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Foods -- Eggs
What causes the yolk of a hard-cooked egg to turn green? Is it safe to eat and how can this be prevented?
If an egg is not cooled quickly, it might develop a green ring around the yolk. Greening occurs when iron from the egg yolk reacts with sulfur from the egg white. During heating, these two chemicals form ferrous (iron) sulfur, which has a green appearance. Rapid cooling of hard-cooked eggs reduces the formation of ferrous sulfur. SOURCE: Dr. Julie Northcutt, Associate Professor/Food Safety Specialist, Department of Food Science and Human Nutrition, Clemson University, Clemson, SC (April 2008)