he faster you use your eggs, the less time any potential bacteria will have to multiply. However, when properly handled and stored, eggs rarely spoil. Instead, as an egg ages, the white becomes thinner, the yolk becomes flatter and the yolk membrane weakens. Although these changes may affect appearance, they don’t indicate spoilage and don’t have any great effect on the nutritional quality of the egg or its functions in recipes. Rather than spoiling, if you keep eggs long enough, they’re more likely to simply dry up – especially if they’re stored in a moisture-robbing, frost-free refrigerator.
But, like all natural organic matter, eggs can eventually spoil through the action of spoilage organisms. Although they’re unpleasant, spoilage organisms don’t cause foodborne illness. The bacteria Streptococcus, Staphylococcus, Micrococcus and Bacillus may be found on egg shell surfaces because all these species can tolerate dry conditions. As the egg ages, though, these bacteria decline and are replaced by spoilage bacteria, such as coliform and Flavobacterium, but the most common are several types of Pseudomonas. Pseudomonas can grow at temperatures just above refrigeration and below room temperatures and, if they’re present in large numbers, may give eggs a sour or fruity odor and a blue-green coloring.
Although it is more likely for bacteria to cause spoilage during storage, mold growth can occur under very humid storage conditions or if eggs are washed in dirty water. Molds such as Penicillum, Alternaria and Rhizopus may be visible as spots on the shell and can penetrate the shell to reach the egg.
Discard any eggs with shells – or, for hard-cooked eggs, egg white surfaces – that don’t look or feel clean, normally colored and dry. A slimy feel can indicate bacterial growth and, regardless of color, powdery spots that come off on your hand may indicate mold.
Above information from the American Egg Board