Why do the yolks of hard-boiled eggs go green around the edges and not the yolks of soft boiled eggs

asks Modge:
A. The blue-green colour you see on the yolks of hard-boiled is iron sulphide. It is caused by a reaction between iron in the egg yolk and the hydrogen sulphide in the white, which is why you see it where they join up.

Q. Why only in hard-boiled eggs
A. They are cooked long enough to release the iron and hydrogen sulphide. The iron found in an egg yolk is attached to a yolk protein called phosvitin. Protein molecules are long strings of amino acids. When the egg is raw, they are folded into a shape specially designed to help a chick grow inside the egg. If, however, you boil the egg, the heat causes these proteins to partly unfold. This is the action that makes the yolk and the white get harder. It also releases the iron from the phosvitin, and the hydrogen sulphide from the sulphur-containing proteins in the egg white.

Q. Doesn't hydrogen sulphide smell horrible
A. Yes, it does. Next time you have a hard-boiled egg, smell it very carefully - you'll catch a faint whiff of hydrogen sulphide. It's only a faint whiff on a fresh egg. Old, rotten eggs have a far stronger smell of hydrogen sulphide. The reason for that is, when they start to go bad, micro-organisms in the egg create a much more complete breakdown of the sulphur-containing proteins than you'd get if you simply boiled the egg.

Q. Why do some eggs have a darker shade of blue-green than others
A. The shade of blue-green is deeper in older eggs. This is because the yolks of older eggs are slightly more alkaline, which means that more of the iron is released.

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By Sheena Miller