A popcorn kernel is a tiny pressure cooker. Heat it to about 180 °C and the water inside, trapped by a tough shell, finally wins.
A popcorn kernel is a seed. Inside is a dense block of starch with a little water mixed in, wrapped in a hard, smooth hull. The best-popping kernels hold roughly 13 to 14.5 % water. Kernels that are too dry build too little pressure and may not pop at all.
The hull is the key part. It is tough and nearly sealed, so when the kernel heats up, the water has nowhere to go. Water boils at 100 °C in the open, but in a sealed shell it stays liquid while the pressure climbs. This is exactly how a kitchen pressure cooker works.
Drag the temperature slider. The gauge shows the steam pressure that water would reach in a sealed space at that temperature. Watch the kernels near 170 °C and above.
A simplified model: kernel-to-kernel differences are random, tuned so about a third pop at 170 °C and nearly all by 180 °C. The cut-off for dry kernels is illustrative. Pressure is the steam pressure of pure water.
In 2015, physicists Emmanuel Virot and Alexandre Ponomarenko at the École Polytechnique in France heated kernels and counted. At 170 °C only about a third of them popped. At 180 °C, 96 % did. They found the critical temperature is about 180 °C regardless of the size or shape of the kernel.
That number is not a coincidence. At 180 °C, steam in a sealed space reaches about 10 atmospheres, roughly ten times the air pressure around you. Estimates from the same work put the pressure inside a popping kernel at around that figure. That is the point where the hull gives way.
As the kernel heats, the starch absorbs the hot water and softens into a gel. When the hull finally ruptures, the pressure drops almost at once. The superheated water flashes into steam and the molten starch is blown up like a foam. In the open air it cools within a moment and sets, locking the bubbles in place. That solid foam is the white, fluffy flake you eat.
The kernel does not just burst. In the same study, high-speed video showed the hull first pushes out a small starchy “leg” that presses on the surface below and flips the kernel into the air. That is why popcorn jumps.
The loud snap you hear is not the hull cracking. The researchers found the sound comes from the steam rushing out of the kernel. About 20 milligrams of water vapour escapes in a burst, which leaves a brief low-pressure void inside. Air rushes back in and the kernel rings like a tiny bottle.
Every bag has a few unpopped kernels, sometimes called “old maids”. A common cause is a kernel that leaks. If the hull is cracked or has lost its water, it cannot hold enough pressure. Try pulling the water slider down to 10 % and run a new batch. Without enough water to build pressure, the heat does nothing.
So next time you wait for the last pops to fade, you are listening to a pressure cooker the size of a pea, one that only works if it holds water and a sealed shell at the same time.