Compound
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.
Also called 5-O-caffeoylquinic acid.
- Class
- Phenolics
- Formula
- C16H18O9
- How often it matters
- Regularly encountered
What it does in cider
- Serves as the principal substrate for polyphenol oxidase, so it is the compound whose oxidation turns cut apple flesh and fresh juice brown within minutes.
- Generates quinones on oxidation that go on to attack other phenolics and thiols, driving the coupled reactions that fix colour and strip aroma.
- Yields caffeic acid on hydrolysis, which certain lactic bacteria and Dekkera convert onwards to 4-ethylcatechol.
- Contributes a mild bitterness of its own, well below that of the procyanidins it sits alongside.
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
- faint astringency
Individually a weak taste contributor compared with the procyanidins. Its sensory importance is indirect: what it becomes after oxidation matters more than what it tastes like.
Descriptors it is responsible for
Sensory records that name Chlorogenic acid as a cause. Each states the perception and the mechanism behind it.
- Bittersweet apple — The soft, low-acid, faintly earthy ripe-fruit character of traditional West Country cider fruit, inseparable from its phenolic weight.
- Bruised apple — The brown, soft, faintly sweet note of enzymatic browning, met wherever apple tissue or juice has met air.
- Apple pomace — The wet, sweet-sour smell of freshly milled and pressed pomace: cut-apple aldehydes over the first traces of wild fermentation and browning.
- Windfall apple — Bruised, soil-touched ripe fruit character from apples collected off the orchard floor, sitting on the border between depth and taint.
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. |
| 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.
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.
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.
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.
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.
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.
Brettanomyces character
Farmyard, horse-blanket, smoky and sticking-plaster aromas from *Brettanomyces* yeast converting hydroxycinnamic acids into volatile phenols.
Colour loss
A cider left noticeably paler than it should be, usually because fining, filtration or sulphite has removed the phenolic material that gave it colour.
Excessive bitterness
Bitterness that dominates the palate rather than supporting it, usually from a blend weighted too heavily towards high-tannin fruit or extracted too hard.
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.
Lactobacillus collinoides
A lactic acid bacterium first described from cider, and the organism most closely associated with acrolein bitterness through its conversion of glycerol.
About Chlorogenic acid
Of all the phenolic compounds in an apple, chlorogenic acid is usually the most abundant single molecule — an ester of caffeic acid with quinic acid, present in flesh as well as skin, and present in dessert fruit as well as cider fruit. It is not a tannin, contributes only mildly to bitterness, and would be a minor entry were it not for what happens to it in the presence of oxygen.
Polyphenol oxidase works fastest on compounds carrying two adjacent hydroxyl groups on a ring, and chlorogenic acid is the fruit’s main supply of exactly that arrangement. The enzyme converts it to a quinone within seconds of the fruit being cut, and the quinone is far more reactive than the phenol it came from: it condenses with other phenolics to give the brown pigments, it oxidises thiols and can strip aroma compounds out of the juice, and it recruits other phenols into the reaction that would not have oxidised on their own. This coupled oxidation is why juice browns as a cascade rather than a slow fade.
A cidermaker meets this as a decision rather than a problem. Sulphiting the juice promptly, or pressing under inert gas, preserves pale colour and fresh aroma. Allowing the juice to brown deliberately — the traditional practice behind much English and French cider — consumes the oxidisable phenolics before fermentation, so the finished cider is more stable to air afterwards and has a softer, less bitter phenolic profile because a portion of the tannin has been polymerised and dropped out.
The compound has a second career after fermentation. Slowly hydrolysed, it releases free caffeic acid, one of the hydroxycinnamic acids that Dekkera bruxellensis and some lactic bacteria decarboxylate and reduce to volatile phenols. In apple the resulting compound is 4-ethylcatechol, which sits alongside 4-ethylphenol in the leather-and-farmyard end of cider aroma.
Related compounds
Compounds it is formed from, converted into, confused with, or routinely met alongside.
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.
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.
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
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
Total phenolics
The single number used to summarise everything phenolic in a juice, useful for comparing fruit and misleading whenever it is used to predict how a cider will taste.
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.
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.
- When are cider apples harvested — In the northern hemisphere the cider harvest runs from about September to November, with late bittersweets such as Dabinett and Vilberie coming in last. Cider fruit is generally picked riper than dessert fruit, and often gathered from the ground.
- 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.
- Does cider contain antioxidants — Cider carries apple polyphenols, and tannic bittersweet ciders carry considerably more than pale ones made from dessert fruit. What that means for health is a clinical question this site does not adjudicate.
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.