If you often cook eggs, you may have noticed that extremely fresh ones are difficult to peel when hard-boiled. The shell doesn't peel back cleanly and often leaves a pock-marked texture on the surface of the egg. But as eggs grow older, their shells peel more easily. Why is this? In fresh eggs, the albumen (egg white) tends to stick to the inner shell membrane due to the less acidic environment of the egg.
After the eggshell's protective coat slowly wears off, the egg becomes porous, absorbs more air, and releases some of its carbon dioxide. This makes the albumen more acidic, causing it to stick to the inner membrane less. The egg white also shrinks slightly, so the air space between the eggshell and the membrane grows larger, resulting in boiled eggs that are easier to peel.
For ideal peeling, use eggs that are seven to 10 days old and follow Jacques Pepin's tips for boiling the perfect egg.