Why do some hard-cooked eggs have a greenish ring around the yolk?
The greenish-gray ring forms around the yolk due to the chemical reaction between the sulfur (from the egg white) and the iron (from the egg yolk). The reaction usually occurs because of overcooking or high amounts of iron in the water.
You can try to eliminate this greenish ring by not overcooking your egg; also, cool the eggs quickly after cooking by running cold water over them immediately or putting them in an ice water bath. You can place hard-boiled eggs in the fridge for up to a week for use.
