Although hard-boiled eggs appear to be a perfectly packaged picnic snack, they need to be stored below a safe temperature as soon as possible after cooking. There are often no visible sign of spoilage. While raw eggs attract the most concern -- mainly because of salmonella -- a cooked egg that has been left at room temperature can pose just as many health risks.
Steering clear of a hard-boiled egg -- still in its shell -- that floats in water will safeguard against choosing an egg that's spoiled. Even if some perfectly good eggs do float because of an aberration, buoyancy is a sign that the air sack inside the membrane has enlarged as a result of the egg aging beyond its recommended storage span.
For the same reason, fresher eggs are harder to peel than those that might be past their prime. Because air accumulates in older eggs, those eggs whose shells slip off easily might be spoiled, although other tests should be performed.
As a supplementary test, spoiled eggs announce themselves with a strong, sulfurous odor once opened. The smell is so potent that there is little guesswork required in deciding whether to press on with enjoying the egg.
Color is a less reliable gauge. Hard-boiled eggs that have been overcooked will have a green tinge to the yolk, caused by sulfur and iron deposits reacting with the proteins. The color might be unappealing, and the taste slightly metallic, but the egg is still good to eat.
Other factors, such as egg white emerging from the shell, cracks or traces of blood in the yolk, are purely aesthetic. As long as the egg was stored properly and cooked by the recommended date, it cannot turn bad during cooking.
Although boiling an egg to an internal temperature of 160 degrees Fahrenheit destroys salmonella, the process also destroys the protective shell coating that keeps out other bacteria. As soon as the egg is boiled, harmful bacteria are able to penetrate the porous shell.