It has a delicate yet tough outer shell which, while providing protection of the growing life inside, is at the same time porous, meaning the air can penetrate and allow the growing chick to breath.It's the amount of air inside the egg that the cook needs to be concerned with. If you look at the photograph to the left, you'll see the construction of the egg includes a space for the air to collect at the wide end, and it's the amount of air in this space that determines the age and quality of the egg and how best to cook it. In newly laid eggs, the air pocket is hardly there, but as the days or weeks pass, more air gets in and the air pocket grows; at the same time, the moisture content of the egg begins to evaporate.
All this affects the composition of the egg, so if you want to cook it perfectly it is vital to determine how old the egg is.
An egg at its freshest, with a rounded, plump yolk that sits up proudly. The white has a thicker, gelatinous layer that clings all around the yolk, and a thinner outer layer. After a week the yolk is flatter and the two separate textures of white are not quite so visible.
Eggs need to be fresh if you want to fry or poach it, because what you will get is a lovely, neat, rounded shape. A stale egg will spread itself more thinly and what you will end up with if you are frying it is a very thin pancake with a yellow centre. If you put it into water to poach it, it would probably disintegrate, with the yolk and white parting company. Separating eggs is yet another hazard if the eggs are too old, because initially the yolk is held inside a fairly tough, transparent membrane, but this weakens with age and so breaks more easily.
For more in-depth information on eggs, click here for the Delia Online Cookery School Study Notes
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