Have you ever noticed a green ring around the yolk of a hard-boiled egg? This colour change can happen when you overcook an egg or cook it at a high temperature for too long.
The green ring forms when sulfur in the egg white reacts with iron in the yolk. This reaction creates a harmless compound called ferrous sulfide, which gives the yolk its greenish colour.
Although the green ring may not look appealing, the egg is still safe to eat. The colour change does not affect the egg’s nutritional value or flavour.