Fault
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.
Also called juice browning, PPO browning.
- Severity
- Quality — the drink is worse for it
- Where it starts
- Fruit preparation, Juice treatment, Pressing, Harvest
- Can it be fixed?
- Yes, if caught in time
- When you notice it
- Fruit preparation, Pressing, Juice treatment
- Signs
- 5 recorded
What it is
Enzymatic browning is what happens in the minutes after fruit is cut or milled: an enzyme released from damaged cells meets phenolic compounds and oxygen, and the juice turns from pale to deep amber. It is the same reaction that browns a cut apple on a plate. In cidermaking it is a fault only in context — it costs colour and consumes some of the phenolic material that would otherwise contribute to flavour and structure — and in several traditional processes it is allowed or encouraged rather than prevented.
What you notice
Grouped by sense, because that is how the fault presents itself rather than how it works.
To look at
- Pomace darkening visibly in the space of a few minutes after milling
- Juice running deep amber or bronze off the press rather than pale
- A brown crust forming on the exposed surface of a press cheese
On the palate
- Finished cider paler in flavour than the fruit analysis suggested it should be — The phenolics lost to browning are phenolics that never reach the glass.
On the nose
- A cooked or dried-apple note in juice that has stood
The words for it: Bruised apple. Each links to what produces it.
When it appears. Within minutes of the fruit being cut or milled. If the juice ran brown from the press this is what happened; if the cider went brown in the vessel it is something else.
The colour change is the visible half of a phenolic loss that is not visible: procyanidins are consumed in the reaction, so a heavily browned juice has less tannin available to the finished cider than the fruit contained.
Various journals, Andrew Lea
What it is mistaken for, and how to tell
One observation per rival, chosen because it separates them rather than because it describes either. Several of these are not faults at all — a style feature, a normal outcome, or the fruit behaving as it does.
- Oxidation — When it happened. Enzymatic browning is a press-house event driven by polyphenol oxidase on cut tissue; oxidation of a finished cider is a slow chemical process over months.
- Colour from bittersweet fruit — Deep colour in a cider from high-tannin fruit is normal and comes with tannin on the palate. Browning without matching structure means phenolics were lost rather than extracted.
A degree of controlled browning is deliberate practice in some traditions, where allowing the juice to oxidise before fermentation removes the most oxidisable phenolics and gives a cider that is then stable. What is a fault is uncontrolled browning that takes the fruit character with it.
What is actually happening
The chemistry or microbiology behind it. Understanding the mechanism is what makes the prevention make sense rather than being a list of rules.
Apple tissue keeps polyphenol oxidase and its phenolic substrates in separate cellular compartments. Milling destroys that separation. The enzyme, a copper-containing oxidase, then catalyses two reactions on ortho-diphenols such as chlorogenic acid, epicatechin and the procyanidins: hydroxylation, and oxidation to the corresponding ortho-quinones.
The quinones themselves are pale and short-lived. What produces the colour is what they do next — condensing with each other and with other phenolics, amino acids and proteins into progressively larger brown polymers. The reaction is fast because it is enzymatic rather than metal-catalysed, and at ambient temperature a milled apple browns within minutes.
Cultivars differ enormously, in both enzyme activity and substrate concentration. A high-tannin bittersweet has far more oxidisable material than a low-phenolic dessert apple, which is why traditional cider fruit browns so dramatically and why the pale juice a cidermaker sees from culinary fruit is not evidence of better handling.
Two interventions stop it. Sulphur dioxide inhibits the enzyme and reduces quinones back to phenols, which is why sulphited juice stays pale. Heat denatures the enzyme, which is what pasteurisation of juice achieves. Ascorbic acid reduces quinones but is consumed as it does so, so its effect is temporary.
How it happens
| Cause | Stage | How often |
|---|---|---|
| Milled pomace held in air before pressingMaceration deliberately extends this period to extract more phenolic material, accepting the browning that comes with it. | Fruit preparation | common |
| Juice standing in open tubs between press and vessel | Juice treatment | common |
| High-phenolic bittersweet fruit, which browns faster and further | Fruit preparation | common |
| No sulphite applied at the juice stage | Juice treatment | occasional |
| Warm pressing-day conditions raising enzyme activity | Pressing | occasional |
| Bruised or damaged fruit in which browning has already begun on the tree or in store | Harvest | occasional |
Can it be put right?
Most cider faults cannot be reversed. Where that is the answer it is given plainly — an honest 'this batch is what it is' is more useful than a procedure that will not work.
| Option | Effectiveness | What it involves |
|---|---|---|
| Accept it and adjust the blend | Reliable | Where colour matters, the practical answer is fruit choice and process speed next season rather than treatment of this batch. |
| Allow the browned material to settle and be racked away | Partial | Some of the polymerised phenolic matter drops out during settling and after fermentation, and a heavily browned juice often ferments to a cider paler than the juice looked. This is normal rather than a rescue. |
| Fine the finished cider to lift colour | Partial | Fining does remove some coloured polymer. It also removes tannin, so the cider gets paler and thinner together. Bench trial before treating a batch. |
| Restore the colour of browned juice | Not possible | The brown polymers are stable and cannot be converted back. Colour lost at the press is lost. |
CiderHQ gives no additive dosage figures. An addition is a food-safety decision that depends on legal limits, on the juice in front of you and on what you are protecting against, and the right figure comes from your supplier’s or regulator’s own guidance rather than from a general reference.
Preventing it
- Shorten the time between mill and press, and move juice into its fermentation vessel promptly rather than letting it stand.
- Where sulphite is used, add it to the juice as it comes off the press, at a rate taken from the supplier’s table for the measured pH.
- Fill vessels from the bottom and avoid splashing juice into air.
- Keep fruit, pomace and juice as cool as the day allows; enzyme activity falls with temperature.
- Decide deliberately whether browning is wanted. Maceration and keeving both work with the phenolic chemistry rather than against it, and a maker choosing them is not making a mistake.
Enzymatic browning is the fault that most often is not one. Traditional West Country practice, French keeving and maceration all involve extended contact between pomace and air, and the resulting colour is part of how those ciders have always looked. A maker aiming for a pale, fresh, modern style is fighting the same chemistry that a maker aiming for a traditional one is using.
What makes it worth treating as a fault entry is the hidden cost. Phenolic material consumed by the enzyme before fermentation is not available to the finished cider, so heavy uncontrolled browning takes structure as well as colour. A bittersweet blend that browns hard on a warm pressing day gives a thinner cider than the fruit analysis predicted.
The practical distinction from ordinary oxidation is speed and cause. Enzymatic browning is an enzyme acting within minutes on fresh tissue; the oxidation of finished cider is a slow metal-catalysed process over months. Sulphite prevents both but by different routes, and heat stops one while doing nothing useful for the other.
It is also the clearest visual demonstration of how much phenolic material cider fruit carries. A milled dessert apple browns; a milled bittersweet turns the colour of strong tea, and that difference is the whole reason those cultivars exist.
The 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.
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.
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.
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.
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.
Where in the process it arises
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.
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 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.
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.
Pressing yield
The proportion of a fruit charge recovered as juice, and why a single headline figure for it is not a meaningful number.
Faults it is confused with
These present similarly. What separates them is set out on each page.
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.
Excessive astringency
A drying, roughening, mouth-puckering sensation that outstays its welcome, produced by larger phenolic polymers precipitating salivary proteins.
Where this comes up in a guide
The link lands on the step where the fault arises rather than at the top of the pathway, because that is where the decision that causes it is taken.
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
- 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.
- 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.
- 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.
Where to go next
- The fault finder — Describe what you can smell and see, and narrow it down from the signs.
- All faults — Grouped by where they come from and how serious they are.
- Cider science — The chemistry and microbiology these faults come out of.
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.
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.