Why popcorn pops

Every popcorn kernel is a tiny pressure cooker. Heat turns a little water inside it to steam, until the hull gives way and the inside turns to foam.

Popcorn is a kind of maize, and its secret is in the kernel. The hull is strong and doesn't let moisture through, and the starch inside is almost all of a hard type. Growers dry the kernels to about 14 to 15% water by weight, which turns out to be just right.

When the kernel is heated, that water turns to steam. The hull holds it in, so the pressure inside climbs, while the heat softens the starch into a gel.

At around 180 °C the hull gives way. By then the pressure inside is about 930 kilopascals, roughly nine times the pressure of the air around us.

From kernel to foam

Once the hull breaks, the pressure drops at once and the steam expands, puffing the softened starch and proteins into a foam. Each kernel grows to 20 to 50 times its original size. The foam cools in moments and sets into the crisp, white shape we eat.

In a 2015 study, two physicists in France filmed kernels popping and recorded the sound at the same time. The bursting, jumping and popping all happen within a few hundredths of a second. The kernel jumps because a leg of starch pushes out and presses against the pan, and the familiar pop, they proposed, comes from the release of water vapour.

The same physics explains the duds at the bottom of the bowl. Kernels that stay unpopped usually don't have enough water to build up the pressure. Fresh kernels with too much water pop poorly and turn chewy, and over-dried ones expand less.

The speed of heating matters too. Too fast, and the steam near the outside forces the hull open before the centre has softened, leaving a hard middle. Too slow, and steam leaks out through the tip of the kernel, so it may never pop at all.

People enjoyed the result long before anyone measured it. Archaeologists have found evidence of popcorn in Peru from about 4700 BCE.