And how long can they be used after the date on the carton. I have heard that if a egg floats in a glass of water it is not to be consumed. Is this test a true way to evaluate a egg?
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How long an egg lasts is very much dependent upon its care and handling. Time, temperature, humidity and jostling during transport all have an effect on egg quality. (Agitation during storage is an issue too, which is why you shouldn't keep them in your refrigerator door.)
USDA regulations state the sell-by date may be up to 45 days from the pack date (some state regs may be less) and recommends using them (properly refrigerated) within 3-5 weeks of purchase. Sometimes you'll find a packing date in Julian format. USDA grading is one indication of quality but since the system isn't foolproof, there are variations allowed.
The float test will give you an idea of how fresh an egg is. As an egg ages, it loses moisture due to evaporation. As that happens, the air cell inside the shell expands and the egg has an increasing tendency to float. Floaters are old.
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I'm glad ChefOno still has the boilerplate down on this topic because it is exactly right.