How to tell how old an egg is
1. It's the amount of air inside an egg that the cook needs to be concerned with. You can see that 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 pocket grows; at the same time, the moisture content of the egg begins to evaporate.
2. At the top of the picture 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, on the right, the yolk is flatter and the two separate textures of white are not quite so visible. Now all is revealed! You can see very clearly why an egg needs to be fresh if you want to fry or poach it, because what you will get is a lovely neat, rounded shape. Alas, a stale egg will spread itself more thinly and if you are frying it you will end up with a very thin pancake with a yellow centre. If you put it into water to poach, it would probably disintegrate, with the yolk and white parting company.
3. How to tell how old a raw egg is while it is safely tucked away in its shell could seem a bit tricky, but not so. Remember the air pocket? There is a simple test that tells you exactly how much air there is. All you do is place the egg in a tumbler of cold water: if it sinks to a completely horizontal position, it is very fresh; if it tilts up slightly or to a semi-horizontal position, it could be up to a week old; if it floats into a vertical position, then it is stale. The only reason this test would not work is if the egg had a hairline crack, which would allow more air in. That said, 99 per cent of the time the cook could do this simple test and know precisely how the egg will behave. In my opinion, all eggs should be used within two weeks if at all possible. An extra week is okay, but three weeks is the maximum keeping time.
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