People are always asking me how the float test works in determining the freshness of an egg. To show how it works, I drew this simple diagram. Eggs that lay flat on the bottom are fresh. If the fat end is starting to stand up they are about a week old, and a few weeks old if they are standing on end. When doing this test, it is very possible you will see eggs floating somewhere between the standing on end and the floating above water stage. This just means the air-cell size is getting bigger in the egg. The bigger the air cell, the higher it will float, the older the egg will be. Another way to estimate the egg's age is to break it open and examine the contents. A fresh egg has a firm yolk standing high surrounded by an egg white with a cloudy firm appearance. As the egg ages, the egg white, or albumen, will becomes clear and watery and the yoke begins to flatten. The older the egg gets, the more likely it is the yolk will break when you crack open the shell. This happens because water moves from the albumen (egg white) to the yolk causing the yolk membrane to stretch and weaken. It eventually breaks. If you want still another way to test freshness, smell it! If it has a foul rotten-egg smell, it's bad! This smell results from the emission of hydrogen sulfide.
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