Why is there a green color around the yolk of hard boiled egg?

The green ring around the yolk of a hard-boiled egg is a result of a chemical reaction called iron sulfide formation.

Here's how it happens:

1. Iron: Egg yolks contain iron, primarily in the form of iron(II) sulfide.

2. Hydrogen sulfide: The white of the egg contains hydrogen sulfide (H2S), which is a colorless gas with a characteristic rotten egg smell.

3. Overcooking: When eggs are overcooked, especially at high temperatures, the heat breaks down the protein in the egg white, releasing the hydrogen sulfide gas.

4. Reaction: The hydrogen sulfide gas reacts with the iron in the yolk, forming iron sulfide (FeS). Iron sulfide is a green-colored compound that causes the green ring around the yolk.

Why does this happen more often with older eggs?

Older eggs tend to have a higher concentration of hydrogen sulfide in the white, which is why overcooked older eggs are more likely to have a green ring.

Note: While harmless, a green ring can indicate that the egg was overcooked, which can make the yolk rubbery and less flavorful.