Fault
Metal haze
A haze or dark cast caused by dissolved iron or copper picked up from equipment, forming insoluble complexes with phosphate or with the cider’s phenolics.
Also called iron casse, copper casse, metal casse.
- Severity
- Quality — the drink is worse for it
- Where it starts
- Pressing, Maturation, Stabilisation, Orchard, Harvest, Juice treatment
- Can it be fixed?
- Yes, if caught in time
- When you notice it
- Pressing, Maturation, Packaging
- Signs
- 5 recorded
What it is
Metal haze is a clarity fault caused by metal ions in solution rather than by anything biological or from the fruit. Iron picked up from bare steel, from an old press or from unsuitable fittings forms insoluble complexes with phosphate and with phenolic material, giving a bluish-grey or inky cast; copper from brass fittings or from copper treatment can form a reddish-brown haze under reducing conditions. Both are rarer than they once were, because stainless steel and food-grade plastic have replaced the equipment that caused them.
What you notice
Grouped by sense, because that is how the fault presents itself rather than how it works.
To look at
- A grey, bluish or inky cast rather than a simple cloudiness
- A brown-red haze appearing in bottle after a period under reducing conditions — The copper form, and it often appears after some time in a sealed bottle rather than in tank.
How the batch behaves
- Haze developing after the cider was exposed to air, in the iron case
- A cider handled through brass taps, an old press or bare steel
On the palate
- A metallic, inky taste alongside the haze
When it appears. Some time after contact with the metal responsible, which may have been a press, a fitting, a tank or a single unsuitable tap.
The diagnosis is usually made from the equipment rather than from the glass: a haze that appeared after a change of press, fitting or vessel is worth investigating as a metal problem before anything else.
Peter Mitchell / Cider and Perry Academy, 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.
- Protein haze — Whether anything tastes metallic. A metal haze usually brings a palate finding with it; a protein haze does not.
- Metallic taint — The same contamination can produce either or both. The haze is the visible half and the taint is the palate half, and a cider can have one without the other.
- Pinking — Colour. Metal contamination can cast a cider grey or blue-grey; pinking is a pink or salmon shift with a different cause.
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.
Iron in cider is mostly present as ferrous iron. On exposure to oxygen it is oxidised to ferric iron, which is far less soluble and readily forms complexes: with phosphate, giving the classic white-to-grey ferric phosphate casse, and with phenolic compounds, giving the darker, inky ferric-tannate colouration familiar from iron gall ink. The reaction is the same chemistry, in a drink.
Copper behaves in the opposite direction. Under the reducing conditions inside a sealed bottle, cupric copper is reduced to cuprous, which combines with proteins and with sulphur compounds to form a colloid that appears as a reddish-brown haze. This is why copper casse tends to appear after bottling rather than in tank.
Both metals are also powerful catalysts of oxidation, cycling between oxidation states and allowing oxygen to be reduced to hydrogen peroxide. A cider carrying enough iron or copper to haze is a cider that will also oxidise faster than it should, and the metallic taint and the haze usually arrive together.
Citric acid and other chelators bind the metal ions and keep them in solution, which is the basis of the classical remedy. Removal proper requires a treatment that binds and precipitates the metal, and those are regulated and analytically controlled operations rather than home procedures.
How it happens
| Cause | Stage | How often |
|---|---|---|
| Bare iron or mild steel anywhere in the cider pathOld presses, unpainted frames and rusted fittings are the classic sources. | Pressing | occasional |
| Brass taps and fittings, which release copper and zinc | Maturation | occasional |
| Copper treatment for sulphur compounds, leaving residual copper | Stabilisation | rare |
| Copper-containing orchard sprays carried in on the fruit | Orchard | rare |
| Soil and grit on windfall fruit contributing iron | Harvest | rare |
| Galvanised or unsuitable containers used for juice or cider | Juice treatment | rare |
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 |
|---|---|---|
| Remove the source and prevent recurrence | Reliable | The only action that reliably solves the problem, and it applies to next season rather than to this batch. |
| Fine or filter out the precipitated haze | Partial | Removes the visible complex. The dissolved metal remains and can haze again, and continues to catalyse oxidation. |
| Add citric acid to chelate the metal | Partial | The traditional treatment for iron casse, and it does hold the metal in solution. It adds acidity, which may or may not suit the cider, and any addition should be small and trialled on a sample first. |
| Blend into a larger clean volume | Partial | Lowers the metal concentration below the level at which it hazes, if a suitable batch is available. |
| Treat with a metal-removing agent | Unlikely to work | Such products exist, are regulated, and require an analytical measurement of the metal concentration to be used correctly. Not a home procedure, and CiderHQ publishes no dose. |
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
- Keep the entire cider path in stainless steel, food-grade plastic, glass or wood, and eliminate brass and bare iron.
- Wash fruit properly, particularly windfalls, so soil and grit do not reach the mill.
- Avoid copper treatments unless working under laboratory control with a specific analytical result.
- Observe harvest intervals for copper-containing orchard products.
- Inspect old equipment for rust and worn plating before each pressing season.
Metal haze is largely a historical fault, and the reason is worth stating: the equipment that caused it has mostly been replaced. A cellar of stainless steel, plastic and wood does not put iron or copper into cider, and a maker who has never used a brass tap will probably never see this.
It survives in two places. The first is old equipment, particularly presses and mills that have been in a barn for decades and have rusted or lost their plating. The second is deliberate copper treatment for sulphur faults, which introduces the metal on purpose and leaves whatever was not consumed.
The haze is not the main problem it announces. Iron and copper are catalysts of oxidation, and their presence at hazing concentrations means the cider is being oxidised faster than it should be. A cider with a metal haze usually also has a metallic taste and a prematurely aged character.
The chemistry has a pleasing historical resonance. Ferric-tannate complexes, which give the inky cast of iron casse, are the same compounds as iron gall ink — the ink of medieval manuscripts. It is the same reaction between iron and plant phenolics, occurring where nobody wanted it.
The compounds involved
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.
Procyanidins
The condensed tannins of cider fruit, whose chain length — not their quantity — decides whether the mouth registers bitterness or astringency.
Citric acid
A minor acid in apples and a much more significant one in pears, whose metabolism by lactic bacteria is the reason perry gains more butter and more vinegar from malolactic fermentation than cider does.
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.
Tannin–protein complexes
What astringency physically is: long tannin chains cross-linking salivary proteins and precipitating them, which is also the mechanism behind fining, protein haze and the classic cider-and-cheese pairing.
Where in the process it arises
Traditional screw pressing
The wooden farm press in which one or two hand-turned screws drive a follower down onto a built cheese of pomace, tightened by degrees over hours.
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.
Filtration
Passing cider through a porous medium to reach a stated visual brightness, at a real cost in colloidal material, body and aroma.
Fermentation vessels
The container a cider ferments in — wood, stainless, plastic, glass or concrete — and how its permeability, thermal mass and resident microflora shape the result.
Fruit washing
Removing soil, grass, stones and surface contamination from gathered fruit, usually in a water flume, before it reaches the mill.
Faults it is confused with
These present similarly. What separates them is set out on each page.
Metallic taint
A metallic, inky or blood-like taste from dissolved iron, copper or zinc picked up from equipment, usually accompanied by accelerated oxidation.
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.
Protein haze
A fine haze that forms as protein and tannin combine into insoluble complexes, often appearing in a cider that had already been clear.
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.
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.
- 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 oxidation in cider
- What should i ferment cider in — Anything inert, cleanable and closable: glass demijohns, food-grade plastic, stainless steel, or a wooden cask if you can keep it sound. Vessel shape and material change how much oxygen the cider sees and how fast it clears.
- 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.
- Do i need a filter to make cider
- What is a traditional screw press
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.
Cider and perry production technical training material
Peter Mitchell / Cider and Perry Academy · reference work · registered as competent for this subject
Searched again on 2026-08-25 and it remains the one printed source here that a bibliographic check cannot fix, because it is not a published work with an edition. It is course material issued to participants of a training programme, so there is no catalogue record to verify, no ISBN, and no lawful public copy. Its state stays `registered` for that reason rather than through inattention.
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.