Why Does Popcorn Pop?

Not every kind of corn pops. Popcorn is a special variety whose kernels act like tiny pressure cookers.

Inside a kernel

• A hard, waterproof outer shell called the pericarp.
• Starchy endosperm inside.
• A small amount of water — ideally about 13–14% of the kernel’s weight.

The pop

As the kernel heats, the water turns to steam. The tough hull traps it, so pressure builds while the starch inside softens into a hot gel. At around 180°C, the pressure becomes too much and the hull ruptures. The sudden drop in pressure makes the steam expand instantly, puffing the gelatinised starch into a foam that sets as it cools — the fluffy white flake.

Why some kernels don’t pop

“Old maids” usually have too little moisture (they dried out in storage) or a cracked hull that lets steam leak out slowly. Keep popcorn in an airtight jar for fewer duds.

Butterfly or mushroom

Popcorn comes in two main shapes: “butterfly” flakes with wings (common in cinemas) and round “mushroom” flakes, which are sturdier for caramel coating.

An old snack

Archaeologists have found popped corn in Peru thousands of years old — people have enjoyed this bit of physics for a very long time.