A popcorn kernel is a remarkably well-designed little pressure vessel.
Inside the hard outer shell are starch and a small amount of water. Heat the kernel, and that water becomes the key to the entire transformation.
A popcorn kernel traps water inside
Popcorn is a particular type of corn with a hard, relatively moisture-tight outer layer called the pericarp, often simply called the hull.
The inside contains starchy endosperm and water. Proper popping depends on the kernel retaining enough moisture after harvest and storage.
When the kernel is heated, the water gets hotter and eventually produces high-pressure water vapor inside the sealed hull.
The steam cannot simply drift away because the outer layer is tough.
Pressure builds.
Heat also softens the starch
While the pressure rises, the starch inside the kernel becomes hot and soft.
Eventually the hull can no longer contain the pressure. It ruptures suddenly.
The pressure inside the kernel drops almost instantly, allowing the superheated water to expand into steam. The softened starch expands outward with it.
The kernel essentially turns itself inside out.
As the expanded starch cools, it solidifies into the light, crunchy structure we call popcorn.
That is a fairly dramatic career change for a grain of corn.
Why does popcorn jump when it pops?
The rupture is not perfectly symmetrical. As the hull breaks and the starchy material begins expanding, the forces can push against the surface beneath the kernel and launch it into the air.
High-speed studies of popping kernels have shown that the opening and expansion can act a little like a tiny leg pushing the kernel upward.
So the jumping is part of the same rapid mechanical event that creates the puff.
Why do some kernels never pop?
Unpopped kernels often have a moisture problem or a damaged hull.
If the outer shell is cracked, steam can leak out before pressure becomes high enough to rupture the kernel explosively.
If the kernel has dried too much, there may not be enough internal water to create the necessary pressure and expansion.
That is why popcorn is stored so that it does not become excessively dry.
The sad hard kernels at the bottom of the bowl are not necessarily defective corn. Some simply failed the pressure test.
Why does ordinary sweet corn not pop the same way?
Different types of corn have different hulls and internal structures. Popcorn has the particular combination of a strong outer layer and suitable starchy interior needed to trap pressure and expand dramatically.
Other corn varieties may split or crack when heated, but they generally do not produce the same fluffy result.
Does microwave popcorn pop differently?
The basic physics is the same whether popcorn is heated in a pan, hot-air popper, or microwave bag.
The method changes how energy reaches the kernel. The kernel still needs internal water, a strong hull, heat, and enough pressure to burst.
So popcorn is not exploding because the microwave has a special “popcorn frequency.”
The kernel already contains the equipment.
The heat simply starts the countdown.
A related bit of kitchen science appears in Why do onions make you cry?, where a different familiar food has its own chemistry.
And that’s the short explanation.
Related explanations: Why do onions make you cry?, Why do apples turn brown?, and Why does organic milk last longer?