Short answer: a popcorn kernel traps water inside a strong outer hull. As the kernel heats, that water becomes superheated and pressure rises. At roughly 177°C in classic measurements, the hull can no longer contain the pressure and ruptures; the hot starch expands rapidly as steam escapes.
The hull is the pressure vessel
Popcorn is special because its pericarp—the outer shell—is tough and relatively impermeable. Ordinary corn does not build pressure in quite the same way. A classic study in the Journal of Cereal Science measured popping near 177°C and estimated internal pressure around 135 psi at rupture.
Why does it turn inside-out and fluffy?
Inside the kernel, starch has been heated and softened. When the hull fails, pressure drops suddenly. Superheated water flashes into expanding vapor and the starch matrix inflates, then cools into the familiar white foam.
That is why moisture content matters: too little water means too little steam pressure, while damage to the hull lets pressure leak away before the dramatic pop.
Quick follow-ups
FAQ
Why do some kernels stay unpopped?
A damaged hull, unsuitable moisture content or insufficient heating can prevent the kernel from building and releasing pressure in the right way.
Is the pop caused by the kernel exploding?
In a mechanical sense the hull ruptures under internal pressure, but the edible white part is rapidly expanded starch rather than fragments of a hollow shell.
Check the evidence
Sources
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