Ferrous Gluconate
The short version
An iron(II) gluconate — the blackening agent for cured black olives.
It's widely considered safe at the amounts used in food, so we mark it green. The Safety section below has the details.
Where it comes from
Manufactured by neutralizing gluconic acid with iron(II) carbonate or hydroxide. Available as the dihydrate (food-grade).
How it's used
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.
Safety profile
ADI not specified — considered safe at typical use levels. 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.
Physical & chemical properties
Yellow-gray to greenish powder, soluble in water with characteristic iron taste. The dihydrate form is the food-grade standard.
Regulation worldwide
Permitted globally for olive blackening. Also a permitted iron-fortification ingredient in some markets.
Educational information only. Not medical or dietary advice.