Hard-boiled eggs: The mistake that turns the yolk green and how to avoid it easily

The green ring around the yolk of a hard-boiled egg is caused by a chemical reaction between sulfur from the white and iron from the yolk, forming iron sulfide. This reaction is triggered by overcooking or slow cooling. The common mistake is leaving eggs in hot water after cooking. To avoid it, immediately plunge the eggs into cold or ice water to stop the cooking.