E-numbers / Other additives / E579
also: Iron(II) gluconate
An iron salt used almost exclusively to fix the black colour of processed olives. Also used in iron supplements.
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INGREDIENTS: Black Olives, Green Olives, Salt, Rapeseed Oil, Concentrated Lemon Juice, Stabiliser (Ferrous Gluconate).
Ferrous gluconate is the iron(II) salt of gluconic acid, a naturally occurring acid derived from glucose.
39
of the 43,325 UK supermarket foods we checked declare E579 in their ingredients
That is under 0.1%, counted by its code or its name.
Counted by NutraSafe from UK supermarket ingredient lists, September 2026. How we counted
Ferrous gluconate is the iron(II) salt of gluconic acid, a naturally occurring acid derived from glucose. It appears as a yellowish-grey to pale yellow powder and is one of two iron salts approved in the UK and EU for use as a food additive (alongside ferrous lactate, E585). It is also widely used in pharmaceutical iron supplements.
In food, its role is purely as a colour stabiliser. Processed black olives are not naturally black: they are green olives darkened by oxidation treatment. Ferrous gluconate reacts with plant tannins in the olive flesh to form a stable iron-tannin complex that locks in the black colour and prevents fading or greying during storage. It does not add flavour, preserve the product, or act as a nutrient in the food context.
Almost exclusively in canned or jarred black olives, particularly the smooth-textured California-style or Spanish-style oxidised olives used in catering and pizza toppings. In the EU and UK its permitted use as a food additive is restricted to olives processed to a black colour. On the label it appears as 'ferrous gluconate' or 'E579'.
The amount of ferrous gluconate permitted in olives is capped at 150mg/kg. Typical olive servings are small, so the contribution to daily iron intake from this additive is very low compared with dietary iron from meat, legumes, or fortified cereals. The JECFA established a provisional maximum tolerable daily intake for total iron from all sources of 0.8mg per kilogram of body weight. This figure covers all iron intake, not additive use alone.
JECFA set a provisional maximum tolerable daily intake (PMTDI) of 0.8mg/kg body weight for iron, expressed as elemental iron, covering all dietary sources.
Iron is an essential nutrient but can be toxic at high doses. Acute iron poisoning, primarily a concern with supplement overdose rather than food additives, can cause serious gastrointestinal and systemic effects. The iron supplied through ferrous gluconate in olives is far below any level associated with toxicity in normal dietary conditions. Excess iron from supplements remains a relevant concern, particularly for people with hereditary haemochromatosis, but this is a supplement issue, not a food-additive issue.
High-dose iron supplementation can cause gastrointestinal irritation and, in cases of acute overdose, systemic toxicity. Individuals with iron-overload conditions such as haemochromatosis are advised to avoid iron supplements.
People with hereditary haemochromatosis (iron overload disorder) are routinely advised to be cautious about any additional iron source, including from food. In practice the amounts from olives are very small. Anyone managing iron intake for a medical reason should check with their doctor about total dietary iron. Look for 'ferrous gluconate' or 'E579' on the ingredients list.
This is a narrowly used additive with a single primary function: keeping processed black olives black. Its chemistry is straightforward and its permitted level in food is low. The science on iron toxicity is well established at supplement doses, not additive doses. EFSA's 2023 call for data is a routine part of its scheduled re-evaluation programme for all authorised additives, not a response to new safety signals. The main real-world consideration is for people with iron-overload conditions, who are already managing total dietary iron.
No. Ferrous gluconate is on the UK FSA approved-additives list and is permitted as a colour stabiliser in oxidised black olives at up to 150mg/kg.
Black olives sold in tins or jars are not naturally black. They are green olives darkened by an oxidation process. Without a colour stabiliser the flesh would turn grey or brown during storage. Ferrous gluconate reacts with tannins in the olive to lock in the black colour.
In the UK and EU, E579 is only permitted in olives that have been processed to a black colour. You will find it listed on tins or jars of smooth-textured black olives. It is not permitted in other food categories as a food additive, though it is used separately as an iron supplement ingredient in capsules and syrups.
Yes. Ferrous gluconate is synthesised from iron salts and gluconic acid without any animal-derived raw materials.
Last reviewed: 20 June 2026
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