Why does popcorn pop?
Because of trapped steam. Heat turns the water inside each kernel into steam, pressure builds until the hull bursts at about 180°C (356°F), and the starch inside puffs out into a foam that sets into a flake.
Quick Answer
Popcorn pops because of steam. A popcorn kernel has a hard outer hull that keeps moisture in, and inside it are starch and about 14% water. As the kernel heats up past the boiling point, the water turns to superheated steam, pressure builds inside the sealed hull and the starch softens. At about 180°C (356°F), the hull gives way. The pressure drops, the steam expands and the softened starch puffs into an airy foam that cools and sets into the fluffy shape we eat. In a 2015 study in the Journal of the Royal Society Interface, researchers Emmanuel Virot and Alexandre Ponomarenko found that the critical temperature is about 180°C regardless of a kernel’s size or shape, that the pop comes from the release of trapped steam, and that the new popcorn launches itself with a starchy “leg” that pushes off the surface, like a gymnast.
By the Numbers
Key Facts
- What pops it
- Steam from the water inside the kernel
- Temperature
- About 180°C (356°F), whatever the kernel’s size or shape
- The sound
- The release of trapped steam, according to a 2015 study
- The jump
- A starchy “leg” that pushes the kernel off the surface
Inside a popping kernel
- Heat
The water inside the kernel boils and turns to steam.
- Pressure builds
The hull traps the steam, and the starch softens.
- The hull bursts
At about 180°C, the pressure breaks the hull open.
- The starch puffs
The expanding steam turns the starch into a foam that sets as it cools.
A pressure cooker the size of a pea
A popcorn kernel’s hull is hard and nearly waterproof, which lets steam build up instead of leaking out. Past 100°C, the water inside boils, and as the vapor pressure rises, the starch gelatinizes. Above a critical pressure, the hull breaks.
Why some kernels never pop
Popcorn needs the right balance. If a kernel heats too slowly, or its hull leaks, steam escapes before enough pressure builds, and the kernel stays a kernel.
The jump and the sound
Virot and Ponomarenko filmed popcorn with high-speed cameras. They found that the pop comes from the release of trapped steam, and that a bent “leg” of starch on the new popcorn pushes against the surface and launches it, much as a gymnast springs off a floor.
Myth vs. Fact
MythThe pop is the sound of the hull cracking.
FactAccording to the 2015 study, the sound comes from the release of trapped steam.
MythAny corn can be popped.
FactPopcorn is a variety with a hard, nearly waterproof hull that holds the steam in until the pressure is high enough.
The Bottom Line
Popcorn pops because heat turns the water inside each kernel into steam. The hull holds it until the pressure bursts it at about 180°C, and the starch puffs out into foam.