Sulphur impacts on quality of eggs
Patricia Brown asked:
I hear that eggs have sulphur in them. Is it in the yolk or the white?
Fiona Nyhof, a food scientist at the University of Otago, responded:
Eggs do contain sulphur in the egg white.
Egg white is a mixture of water and protein called albumen. The most abundant protein in eggwhite albumen is ovalbumin, which contains sulphur, so the way ovalbumin reacts and changes during storage and cooking has a big effect on the eating quality of eggs.
Proteins are complex folded molecules comprising many amino acid groups joined by peptide bonds. A sulphurcontaining amino acid called cysteine forms part of the structure of the ovalbumin protein in egg white. When eggs are cooked, heat causes cysteine to release its sulphur in the form of strong smelling hydrogen sulphide (H2S) gas. In older eggs, the production of hydrogen sulphide happens even more readily than when the eggs are fresh. As eggs age, the albumen becomes more alkaline (pH>7). Proteins are very sensitive to pH changes and they tend to ‘‘unfold’’. When ovalbumin unfolds, sulphurcontaining cysteine, previously buried inside the protein structure, gets exposed and is, therefore, more able to react, resulting in higher concentrations of the hydrogen sulphide gas that is responsible for strong sulphurous ‘‘eggy’’ smells that most people find objectionable.
Another interesting chemical reaction involving sulphur in eggs happens when eggs are boiled in water for a long time, for instance 15min. People often ask ‘‘what is that greengrey discoloration seen at the edge of the white and the yolk, or even right through the yolk?’’ The undesirable colour is due to the formation of iron sulphide (FeS). The sulphur (S) comes from the hydrogen sulphide released by cysteine and the iron (Fe) is present in the yolk.
Harold McGee in his famous book On food and cooking: the science and lore of the kitchen, suggests that to avoid discoloration, cook eggs only as long as necessary to set the yolk, then to plunge them into cold water immediately after cooking, and peel them promptly, so that the hydrogen sulphide gas can escape.
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