How to tell if an egg is bad

Food poisoning is one of the worst experiences a person can go through, and it can even be lethal. Eggs can be the source of some superb meals, but they can also be the source of food poisoning if they are eaten when they've gone bad. How do you tell when an egg is fresh or if it has gone off, without cracking it open to find out?

Steps:
1.Place the egg into a bowl of water. The water level should be deeper than the egg is long.
2.Observe what the egg does.

• Fresh eggs will sink to the bottom of the bowl and lie on their sides.


• Slightly older eggs (about one week) will lie on the bottom but bob slightly.


• If the egg balances on its small end, with the large end reaching for the sky, it's probably around three weeks old.


• Eggs that float at the surface are bad and should not be consumed.

3.Crack the egg open and look carefully.

• Blood spots (also referred to as "meat" spots) don't signify a bad or fertilized egg. It's caused by a ruptured blood vessel during the formation of the egg. Since blood spots are diluted as the egg ages, their presence actually means you have a fresh egg. You can eat it safely, or remove the blood spot with the tip of a knife, if it makes you feel better.[1]


• Stringy, rope-like strands of egg white are chalazae which are present in every egg to keep the yolk centered. They're not a sign that the egg is bad or fertilized, and they can be consumed safely or removed.[1]


• An egg white that is cloudy or has a yellow or greenish cast to it is caused by carbon dioxide not having had enough time to escape from the shell and is especially common in fresh eggs.[1]


• Smell the egg. With time, bacteria break down the proteins in the whites of the egg and create a gas. This gas is hydrogen sulphide, better known as "rotten egg gas."

Tips:

• The floating test works because the air pocket inside the egg gets bigger with time as the egg contents lose both moisture and carbon dioxide. As the air pocket gets bigger, the egg is more likely to float.


• When a recipe calls for a lot of yolks or whites, separate the eggs in a different bowl then dump the contents with the rest of the egg yolks (or whites). There's nothing more wasteful than cracking open egg number 14 in a 15 yolk recipe and finding out that it is a bad egg. You could also test all the eggs for freshness using the floating technique.


• If you're worried about the egg being fertilized, keep in mind that most eggs come from large-scale operations where laying hens are never exposed to a rooster. If the eggs come from a smaller farm where roosters are present, there is a chance they might be fertilized, but the only way to tell is by candling (holding a light up to the egg in a dark room and looking for blood vessels, movement, and chick development). That being said, fertilized eggs are no more or less nutritious than unfertilized eggs and both are safe to eat.

Warnings:

• Do it in the sink; it is easier to clean up an egg in the sink than on the bench or floor.


• Be careful with the eggs; if you have a rotten egg and you break it, you'll have more than the egg mess to clean up. The smell is quite unpleasant.


• If you're the least bit doubtful about the egg, it's better to be safe than sorry - skip it and try another one.


• Avoid eating eggs past their expiration date. Check the date stamped on the carton or the egg itself.
  

source : http://www.wikihow.com/Tell-if-an-Egg-is-Bad