Dlaczego jajka na twardo mają dziwny zielony pierścień wokół żółtka?

The greenish ring that sometimes forms around the yolk of a hard-boiled egg is caused by a chemical reaction between sulfur in the egg white and iron in the yolk. This reaction typically occurs when eggs are overcooked.

When eggs are boiled too long or at too high of a temperature, the sulfur in the egg white reacts with the iron in the yolk to form iron sulfide, which has a greenish-gray color. The longer the eggs are cooked, the more likely this reaction is to happen, especially if the eggs are cooked at a high temperature for a prolonged period.

To prevent the green ring, you can try: