The green ring is the result of a chemical reaction in which the sulphur in the egg white and the iron in the egg yolk form a new green compound.

• Eggs are an important dietary source of sulphur, which is beneficial for hair and nails. The sulphur in eggs is mainly concentrated in the egg white, captured in the amino acids methionine and cysteine.
• Methionine and cysteine are stable at room temperature, but when you boil an egg, a small amount of these amino acids is broken down in the protein.
• broken down in the protein. This allows the hydrogen and sulphur in the egg white to react and form hydrogen sulphide.
• At the junction of the egg white and the yolk, the hydrogen sulphide reacts with the iron in the yolk to form iron sulphide, which has that characteristic green colour.

The longer an egg is cooked, the more sulphur is released, the more iron sulphide is formed and the ‘greener’ and darker the surface of the yolk becomes (and the worse it smells).

• Scrambled eggs can also turn green if you leave them on the heat for too long or keep them warm for a long time, which is often the case in breakfast buffets.
• Using a cast iron pan to cook your eggs can also increase the risk of green discolouration.
• Finally, using the microwave can trigger the same phenomenon. An omelette or quiche heated too long in the microwave can turn a greenish grey colour.

The risk of a green edge increases if the egg is cooked for too long or if it is not cooled immediately after cooking, allowing the reaction to continue. So it’s easy to prevent your eggs from turning green:

Watch the cooking time:

• Hard-boiled eggs should only cook for 9 to 12 minutes, depending on their size. Set a timer to avoid overcooking them.
• If you’re making an omelette, don’t prepare it too far in advance if you don’t need to keep it warm or reheat it for a long time. Serve immediately for best results.

Cool boiled eggs quickly in water:

Immediately immerse hard-boiled eggs in a bowl of ice water after cooking. This will stop the cooking reaction and reduce the risk of a chemical reaction between the sulphur and iron.