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E579ColorantLow concern
Ferrous Gluconate
In plain words
The short version
An iron(II) gluconate — the blackening agent for cured black olives.
Why it's green
It's widely considered safe at the amounts used in food, so we mark it green. The Safety section below has the details.
Safety profile
JECFA includes ferrous gluconate in a group provisional maximum tolerable daily intake (PMTDI) for iron of 0.8 mg/kg body weight, expressed as iron (1987 evaluation). The iron contribution to the daily diet from olive blackening is small. Pharmaceutical doses (15–60 mg iron daily for anemia) are much higher than dietary additive levels.
The detail
Manufactured by neutralizing gluconic acid with iron(II) carbonate or hydroxide. Available as the dihydrate (food-grade).
The standard blackening agent for California-style black olives — the olives are pre-treated to remove bitterness, then dipped in ferrous gluconate solution which reacts with residual polyphenols to produce the deep matte-black color. Also a common iron supplement in pharmaceutical-grade form.
Yellow-gray to greenish powder, soluble in water with characteristic iron taste. The dihydrate form is the food-grade standard.
Permitted globally for olive blackening. Also a permitted iron-fortification ingredient in some markets.
Sources
- JECFA database: Ferrous gluconate (INS 579), 1987 evaluation — WHO / FAO Joint Expert Committee on Food Additives, 1987. Group PMTDI 0.8 mg/kg bw for iron
- Regulation (EC) No 1333/2008, consolidated text of 18.08.2026: E 579 in categories 04.2.2 and 04.2.3 — EUR-Lex, 2026. EU use only for olives darkened by oxidation, 150 mg/kg as Fe
- 21 CFR 73.160 Ferrous gluconate (color additive exempt from certification), CFR 2025 edition — US Government Publishing Office (govinfo), 2025. US permitted for coloring ripe olives
- 21 CFR 184.1308 Ferrous gluconate (affirmed GRAS), CFR 2025 edition — US Government Publishing Office (govinfo), 2025. US GRAS as nutrient supplement (iron fortification)
Checked against these sources on .
Educational information only. Not medical, dietary or veterinary advice. Spotted an error? Tell us.