Many home cooks are often surprised to find a green or gray ring around the yolk of their hard-boiled eggs. While it may look unappetizing, it’s actually the result of a harmless chemical reaction between iron in the yolk and sulfur in the egg white, caused by overcooking.

The good news is that this reaction is easy to prevent. The key lies in cooking the eggs gently and not for too long. Start by placing the eggs in a pot in a single layer, then cover them with cold water by about 3 to 5 centimeters.

Next, heat the pot over medium heat. As soon as the water reaches a full, rolling boil, remove the pot from the heat immediately. Cover it with a lid to allow the eggs to cook gently in the residual heat.

Let the eggs sit in the covered pot for 9 to 12 minutes, depending on their size and how firm you want the yolk. Nine minutes will result in a slightly soft center, while 12 minutes will give you a fully firm yolk.

Once the time is up, quickly transfer the eggs to a bowl of ice water or very cold water. This halts the cooking process, preventing the green ring and also helping make peeling easier.

Allow the eggs to cool for about 5 minutes before peeling. Peeling them under running cold water can further ease the process.

For best results, use eggs that are 5 to 7 days old, add a little salt or vinegar to the water to prevent cracking, and store cooked eggs properly for freshness.