Juice treatment
Juice oxidation
The enzymic browning that begins the moment apple juice meets air, which some traditions deliberately allow and others suppress.
Also called Browning down, Enzymic browning of juice, Oxidative juice handling.
- Stage
- Juice treatment
- Traditional in
- Somerset, Devon, Herefordshire, Pays d’Auge and 1 more
- What it most changes
- Phenolic character down, astringency down
- Safety
- None recorded
What it is
Fresh apple juice darkens within minutes of leaving the press. The reaction is enzymic: polyphenol oxidase, released from the fruit cells during milling, uses dissolved oxygen to convert the juice’s phenolic compounds into quinones, which are reactive and go on to condense with one another and with protein. The visible result is a shift from pale gold through amber to deep brown, followed — if the juice is left standing — by the settling of a dark pigmented deposit. Cider makers treat this either as a fault to be prevented or as a step in the method, and both positions are defensible depending on the fruit.
Why it is used
- Allowing oxidation strips out the most aggressively astringent phenolic fractions before fermentation, so a high-tannin bittersweet juice arrives at the ferment with its rough edge already knocked off.
- The pigment formed carries protein and phenolic material down with it as it settles, giving a juice that is easier to clarify and a cider less prone to browning further in the bottle.
- Suppressing oxidation, by contrast, protects the varietal aroma compounds and the pale colour on which a modern aromatic or single-varietal cider depends.
- Either way the decision sets the phenolic ceiling for everything downstream, because phenolic material lost at the juice stage cannot be recovered later.
How it works
- Polyphenol oxidase is a copper-containing enzyme held in the fruit’s plastids; milling ruptures the cells and brings it into contact with phenolics stored in the vacuole, so the reaction begins at the mill rather than at the press.
- Its substrates in apple are the ortho-diphenols — chlorogenic acid above all, and the flavan-3-ols catechin and epicatechin. It oxidises them to ortho-quinones, consuming dissolved oxygen as it does so.
- Quinones are electrophilic and do not stay put: they couple with one another into coloured polymers, and they react with the thiol and amine groups of proteins and amino acids to form insoluble complexes that settle out.
- Compounds the enzyme cannot attack directly, including the larger procyanidins and phloridzin, are drawn into the reaction anyway by coupled oxidation from quinones already formed, so total phenolic content falls further than the substrate list alone suggests.
- Sulphur dioxide, ascorbic acid, chilling and exclusion of oxygen each interrupt a different point in the sequence — sulphite by reducing quinones back and inhibiting the enzyme, inert gas by removing the oxidant altogether.
What it changes
The direction this step pushes the finished drink in, dimension by dimension. A direction, not a measurement: how far it moves depends on the juice, the temperature and how the step is carried out.
| Dimension | Direction | Why |
|---|---|---|
| Phenolic character | Lowers | Quinone polymers precipitate together with the protein they bind, so phenolic material physically leaves the juice in the settled deposit rather than merely changing form. |
| Astringency | Lowers | The procyanidins most able to cross-link salivary proteins are also those most readily drawn into coupled oxidation and removed, so what remains binds protein less strongly on the palate. |
| Fruit character | Lowers | Quinones react with thiol-bearing aroma precursors, and the same oxidative conditions degrade the volatile esters and six-carbon aldehydes that carry fresh-apple character. |
| Oxidative character | Raises | Acetaldehyde and related carbonyls accumulate alongside the pigment, giving the bruised-apple and nutty notes that read as oxidative in the finished cider. |
The chemistry and the organisms
What is actually being changed, and by what. Each entry says what that compound or organism does in cider generally; this page is one place it does it.
Compounds involved
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.
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.
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.
Epicatechin
The flavan-3-ol that apple procyanidins are almost entirely built from, and the most bitter of the phenolic monomers a cider contains.
Catechin
The minor flavan-3-ol of apple, present largely as the terminal unit of procyanidin chains, and consequently a useful analytical handle on chain length.
Procyanidins
The condensed tannins of cider fruit, whose chain length — not their quantity — decides whether the mouth registers bitterness or astringency.
Phloridzin
A dihydrochalcone found in apple and essentially nowhere else, which makes it both a contributor to bitterness and the standard chemical proof that a juice is apple juice.
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.
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.
Where it is traditional
The places this step belongs to as a matter of practice. It is not a claim of exclusivity — a method can be traditional in one region and perfectly ordinary in another.
What it is done with
Dissolved oxygen meters
A dissolved oxygen meter reads the oxygen actually in the liquid, using either an electrochemical membrane cell or an optical sensor whose luminescence is quenched by oxygen — the only instrument that turns an argument about oxygen pickup into a number.
Inert gas: carbon dioxide, nitrogen and argon
Gas is used to push air out of tanks, bottles, kegs and lines, and the three common gases are not interchangeable: carbon dioxide dissolves readily and will carbonate a cider, nitrogen barely dissolves at all, and argon is heavy enough to lie on a surface.
What can go wrong
Faults that begin at this step, or that this step makes more likely. Each one is set out with its causes and whether it can be put right.
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.
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.
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.
Thin body
A cider with no weight or texture in the mouth, most often from over-watered pomace, low-gravity juice or a blend with nothing to give it substance.
Styles it produces
Categories in which this step is characteristic or required. Some name it in their definition; for others it is simply how they have always been made.
West Country farmhouse cider
Cider made on the farm from tannic bittersweet fruit, wild-fermented in wood and sold still and unfiltered, in a tradition whose variability is one of its defining features.
Cidre Pays d’Auge
The controlled appellation cider of the Pays d’Auge in Calvados, made by keeving from bitter Norman fruit and bottle-conditioned to a low alcohol and a high residual sugar.
Scrumpy
A colloquial West Country word for rough farm cider that has never had a fixed definition and now carries as much marketing weight as descriptive meaning.
Keeved cider
Cider clarified before fermentation by a pectin gel that strips nutrients from the juice, producing a slow ferment that stops naturally with sugar still in solution.
More on juice oxidation
The first thing to understand about browning is that it is not slow spoilage creeping in over days. It is a fast enzymic reaction already under way in the mill hopper. Polyphenol oxidase sits in the fruit’s plastids and its substrates sit in the vacuole; the two meet the instant the cell wall breaks, and oxygen entrained by milling and pressing supplies the rest. Because the substrates are the ortho-diphenols — chlorogenic acid, catechin, epicatechin — and because those are also the building blocks of the procyanidins that give bittersweet cider its structure, the reaction is not attacking some peripheral fraction of the juice. It is attacking exactly the phenolic material the maker either wants to keep or wants to be rid of.
What happens next surprises people who assume browning simply makes juice darker. The quinones formed are unstable and reactive. They polymerise into pigment, and they couple with proteins and amino acids into complexes that are no longer soluble. Left standing, that pigment falls, and when it falls it takes phenolic material out of the juice with it. So a juice that has browned heavily and then been racked off its deposit is not a browner juice than it started: it is a paler, softer, less astringent juice carrying a smaller phenolic budget than the fruit originally offered. That mechanism is what lies behind the old West Country instruction to let the juice brown down before pitching yeast.
Traditions diverge sharply here, and neither side is simply failing to understand the other. Traditional West Country and Normandy practice worked with unsulphited juice in wood, with high-tannin bittersweet fruit whose raw astringency would have been punishing, and browning served as a free, self-limiting tannin management step; in the French défécation and keeving route the pigmented material is carried up into the chapeau brun and removed along with the pectin gel. Modern aromatic practice — much of the Pacific Northwest, the northeastern United States, and anyone working dessert or sharp fruit towards a pale fruit-forward cider — moves the other way, sulphiting at the press or blanketing with carbon dioxide, because in low-tannin fruit there is little astringency to remove and a great deal of varietal aroma to lose.
The practical decision is therefore one question asked honestly of the fruit in front of you: are these phenolics an asset to protect, or a rough edge to knock off? Get it wrong in one direction and a delicate juice reaches the ferment already dull-coloured and stripped of the aroma that was the whole reason for growing that cultivar. Get it wrong in the other and a hard bittersweet juice ferments with its full astringent load intact, giving a cider that needs long maturation, or fining, before it is approachable. There is also a middle route many makers use without naming it: a short controlled stand at the press, a partial brown, then sulphite to arrest the reaction at the point they judge right.
Related processes
Steps that sit alongside this one, replace it, or depend on it having been done.
Juice treatment
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.
Fruit preparation
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.
Juice treatment
Juice settling
Letting freshly pressed juice stand cold and undisturbed so that gross solids fall, then racking the cleaner juice off the deposit before pitching.
Fermentation
Keeving
Forming a floating pectin gel that lifts nutrients and solids out of the juice, so that the ferment starves before it finishes and leaves natural sweetness behind.
Juice treatment
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.
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 keeving — Keeving is a technique for starving a ferment of nitrogen so that it stops before all the sugar is gone, leaving a naturally sweet cider. Pectin is made to gel and float as a brown cap, carrying nutrients and yeast out of the juice with it.
- 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.
- What are carbon dioxide, nitrogen and argon used for in cider making, and how dangerous are they — Air is what damages finished cider, and the cheapest way to remove air is to displace it with a gas that will not react with the cider. That is the whole purpose of inert gas in a cider house.
- 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 keeved cider — A cider made naturally sweet by starving the yeast rather than by adding sugar. Keeving strips nutrients out of the juice before fermentation, so the ferment slows and stops with fruit sugar still in it, and nothing has been put back.
- What is oxidation in cider
Where to go next
- How cider is made — The whole sequence, stage by stage, with the choices open at each one.
- Cider science — The chemistry and microbiology the methods on this page rest on.
- Troubleshooting — What goes wrong, how to recognise it, and whether it can be reversed.
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.
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.
Washington State University cider research programme
Washington State University Northwestern Washington Research and Extension Center · university · passage verified 2026-08-24
The single most useful open cider dataset CiderHQ has found. The programme’s cultivar performance database gives juice chemistry, orchard behaviour, bloom and harvest timing and cider-maker tasting notes for 73 cultivars grown at one maritime site over fifteen years, and — unusually — publishes its classification thresholds alongside its figures, so the classification can be checked rather than taken on trust. It also states outright that its results diverge from the English ones. Read in full on 2026-08-24 and transcribed into `data/trials/wsu-mount-vernon.ts`; four published figures were found to be impossible and are withheld there with their reasons.