Ferrous Gluconate
What it does
Blackens olives; iron source.
How it's made
Manufactured iron salt.
Ferrous gluconate is an iron compound used both as a colour stabiliser — notably to keep black olives a uniform dark colour — and as a source of iron for fortifying foods and in supplements. It is the iron salt of gluconic acid, a mild organic acid.
It is made by reacting gluconic acid (produced by the fermentation of glucose) with an iron source such as iron carbonate or iron sulphate, forming ferrous gluconate. The reaction combines the iron with the gluconate to give a compound in which the iron is in its readily-absorbed ferrous (iron-II) form.
The salt is purified and crystallised into a yellowish-grey to greenish powder. In black olives it works by reacting with natural compounds to fix and even out the black colour, while in fortification and supplements it delivers well-absorbed iron; in both roles it is used in controlled, food-grade amounts.
Possible reactions
Iron content — excess not advised.