Abel Stracker, of Westland High School, asks :-

Kate Johnstone, of Ardgowan School, asked:- Koko Hema-Taylor, of Ardgowan School, asked:- Eva Cossan, of Stirling, asked:-

Why do rotten eggs smell like sulphur? Why do eggs go rotten?

John Walker, a biochemist at the University of Canterbury, responded.

Eggs are rich in proteins and this is why they are such a valuable foodstuff. The 'building blocks' of proteins are chemicals called amino acids and the egg's proteins are particularly rich in the sulphur-containing amino acids cysteine and methionine.

When an egg is stored carbon dioxide slowly diffuses through the pores of the shell and this leads to a rise in pH (e.g. becomes more alkaline), especially the egg white or albumin. The egg also loses water by evaporation through the shell which allows the internal air cell to enlarge. You will probably have noticed how these changes effect the quality and appearance of the egg yolk and white when comparing new-laid versus stored eggs. These changes can be minimised by storing the eggs at near-freezing temperatures and at 85-90 per cent humidity; this is what the commercial egg producers do. Your grandparents may recall a pre-refrigerator method of storing eggs in water-glass, which is sodium silicate dissolved in water.

If eggs are not stored properly the proteins may start to decompose, probably as a result of bacterial action. These decomposition reactions release a number of rather smelly compounds and in particular the sulphur-containing amino acids yield the foul-smelling gas 'hydrogen sulphide' (H2S) sometimes called "bad-egg gas". It is this gas that people associate with sulphur, hot-springs, volcanoes and bad-eggs.