Darkening of Egg Yolk in Hard-Boiled Egg

Correct Answer: Iron sulfide (FeS)

The dark greenish-grey ring around the yolk of a hard-boiled egg is caused by Iron Sulfide (FeS), which forms when hydrogen sulfide (H₂S) — released from the albumin during overcooking — reacts with iron (Fe) present in the yolk.

Chemical Reaction

H₂S (from albumin) + Fe (from yolk) → FeS (Iron Sulfide) — dark compound

Prevention

• Do not overcook eggs.
• Place eggs in cold water immediately after boiling — this stops further H₂S diffusion into the yolk.

The H₂S is generated by the breakdown of sulfur-containing amino acids (cysteine, methionine) in albumin upon heating.