Compound
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.
Also called PPO, Catechol oxidase, Tyrosinase.
- Class
- Enzymes
- Formula
- Not a single molecule — see below
- How often it matters
- Present in every cider
What it does in cider
- Oxidises phenolic compounds carrying two adjacent ring hydroxyls to quinones, using molecular oxygen, at a rate measured in seconds rather than hours.
- Sits harmlessly separated from its substrates in intact fruit and meets them only when cells are ruptured by bruising, milling or pressing.
- Sets off a cascade rather than a single reaction: the quinones it makes go on to oxidise phenols the enzyme cannot touch, and to condense into brown pigment.
- Strips aroma as it goes, because quinones react with thiols and with other nucleophiles that would otherwise have contributed to smell.
- Precipitates a portion of the tannin as its products polymerise, which softens a cider’s phenolic profile before fermentation even begins.
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.
- browning
- loss of fresh aroma
- softened tannin
- nutty depth in deliberately oxidised juice
Not perceived itself. What is perceived is the difference between two ciders from the same fruit — one sulphited at the press and one allowed to brown — which is among the largest single differences a cidermaker can create without changing the fruit.
Descriptors it is responsible for
Sensory records that name Polyphenol oxidase as a cause. Each states the perception and the mechanism behind it.
- 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.
- Black tea — A dry, tannic, faintly leafy note from polymerised procyanidins with mild oxidative development.
- Dried apple — Concentrated, leathery apple character from moderate oxidation, sitting between fresh fruit and full oxidative development.
- Pear skin — A dry, gritty, faintly tannic pear note from the fruit’s skin and stone cells, prominent in traditional perry pears.
- 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 |
|---|---|
| Oxidative character | Nutty, bruised-apple, sherry-like or cardboard notes from exposure to air. |
| Bitterness | A taste sensed at the back of the tongue, distinct from the drying grip of astringency. |
| Astringency | The drying, rough sensation left after swallowing. |
| 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.
Milling
Reducing whole fruit to a pulp so that the press has cell walls it can drain, rather than intact apples it can only bruise.
Pressing yield
The proportion of a fruit charge recovered as juice, and why a single headline figure for it is not a meaningful number.
Juice oxidation
The enzymic browning that begins the moment apple juice meets air, which some traditions deliberately allow and others suppress.
What removes or limits it
Processes that reduce it, hold it below a threshold, or stop it forming in the first place.
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.
Juice pasteurisation
Heat treatment of juice before fermentation, which inactivates enzymes and microorganisms, and in doing so removes the wild flora that would otherwise ferment 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.
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.
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.
Cooked character
A stewed-apple, caramel or jam-like character from heat applied to juice or cider, most often through pasteurisation or hot storage.
About Polyphenol oxidase
Polyphenol oxidase is a copper-containing enzyme that lives in the plastids of apple cells while its phenolic substrates live in the vacuole. As long as the cell is intact the two never meet. Break the cell — a bruise in the orchard, a mill, a press — and they meet immediately, with oxygen from the air, and the enzyme converts o-diphenols such as chlorogenic acid and epicatechin into o-quinones. This is why a cut apple browns in a minute and why the first juice out of a press changes colour while you watch.
The enzyme itself does only that one step. What follows is chemistry rather than enzymology, and it is the part that matters. Quinones are strongly electrophilic: they condense with each other and with amino acids into the brown polymeric pigments, they oxidise other phenols that polyphenol oxidase cannot use directly and pull them into the reaction, and they react with thiols and other aroma-relevant nucleophiles, removing them from the juice. Browning is therefore never just a colour change. It is a colour change accompanied by aroma loss and by a shift in the phenolic composition of the juice.
Apple juice pH sits close to the enzyme’s working optimum, so acidity gives little protection. The reliable inhibitors are sulphur dioxide, which both reduces quinones back to phenols and inhibits the enzyme directly, and heat, which denatures it. Excluding oxygen works too, which is why some modern producers mill and press under nitrogen. Each of these is a deliberate act, and doing nothing is also a decision.
Traditional practice in England and Normandy has generally chosen to let it run. Juice left to stand and brown before fermentation loses its oxidisable phenolics early and in a controlled way, and a portion of the tannin polymerises and settles out with the gross lees. The cider that results is softer, less bitter, deeper in colour and markedly more resistant to air afterwards, because the material that would have oxidised in the bottle has already been consumed. A cider from sulphited juice keeps its pale colour, its fresh fruit aroma and its full phenolic bite — and remains vulnerable to the oxygen it meets later. Both are legitimate; they are different products, and the choice is made in the hour after pressing.
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
Epicatechin
The flavan-3-ol that apple procyanidins are almost entirely built from, and the most bitter of the phenolic monomers a cider contains.
Phenolics
Procyanidins
The condensed tannins of cider fruit, whose chain length — not their quantity — decides whether the mouth registers bitterness or astringency.
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.
Additives and processing aids
Sulphur dioxide
The antimicrobial and antioxidant on which most modern cidermaking depends, and whose effectiveness collapses as pH rises — which makes every sulphiting decision a pH decision first.
Gases
Dissolved oxygen
Essential to a healthy yeast population at the start of fermentation and the principal enemy of a cider from the moment fermentation ends.
Additives and processing aids
Ascorbic acid
An oxygen scavenger that protects colour and aroma while it lasts and can promote browning once it is exhausted, which is why it is never used without sulphur dioxide.
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.
- Does cider contain sulphites — Most does. Sulphur dioxide is added to protect juice and finished cider, and fermentation itself produces a small amount even when none is added. European labels must declare it above 10 mg per litre.
- What are procyanidins in cider — They are the condensed tannins of apples: chains of catechin-type units whose length decides how much of the phenolic load reads as bitterness and how much as astringency. Two ciders with identical total tannin can taste nothing alike.
- What is oxidation in cider
- How are apples milled for cider
- What is pressing yield — Pressing yield is the proportion of fruit weight recovered as juice. It varies from under half on a poorly loaded basket press to around seventy per cent on a well-run rack-and-cloth or belt press.
- 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 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 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.
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.
Hochschule Geisenheim University — beverage technology
Hochschule Geisenheim · university · retrieved 2026-08-24
German beverage-technology research covering apple wine and fruit juice processing, including the enzymology of clarification.