
Enzymes
Polyphenol oxidase
The copper enzyme that turns cut apple brown within seconds, and the reason a cidermaker has to decide, at the press, whether to let the juice oxidise or to stop it.
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 oxidation in cider
- Is cider vegan — Cider itself is a plant product, but some producers clarify it with animal-derived finings such as gelatine, isinglass or chitosan. Vegan status therefore depends on the fining regime, which is why some ciders are certified and others are not.
- 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 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.
- 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.
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.
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.
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 apple phenolics and cider sensory perception
Various journals · peer-reviewed literature · registered as competent for this subject
Registered as a class rather than as one paper, because the mechanisms CiderHQ describes — tannin chain length driving the split between bitterness and astringency, salivary protein precipitation, enzymatic browning — are established across many studies rather than resting on any single one. Individual papers are cited where a specific number is quoted.
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.