It comes up in our house a lot. I'm guessing it might in yours, too. It goes something like this: one of us is holding up X ingredient to the other asking, "do you think this is still good?" What follows is a delicate dance of sniffing, staring, scratching our heads and making a quick decision that's often more guess than certain knowledge. But with eggs, that decision can be much easier.
Now if you've ever encountered a really bad egg, you'd know. They do smell and it's not pretty. But for that in-between zone when you're just not too sure, food scientist Harold McGee suggests to put them in a bowl of water. In his book, On Food and Cooking, McGee states that "The moment the egg leaves the hen, it begins to deteriorate in important ways." That being said, eggs last a pretty darn long time in the refrigerator, but there is one way in which he suggests to test them at home. McGee says that eggs lose moisture through their shell so the inside contents of the egg shrinks and the air cells expands. When this happens, an older egg is more likely to float.
This doesn't necessarily tell you that the egg is so bad that it shouldn't be eaten, but it does tell you it's been hanging out for a while. A few other tests you can do at home to determine the age of your eggs:
Is My Egg Fresh?
• Look: A fresh egg boasts a bright-yellow yolk that should be round and perky. As the egg ages, the white becomes runny and the yolk becomes much more flat and slightly pale.
• Listen: Take an egg from the refrigerator and hold it up to your ear. Shake it gently. If it's fresh, it won't make a sound. If it's on the older side, the egg will make a slight rattle.
• Peel: Fresher eggs are harder to peel which is why many people let their eggs age for a while in the refrigerator before making deviled eggs.