that you’ve overcooked them.

Don’t worry, such an egg is neither spoiled nor poisonous, nor does it contain any nutrients. It’s pure chemistry, which we would describe in a kitchen lab as follows:

What actually happened?

The grayish-green to black ring on the surface of the yolk is caused by a reaction between the iron in the yolk and the sulfur in the egg white.

If you cook an egg for longer than 10–12 minutes, the heat releases hydrogen and sulfur in the egg white, producing a gas (sulfane).

This gas reacts with the iron at the edge of the yolk to form iron(II) sulfide (FeS).

The result is this visually unappealing, but healthy, green ring. How can you avoid this next time?

For a beautiful, deep yellow yolk without an unsightly “camouflage” effect, try the following:

Pay attention to the time: For hard-boiled eggs, 9 to 10 minutes from the time the water boils is sufficient.

Shock therapy: Cool the eggs in ice water immediately after boiling. This stops the cooking process and prevents the further release of gases that cause discoloration.

Don’t use eggs that are too old: The older the egg, the higher its pH value, which accelerates the chemical reaction.

A little tip: If the yolk is not only green after cutting it open, but also smells strongly (like rotten eggs), or if the egg white is pink, you should probably throw the egg away. The typical “overcooked” rim, however, is only a cosmetic flaw.

Want to know how to determine if an egg is fresh before cracking it with a simple water test?