A green olive turns dark as it ripens. The color of the olive depends on its ripeness. Green olives are picked from the tree before they have time to ripen further and turn dark. The color is therefore determined by the time of harvest. In addition to green olives, stores also offer dark and pitch black olives. The color of flat black olives comes from iron gluconate, known by the e-code E579. Iron gluconate colors the olives pitch black.
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