Every Popcorn Kernel Is a Tiny Pressure Cooker

Pour a handful of popcorn kernels into a hot pan and for a while nothing happens. Then one jumps, then three, then the whole pan erupts into a rattling white storm. Each of those small hard seeds has turned itself inside out into a crunchy cloud. The reason is that every kernel is a tiny sealed pressure cooker, and the pan is simply waiting for the pressure to win.

Inside each kernel is soft starch with a little water in it. Popcorn is dried until that water makes up about 14 percent of the kernel, which turns out to be the sweet spot. Around the starch sits the pericarp, the kernel's hard, shiny outer skin. In popcorn it is unusually strong and nearly watertight, and that is the whole trick.

As the kernel heats up, the water inside turns to steam. Normally steam would leak away, but the tough pericarp holds it in. Pressure builds, and it can reach about 9.2 atmospheres, roughly 135 pounds per square inch, before the shell gives way. Meanwhile the hot steam softens the starch into a thick, jelly-like goo. When the pericarp finally bursts, the goo rushes out, puffs up and sets as it cools into the familiar white foam. A popped kernel ends up 40 to 50 times bigger than it started.

There is a surprisingly exact moment for all this. In 2015 two physicists published a study in the Journal of the Royal Society Interface in which they heated kernels, filmed them and recorded the sound at the same time. They found that almost all the kernels popped once they reached about 180 degrees Celsius, the temperature at which the pressure inside beats the strength of the shell.

Their footage held two more surprises. The jump is not a little rocket blast of steam. As the kernel bursts, a leg of starch shoots out, presses against the pan and flips the kernel into the air, sometimes turning it almost a full somersault. And the pop itself does not come from the shell cracking. By matching the sound to the video, they showed it comes from the release of water vapour, which makes the kernel ring for a moment like a tiny bottle blown across its top, a few thousandths of a second after the crack.

This also explains why sweet corn, the kind eaten on the cob, will not pop. Its outer skin is not strong enough to hold the steam, so the pressure simply leaks away and the kernel scorches. And those hard, unpopped leftovers at the bottom of the bowl usually had too little water inside, so they never built enough pressure to burst.

So the next time a pan starts to crackle, listen closely. Each pop is a little burst of steam escaping a seed that could not hold it in any longer, followed by a starchy acrobat leaping for freedom. Few snacks come with their own physics experiment, and even fewer with applause.