Eva Crossan of Stirling asks :-

Why is sulphur in eggs and how does it get there?

Geoff Jameson, a biochemist at Massey University, responded.

Firstly I need to dispel a few myths regarding sulphur and eggs. The yellow colour of eggs does NOT come from sulphur, neither the pure element (which is indeed yellow) nor even sulphur-containing compounds, as these are mostly colourless.

Rather, the yellow colour comes from a family of molecules, xanthophylls, molecules similar to that (carotene) which gives carrots their orange colour. As animals, in this case the hen, do not make xanthophylls, the yellow colour of eggs comes from what the mother hen eats.

So, where does the sulfur that gives rotten eggs their characteristic smell come from? Eggs, both egg white and egg yolk, contain considerable amounts of protein. Two of the constituent amino acids of proteins, methionine and cysteine, contain sulphur. When the hen makes an egg, proteins from the mother hen are used to make the proteins of the egg. There are a number of other non-protein sulphur-containing compounds in eggs, but these are present in very small amounts. The bulk of sulphur in eggs is incorporated in proteins.

When microorganisms invade an egg, the breakdown of egg proteins releases hydrogen sulphide, giving the characteristic smell of rotten eggs. Hydrogen sulphide is an extremely smelly (and very toxic) molecule, so very small quantities are easily detected by our noses.

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