Science Puzzle
Why Does Popcorn Pop?
A popcorn kernel is a small, hard seed. Heat a pan of them and, one by one, they burst with a pop into fluffy white puffs many times their original size. Ordinary sweetcorn kernels just shrivel and burn.
What makes a popcorn kernel explode?
The Answer
Steam. Each kernel is a tiny pressure cooker that eventually bursts.
Inside every kernel is soft starch holding a little water, about 14% of its weight. The outer shell, the hull, is unusually hard and almost waterproof. As the kernel heats up, the water turns to steam, but the steam can't escape through the hull, so the pressure inside climbs. At the same time the heat softens the starch into a hot, gooey gel.
At around 180 °C the pressure reaches about 9 times normal air pressure, and the hull finally gives way. The pressure drops instantly, the trapped steam expands violently and blows the soft starch out into a foam, which sets almost at once as it cools. The kernel turns inside out, and you hear the pop.
That's why only popcorn pops. Other kinds of corn have hulls that let the steam leak out slowly, so the pressure never builds. And it's why old popcorn that has dried out too much leaves lots of unpopped kernels at the bottom of the bowl: without enough water, there isn't enough steam to burst the hull.
The principle: Pressure from trapped steam. Water heated in a sealed space turns to steam and builds up pressure until the container bursts, and the sudden expansion does the work.