A grey or greenish tint on cooked eggs is a chemistry problem, not a safety crisis. It usually appears as a dark ring around a hard boiled yolk or as dull grey scrambled eggs. Here is what causes it and how to prevent it.

The main reaction

• Iron + sulfur = ferrous sulfide
  Egg yolks contain iron and egg whites contain sulfur compounds. When eggs are heated too long or cooled too slowly, sulfur from the whites reacts with iron in the yolk to form ferrous sulfide, which looks grey green and smells a bit like sulfur. The ring on hard boiled yolks is the classic result.

Other contributors

• High heat and long time
  Boiling hard and cooking past doneness accelerates the reaction.
• Slow cooling
  Leaving eggs hot for a long time after cooking gives the reaction more time to proceed.
• Alkaline conditions
  Older eggs and baking soda raise pH in the whites, which frees more sulfur. This can push color toward grey.
• Reactive cookware and water
  Aluminum or cast iron can discolor sulfur rich foods. Iron rich water can also intensify the ring.
• Scrambled eggs
  High heat, overcooking, and lots of incorporated air can dull the yellow and create a grey cast.

Is it safe

• Yes. The color change is cosmetic and harmless. Flavor and smell may be a little stronger.

How to prevent the grey

Hard boiled

1. Start in cold water, bring just to a gentle boil, then cover and turn off heat.
2. Let large eggs sit in the hot water about 10 to 12 minutes.
3. Drain and shock in ice water for at least 5 minutes to stop the reaction.
4. Use nonreactive pots if possible and avoid long holding on a hot surface.

Scrambled

1. Cook on low to medium heat and pull the eggs while still glossy.
2. Stir gently to avoid excessive air and dryness.
3. A splash of milk or cream can buffer heat and keep texture tender.
4. Use stainless steel or nonstick rather than cast iron or aluminum if grey is a problem.

Special cases and tips

• Very fresh eggs resist peeling after boiling. Slightly older eggs peel easier but do not cause greying by themselves. The ring comes from heat and cooling.
• If your tap water has high iron, use filtered or bottled water for boiling.
• A little acid, such as vinegar or lemon juice in the cooking water for hard boiled eggs, can lower pH and limit sulfur release.

Bottom line

Grey or green tones appear when eggs are overcooked or cooled too slowly, allowing sulfur from whites to react with iron in yolks. Control time and temperature, cool quickly, and use nonreactive cookware to keep your eggs bright yellow and appetizing.