When you hard boil and slice an egg, you may notice a dark green ring around a yolk. This is iron sulphide which is formed by chemical reactions that take place if an egg is heated for a long time.
The protein in an egg yolk contains iron which is released as the protein uncoils on heating. Conversely, the egg white protein contains the elements hydrogen and sulphur, which are released as the protein unfolds. The hydrogen and sulphur combine to make the foul smelling substance hydrogen sulphide.
When exposed to prolonged heat, the egg white’s hydrogen sulphide and the egg yolk’s iron then react together to form green iron sulphide. Since the solid yolk separates a little from the egg white, the iron sulphide stays in the space around the yolk. Fortunately it’s not harmful.
To minimize the green ring problem, don’t boil the eggs for longer than necessary. Also, when you remove the egg from the pan, plunge it straight into cold water. The hydrogen sulphide then tends to migrate to the colder shell and away from the yolk.
Hydrogen sulphide has a very strong odour – the classic rotten egg smell. You can sometimes pick up a very faint odour of hydrogen sulphide if you sniff a hard boiled egg. This doesn’t mean the egg is bad. Usually, it’s just the hydrogen sulphide formed from heating the proteins in the egg white.
So next time you hard boil an egg, watch the clock, and make the plunge.