Cracking Eggs 101
How physics can help you when you’re making your next soufflé.
Reis and Lazarus aren’t the only scientists who are intrigued by the unusual properties of eggshells. There is a surprisingly long history of scientific studies of eggshells, not only in the agricultural sciences but also in the aerospace industry during the 1950s and 1960s, when the focus was on failure analysis of the metal shells used to build airplanes. Failure analysis is the study of how things break, exactly how and why brittle fracturing occurs when one drops a glass, or crashes an airplane—and eggshells’ resistance to shattering made them interesting to airplane engineers.
Eggshells aren’t quite as brittle as glass or the metal shell of an airplane, because they are laced with organic material. Any given material gets its properties, like resistance to cracking, from its crystalline structure. Eggshell is similar to tooth enamel or seashells; all of these materials are made up primarily of calcium carbonate (calcite) crystals embedded within a protein matrix. It’s the latter that gives the shell its remarkable toughness, which is bolstered by a thin inner membrane made of collagen. Rather than splitting an eggshell in one clean break, cracks in an eggshell spread bit by bit, millimeter by millimeter. “Cracks propagate with great difficulty and the eggshell doesn’t shatter,” says Michelle Oyen, a materials scientist at Cambridge University. “The damage is very localized.” In that respect, an eggshell is nature’s perfect packaging.
But not infallible. Science supports conventional chef wisdom when it comes to the optimal way to crack an egg without shattering the shell completely. There may be individual variations from cook to cook: Oyen personally favors a firm tap on a flat countertop. But it will always involve a single sharp smack—as opposed to a series of gentle taps, which are more likely to shatter the shell—against a hard surface or edge, targeting the center of the egg. For the one-handed technique, some chefs advise practicing with a golf ball until you get just the right feel for exactly how much pressure is required. No matter which technique you use, you should heed the advice of Sabrina’s persnickety instructor: Be merciful. A fast, clean break is best.
Jennifer Ouellette writes the blog Cocktail Party Physics, and her most recent book is The Calculus Diaries: How Math Can Help You Lose Weight, Win in Vegas, and Survive the Zombie Apocalypse. Find her on Twitter at @JenLucPiquant.