Green Ring

Some chemical reactions that occur during cooking are undesirable because they decrease the quality of the food. The green film formed on hard-boiled egg yolk is one of the best known examples of undesirable chemical reactions. Students of the Gastronomy and Culinary Arts Department at Yeditepe University learns the science behind it with Prof. Dr. Sibel Ozilgen as a part of ‘Culinary Science’ course. When the cooking time is prolonged, the sulfur compounds in the albumin (egg white protein) and the iron in the egg yolk react to form ferrous sulfide on the surface of egg yolk. This compound forms a grayish-greenish thin film around the cooked egg yolk. In order to prevent this reaction, the cooking temperature should be around 70-80 °C, and the cooking time should be approximately 12 minutes for a medium-sized egg and 15-18 minutes for a large egg.