It’s a small chemical reaction that’s behind this strange color. When eggs are cooked for too long or at too high a temperature, the iron in the yolk reacts with the sulfur in the white. The result: a compound called iron sulfide forms, leaving this greenish-gray ring around the center of the egg.
But don’t panic: it’s just a natural reaction, with no health risks. The taste remains unchanged; only the appearance suffers. And if you like perfectly yellow eggs, there’s a foolproof method to avoid this culinary mishap.
The foolproof method for perfect hard-boiled eggs
Always start with cold water
Place your eggs in a saucepan and cover them with 3 to 5 cm of cold water. This trick prevents thermal shock and cracked shells.Heat over medium heat
Let the water heat gently until it boils. As soon as the bubbles rise noticeably, turn off the heat.
Let it rest, without boiling it further.
Cover the saucepan and let the eggs enjoy the gentle heat:
9 minutes for a slightly creamy yolk,
12 minutes for a firm yolk.