Just before a hen lays an egg, she applies an extra layer of protection to the shell called "Egg Bloom". This magical coating seals the egg and prevents anything extra from getting in such as bacteria. It also slows the evaporation of the egg. Egg bloom is a great protector, BUT when we process the eggs commercially, we remove that magical coating. The large egg producers then replace the bloom with a coat of mineral oil. That's why the eggs you buy at the store sometimes look very shiny. Well, mineral oil helps, but when we boil an egg we effectively remove that coating giving the egg a shorter shelf life. The FDA recommends keeping hard-boiled eggs refrigerated, and using them (in the shell or peeled) within 3 to 4 days after cooking".