A Home Test

Finally, the egg as a whole also loses moisture through its porous shell, so the contents of the egg shrink, and the air cell at the wide end expands. Even an oil-coated egg in a humid refrigerator loses 4 milligrams of water to evaporation each day. The cook can use this moisture loss to estimate the freshness of an egg. A new egg with an air space less than ⅛ inch/3 mm deep is denser than water and will sink to the bottom of a bowl of water. As an egg ages and its air cell expands, it gets progressively less dense, and the wide end of the egg rises higher and higher in the water. An egg that actually floats is very old and should be discarded. Around 1750, the English cookbook author Hannah Glasse gave two ways of determining the freshness of an egg, an important talent at a time when it might have been sitting for some time in an odd corner of the yard. One is to feel how warm it is—probably less than reliable—but the second indirectly assays the air cell: “[Another way] to know a good egg, is to put the egg into a pan of cold water; the fresher the egg the sooner it will fall to the bottom; if rotten, it will swim at the top.”