If you’ve ever made hard-boiled eggs, it’s likely you’ve cut into one and discovered that the outer yolk has turned slightly green. It has nothing to do with the egg itself and everything to do with how you cooked the egg. At some point during boiling, the natural iron in the yolk reacts with the natural sulfur in the white. The reaction produces hydrogen sulfide, a harmless, greenish-gray colored mineral that appears right where the yolk meets the white. Since the eggs have usually cooked through when the reaction happens, the green color is a visual cue that they’ve been overcooked. The good news is that yolks with a green ring are perfectly safe to eat, though they may not make the picture-perfect deviled eggs or egg salad. To prevent this from happening, stop cooking the eggs before the sulfur and iron reaction occurs. For perfectly cooked hard-boiled eggs, bring a large pot of water to a rolling boil, ensure there’s enough water to completely cover the eggs by at least an inch, gently lower cold eggs into the boiling water using a slotted spoon, and cook exactly 10 minutes for softer yolks and 12 minutes for firmer ones. Transfer the eggs to a bowl of cold water and ice and let them chill for 10-15 minutes before peeling.
