Everyday
Why does popcorn pop?
The short answer
Because each kernel is a sealed pressure vessel. Water inside the kernel turns to steam but cannot escape through the hard shell, so the pressure builds to around ten times normal air pressure until the shell cracks at about 180 degrees Celsius. The starch bursts outward and sets, and the pop sound is the steam rushing out, not the shell breaking.
A popcorn kernel is unusual among grains: its outer shell is hard and airtight. Inside is starch and about 14 percent water. Heat the kernel and the water wants to boil, but it has nowhere to go, so the kernel behaves exactly like a pressure cooker with the lid clamped down.
Two French physicists filmed kernels on a hot plate at nearly 3,000 frames a second and found a remarkably precise threshold. At 170 degrees Celsius only about a third of kernels popped. At 180 degrees, nearly all of them did, whatever their size or shape. That is the temperature at which the shell can no longer hold roughly ten atmospheres of steam.
The slow-motion footage also answered why popcorn jumps: a flake of starch shoots out first and acts like a leg, pushing off the pan, so the kernel somersaults into the air like a gymnast. And the pop itself is not the crack of the shell. It is the whoosh of steam escaping, which sets the kernel ringing like a tiny drum. You have been listening to a steam whistle this whole time.
Sources
- The science of popcornThe Royal Society
- Popcorn: critical temperature, jump and soundJournal of the Royal Society Interface