A lot of people rely on the date on the packaging to tell them when food has gone bad, even with eggs, but the sell-by dates are often somewhat arbitrary and are not expiration dates. If you've been tossing your eggs based on the dates on your carton—you could be wasting perfectly good food.
Eggs are often still good to eat long after the date on the packaging says to throw them out. If you want to test how fresh they are before finding out the hard way, here are a few methods for testing them.
Your eyes and nose are the best tools for determining freshness with meats, produce and herbs — it's pretty obvious when something's moldy or smelly. But you can't smell and see an egg before you crack it (unless you're highly skilled), so how do you tell if it's still good?
Just fill a bowl with cold water and place your eggs in the bowl. If they sink to the bottom and lay flat on their sides, they're very fresh. If they're a few weeks old but still good to eat, they'll stand on one end at the bottom of the bowl. If they float to the surface, they're no longer fresh enough to eat.
While you could fry or scramble an egg that's on its side or standing upright, when it comes to hard-boiling, you'll want the upright ones, as Yumi points out in her guide to peeling hard-boiled eggs. The extra air in the older eggs will help the peel come off easier after hard boiling.
Below, you can see what a really old egg looks like in comparison to really fresh one. The one on the left is most likely 3 or more months old (from when it was laid, not the date you bought them).
To give you an idea of hold old an egg is, look at the "packed by" dates on the carton, which are in Julian date form by the "sell by" dates. Julian dates range from 1 to 365 days, and since most companies pack their eggs shortly after being laid, it's a good indicator.
The reason this method works is that the eggshells are porous, which means they allow some air to get through. Fresh eggs have less air in them, so they sink to the bottom. But older eggs have had more time for the air to penetrate the shells, so they're more buoyant and will float.
Some people also claim you can hold an egg up to your ear and shake it to test for freshness. If you can hear liquid sloshing sound inside the egg, it's probably gone bad, but if you hear nothing, it's fine to eat. Personally, though, I don't think this method is as reliable.
Additionally, there is the candling method, which is used primarily for testing egg quality before they're sold, but it could help determine freshness too, though it's more difficult to see at later stages.
You can put a flashlight right next to the eggshell to light up the insides, but historically, a piece of cardboard with a small hole in it was used, with a light source behind it and the egg in front.
The above method will let you see the air space and mold, but it's really a difficult technique to get down.
Above you can see a fresh egg (little air space, slightly visible yolk), a slightly old egg (larger air space, slightly darker yolk), a nearly bad egg (really dark yolk, spotty), and a spoiled egg (mixed in yolk, lots of dark), lit up using the candling technique. If you want to see inside your eggs more often, try out this LED egg candle light — it's made to check if an egg is fertilized if you're into that.
If you don't need the shell intact, you can also crack the egg onto a plate or other flat surface to test how fresh it is. If it's fresh, the yolk should be bright yellow or orange, and the white shouldn't spread much. If you're not sure, give it a good sniff: fresh eggs shouldn't have much of a smell at all.
The yolk will be flatter and the white will be much runnier in an older egg. An egg that spreads out when cracked isn't necessarily bad, though, just older (and again, good for hard-boiled eggs). If it's gone bad, you probably won't even need to do the sniff test—even slightly rotten eggs will have a very strong, distinct smell you'll notice right away.
Looking for more eggy tips? Learn everything you need to know about cooking eggs from The Ultimate Guide To Cooking Eggs, by D and P Gramp.
There are also lots of things you can do with those discarded eggshells, like make your teapot super clean, fertilize soil, and make sidewalk chalk. And don't forget about those leftover egg cartons, which make good seed and fire starters, bird feeders, and candle-making molds.
Do you know of any other ways to test whether or not an egg is good to eat? Share your method with us in the comments.