What Makes Popcorn Turn Itself Inside Out?

Every popcorn kernel is a small pressure vessel containing starch and a carefully preserved amount of water.

The kernel’s hard outer shell, called the pericarp, is unusually strong and nearly airtight. When the kernel is heated, the water trapped inside turns into vapor. Because the vapor cannot escape easily, pressure builds while the heat softens the surrounding starch into a flexible, gel like material.

At about 177 degrees Celsius, the internal pressure may reach roughly nine times normal atmospheric pressure. Eventually, the shell develops a fracture. The sudden pressure drop allows the heated water to expand rapidly into steam. That expansion pushes the softened starch through the opening, inflating it into a foam many times larger than the original kernel.

High speed recordings show that the expanding starch also acts like a leg. As it pushes against the pan, the kernel jumps and turns. The familiar popping sound is produced mainly by the release of water vapor and the vibration of cavities within the expanding flake.

Popcorn must contain the right amount of moisture. A damaged shell may leak before sufficient pressure develops, while an overly dry kernel cannot produce enough steam. This is why some kernels remain stubbornly unpopped.

The white part is not material that was hidden beneath the yellow shell at full size. It is cooked starch expanded in a fraction of a second, after the kernel’s hull holds pressure for as long as it possibly can.