How recently an egg was laid has a bearing on its freshness but is only one of many factors. The temperature and humidity level at which the egg is held play their parts as well. These variables are so important that an egg one week old, held under ideal conditions, can be fresher than an egg left at room temperature for one day. The ideal conditions for egg storage are temperatures between 35 and 40ºF (2º to 4ºC) and a relative humidity of 70 to 80%.
Proper handling means promptly gathering and washing the eggs within a few hours after they’ve been laid. Most commercially produced eggs reach supermarkets within a few days of leaving the laying house. If you and your market handle eggs properly, they’ll still be fresh when they reach the table.
It’s a misconception that you can judge freshness by placing an egg in salt water. A carefully controlled brine test is sometimes used to judge shell thickness of eggs for hatching purposes but the test has no application to freshness of table eggs.
How important is freshness? As an egg ages, the white becomes thinner, the yolk becomes flatter and the yolk membrane becomes weaker. These changes don’t 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. However, if you hard-boil eggs that are at least a week old, you’ll find them easier to peel than fresher eggs. The stronger the yolk membrane, the less likely the yolk will break inadvertently.
– See Storing