10 Mind-Blowing Facts About the Science of Boiling an Egg Perfectly

4. The Dreaded Green Yolk? Blame Chemistry (Sulfur + Iron)

That unappetizing greenish-grey ring around an overcooked hard-boiled yolk isn't spoilage; it's simple chemistry! Egg whites contain sulfur-containing amino acids. Overheating causes these to break down and release hydrogen sulfide gas. Simultaneously, the yolk releases iron. When these two meet at the yolk-white boundary, they react to form ferrous sulfide – which is greenish-grey. It’s harmless but visually off-putting and a sign of overcooking. Shorter cooking times and rapid cooling prevent this reaction.