When you boil an egg you will see how often a greenish layer appears surrounding the hard boiled yolk. This is a sulfur and iron compound called ferrous sulfide. It is formed when the iron present in the yolk and the sulfur contained in the white meet at the surface of the yolk at high temperatures reacting and forming this compound.

The sulfur in the white is attached to a protein and only released in the form of a gas at high temperature. Then this gas diffuses through the egg and it is when it meets the iron in the yolk that they form this green layer.

This sulfur gas is responsible, in small quantities, for the pleasant smell of cooked eggs but, in larger quantities, it is the odor we associate with rotten eggs.

The way to minimize this layer forming is to keep the sulfur gas from reaching the surface of the yolk. This can be done in two ways. First, we can avoid it forming at all by cooking the egg at lower temperatures. We need the water to be hot enough so that the egg will cook but not so hot that this gas will be released in big quantities. This means keeping the water at around 85º-90º (185 F-190 F), that is slightly below boiling point. The second thing we can do is avoiding the gas reaching the egg yolk. We can slow the gas down by cooling it, this is, putting the egg under cold water as soon as it is cooked.

I did the following experiment. I cooked one egg in boiling water for 15 minutes and then did not put it under cold water immediately but rather let it cool down slowly. Then I put another egg in boiling water but with the fire turned off so that the water quickly cooled down. I added regularly boiling water to it so that the mix would be hot but under boiling point. I kept doing this for 30 minutes and then cooled down the egg by quickly putting it under cold water. This was the result:

Yellow and green yolks

Yellow and green yolks

The left yolk is the one that was cooked at a temperature lower than the water’s boiling point and the right one was cooked in boiling water. As you can see the green-greyish layer is not present in the egg cooked at a lower temperature.

Depending on what you are cooking it might not be worth going through the hassle of boiling eggs like this, but if you want to present pretty shinny yellow yolks you know what to do :).