We’ve all puzzled, after following someone’s can’t-fail advice, over less-than-perfect hard-cooked eggs—the eggs with rubbery whites, chalky yolks, and that tell-tale green-gray film between yolk and white. The cause? Temperature differential: The white of an egg dropped into boiling water cooks much faster than the yolk at the center, and that’s trouble. By the time the yolk sets, the white is tough. And if the egg stays over high heat too long, or isn’t cooled quickly after cooking, sulfur in the white will react with iron in the yolk, creating that nasty off-colored ring.
Here’s the fix: To keep the temperature of the egg white and yolk close, heat the eggs gradually. Place them in a saucepan, cover them by an inch or two with cold water, and set the pan over high heat. When the water reaches a full boil, remove from heat, cover the pan, and let the eggs stand for 10 minutes. This cooks them gently and keeps the whites from toughening. Peel the eggs immediately under cold running water; or, if you’re not using them right away, set them in an ice water bath. This lowers the eggs’ temperature and minimizes the pressure that causes sulfur rings to form.