Grapes contain very low levels of iron (0 – 5 mg/l), but these can be increased to unstable levels (over 6 mg/l) by contamination of the vintage with soil containing high levels of iron and/or contact with mild steel or cast iron containers or crusher rollers.

Again, this is mostly a problem associated with white winemaking, where the unstable iron forms a white deposit (iron phosphate), but it can cause a blue-black deposit in red wine. As with copper, iron ions can exist in two states in wine:

• A reduced state in which it soluble
• An oxidised state in which it forms insoluble compounds.

This means that wine can be completely clear, but then go hazy with contact with air, for example, when it is bottled. Measurement of iron in wines is normally done using gas chromatography in a laboratory, but a water-testing kit can give some idea of the concentration. The stability test involves leaving a wine sample in the cool and dark in a half-filled bottle (i.e. with excess oxygen). A wine susceptible to iron casse will become turbid in 48 hours.

Wines showing excess or unstable iron may be treated by:

• Gentle aeration
• Potassium ferrocyanide, which precipitates out the iron. Be careful to ensure that the wine still contains some iron, or it may have excess ferrocyanide
• Addition of citric acid, which combines with the iron to form iron citrate, which is very soluble. The maximum amount that can be added in the EU is 1 g/l, so this treatment is not always sufficient
• The addition of gum Arabic, or gum acacia, which is a protective colloid that prevents the iron particles from becoming large enough to be seen. The effect of gum Arabic are short-lived
• The addition of ascorbic acid (Vitamin C), which prevents the oxidation of the iron