Book 9, Question 10
Why Does a Hard-Boiled Egg Get a Green-Gray Ring Around the Yolk?
A hard-boiled egg gets a green-gray ring because sulfur from the white meets iron in the yolk! It happens when eggs cook for a long time. The ring is harmless, and cooling eggs quickly helps prevent it.
More fun facts
- The green-gray ring comes from iron in the yolk reacting with sulfur from the white.
- The green-gray stuff is called iron sulfide, and it is also behind the eggy smell of some boiled eggs.
- The longer you cook an egg, the more likely you are to get a rubbery white and a green yolk.
- Plunging hard-cooked eggs into cold water helps keep the yolk from turning green.
- Cooling eggs fast keeps the sulfur gas in the white so it does not reach the yolk.
- To avoid green yolks, cook eggs just long enough and no more.
Sources: Exploratorium, Exploratorium, Exploratorium
Questions kids ask
Is a green ring safe to eat?
The ring is iron sulfide from overcooking. It changes the look and smell, and cooking eggs less avoids it.