Cooking is all about chemistry, and the familiar experience of boiling an egg can offer great illustrations of some of the properties of gases.
In this Snack, you may have noticed that the ice-cooled egg was the easiest to peel, followed by the room-temperature egg. That’s because of the egg’s internal structure.
Just inside the eggshell are two membranes that cause the egg to stick to its shell when dried out. In between these membranes is a little sac of air called an air cell. When this air cell is heated, the gas inside expands according to Charles’s Law, which states that the volume of a fixed amount of gas increases as its temperature increases. When the egg is rapidly cooled, the volume of the air cell decreases quickly, pulling the membrane away from the shell and making it easier to peel.
The opposite happens inside the egg that stays in the hot water, which cools gradually over a long period of time. When cooled slowly, the membrane stays attached to the egg, making the shell difficult to peel, even after it cools down. The size of the air cell increases as an egg ages, so this effect is most dramatic with older eggs. Fresher eggs have smaller air cells, so may be difficult to peel no matter how they’ve been cooked or cooled.
Once you peel and halve the eggs, you may notice that the slow-cooled egg has a greenish ring between the yolk and the white. The gray-green compound is iron sulfide, and that’s what’s responsible for the “eggy” odor of some hard-boiled eggs. Iron sulfide forms when hydrogen sulfide gas in the egg white reacts with iron compounds in the egg yolk. The solubility of a gas tends to decrease as temperature increases (think of a flat, warm soda versus a fizzy, cold soda that can keep more carbon dioxide in solution). Quickly cooling the egg keeps more of the hydrogen sulfide in the egg white: the gas doesn’t diffuse into the center, so it doesn’t react with the yolk. In the slow-cooled egg, which is warm throughout, the hydrogen sulfide is not as soluble in the white. It migrates to the yolk, where it reacts with iron, giving the egg yolks a green rim. The room-temperature egg should be somewhere in between.
The verdict: For easy-to-peel eggs with bright yellow yolks, use older eggs and try quickly cooling them in an ice-water bath after boiling.