A: The green ring around the yolk of a hard cooked egg happens because hydrogen in the egg white combines with sulfur in the yolk. The cause is most often related to boiling the eggs too hard for too long. The green ring can also be caused by a high amount of iron in the cooking water.

How do you keep hard boiled eggs from turning green?

You can keep the yolk from turning green by only cooking the eggs long enough to harden them and then chilling the eggs as soon as they have finished cooking. One way to do this is by running cold water over the hot eggs as soon as the cooking time has elapsed.

How can you tell if a hard boiled egg is bad?

The most noticeable sign that a hard-cooked egg has gone bad is the odor. What is this? If the egg has any sort of unpleasant, sulfurous, or rotten smell, it has gone bad and should not be consumed. If the hard-boiled egg is still in its shell, you may have to crack it open in order to notice any odor.