Eggs: Test for Freshness

The air cell, or the empty space between the white and shell at the large end of the egg, gets larger as the egg ages. When an egg is first laid, it is warm. As it cools, the contents contract and the inner shell membrane separates from the outer shell membrane to form the air cell. You can see the air cell in the flattened end of a peeled, hard-cooked egg.

If you immerse an egg in (cool) water, you can judge how fresh it is by how high it floats. Really fresh eggs won't float at all and will sink; the higher an egg floats, the less likely it is to be fresh.

Another way is to shake the egg. Fresh eggs make no sound. Old eggs slosh in the shell.

As an egg ages, the white becomes thinner and the yolk becomes flatter. These changes do not have any great effect on the nutritional quality of the egg or its functional cooking properties in recipes. 

Appearance may be affected, though. When poached or fried, the fresher the egg, the more it will hold its shape rather than spread out in the pan. On the other hand, if you hard cook eggs that are at least a week old, you'll find them easier to peel after cooking and cooling than fresher eggs.