Copper is normally only present in very low levels in grapes and wine (0.4 mg/l), but this level can be increased by:

• High levels of copper pesticide residues
• Over-use of copper fining during or just after the alcoholic fermentation in order to eliminate hydrogen sulphide aromas
• Use of bronze or brass equipment and fittings

This can lead to levels of 1-2 mg/l, at which copper can become unstable, producing a light brown sediment in the tank or bottle. The problem is that copper ions in wine can exist in two states:

• The oxidised form, which forms soluble compounds.
• The reduced form, which forms poorly soluble compounds.

Therefore, a wine can contain high levels of dissolved copper that will stay in solution until it has been in bottle for a while, after which some of it can become reduced and precipitate out of solution as sulphides and copper/protein colloidal complexes. This reaction is encouraged by the presence of sulphur dioxide, warmth and the exposure of the wine to sunlight.

Red wines do not suffer from this problem, as the copper is protected from reduction by the high levels of phenolics present and any proteins are quickly flocculated (see protein instability).

Measurement of copper in wines is normally done using gas chromatography in a laboratory, but a water-testing kit can give some idea of the concentration.

Additionally, a stability test can be carried out by completely filling (no headspace) a clear bottle with the wine and placing it on a warm window ledge in direst sunlight for 7 days. A brown deposit at the bottom of the bottle indicates unstable copper. If either test is positive, the wine can be treated by:

• Filling the tank completely, adjusting the free sulphur dioxide level to 30 mg/l, and leaving it for a month or so.
• Bentonite fining, which will reduce the level of unstable protein.
• Treatment with potassium ferrocyanide. This forms insoluble deposits with both forms of copper, which can then be removed by filtration. However, due to the toxicity of this material, it is best if this treatment is supervised by a qualified oenologist and that the wine is tested after treatment to ensure that some copper still remains in it.
• The addition of gum Arabic, or gum acacia, which is a protective colloid that prevents the copper particles from becoming large enough to be seen. The effect of gum Arabic are short-lived