1.6.4 Cooking eggs
Many solids melt as they get hotter, but eggs do not. They start as quite liquid in the fresh state and when they are heated, they go solid. This behaviour is a result of the effect of the heat on the proteins they contain. The proteins of both the egg yolk and white are folded into precise shapes (each molecule forming a minute ball called a globular protein. But when these proteins are heated, some of the interactions holding the protein molecules into their precise globular shapes are broken and the molecules begin to unravel. This enables the separate protein chains to become entangled with one another and new interactions can form, particularly hydrogen bonds and the stronger disulfide bridges, which create a solid three-dimensional network of protein molecules, leading to a soft-boiled or eventually a hard-boiled egg.
Which amino acid forms disulfide bridges and which element does it contain?
Disulfide bridges form between the R side-chains of the amino acid cysteine, which contains sulfur (Figure 8).
When an egg begins to go ‘bad’, its proteins break down and the sulfur from the cysteines forms a gas called hydrogen sulfide (H2S), which has the characteristic and very unpleasant smell of ‘rotten eggs’.
Coagulation (solidification) of proteins can be caused by acidity, as well as by heat. Some people add vinegar or lemon juice to the water in which they are boiling eggs. If the eggshell cracks, and the white of the egg (albumen) begins to escape, the protein in it coagulates more quickly in the slightly acidic water than it would do in ‘plain’ (neutral) water, sealing the crack.
Whisking egg whites also causes the globular proteins to unravel and take up new shapes, but this time air is trapped in the three-dimensional network that they form. Baking the whisked egg white causes coagulation, and the protein becomes solid, forming a meringue. If fat molecules are present, they coat the globular proteins and prevent them from unravelling and tangling when they are whisked, which is why you cannot make a meringue if any of the egg yolk, which contains fat, is present in the egg white.