The reason why the yolk has a green layer?

Actually, the green layer on the egg is due to the way it is boiled, not because the egg has any strange substance. When we boil/steam the egg too much, this green color will appear. That is because under the influence of heat for too long, the hydrogen sulfide (H2S) in the egg white will react with the iron in the egg yolk, forming iron sulfide FeS which is blue-black.

That is why the green layer is created on the yolk, where it comes into contact with the egg white, while the yolk inside remains yellow and does not turn green all over the yolk.

This green color is not harmful to the health of the eater. If you boil the egg too much, the taste of the egg will not be as good as when it is boiled just right, and the egg will be drier, so it will not be as soft and creamy as when it is boiled just right or boiled with the yolk.

If you do not want the green layer on the yolk of a boiled egg, simply do not boil the egg for too long. If you like the rich taste of hard-boiled eggs, stop at 11 minutes and turn off the heat.

If you want your eggs to be well-done, boil them for only 9 minutes. For soft-boiled eggs, 3 minutes is a good amount of time. You can wait 5 minutes if you want the yolk to start to thicken or 7 minutes for the yolk to be cooked but still pink in the middle.