What causes the green ring between yolk and white when hardboiling an egg? The yolk of a hardboiled egg often has a green tinge right at the interface to the white. Besides not looking very good, I think it also maybe contributes a sulfurous odor. What causes this green coloration and how can I prevent it?
This is caused by overcooking the egg. The green color is a result of overheating causing the iron and sulfur compounds in the egg to express. You can prevent it by gently boiling the egg, and plunging it into an ice bath when it is done. This stops the carry-over heat from continuing to cook the egg.