What causes the green “ring” around the yolk in hardcooked eggs?
Cooking for too long or at too high a temperature may cause a chemical reaction between the naturally-occurring iron in the yolk and sulphur in the egg white. The result is the formation of a harmless greenish coating on the yolk’s surface. Proper cooking methods and chilling hardcooked eggs in cold water as soon as they are cooked prevent the formation of this discolouration.
