Just before laying an egg, the hen adds a protective layer called “egg bloom” or cuticle to the outside of the egg. This coating seals the shell pores, prevents bacteria from getting inside the shell, and reduces moisture loss from the egg – all designed to make the egg last longer.
Unfortunately, because of conditions at some large egg operations, commercial eggs are washed right after collection to make them appear clean and presentable. Of course, this destroys the protective egg bloom. To try replacing natural bloom, some commercial packers spray shells with a thin film of mineral oil – that’s why grocery store eggs sometimes appear shiny.