Food · 30 second break
Every unpopped kernel in the bag looks like a small, hard, dry seed. Yet a few minutes over heat turns it into a fluffy flake many times its size, with a bang and a jump. The trigger is not magic and not fire. It is a bit of water that cannot boil.
A popcorn kernel is a seed with a tough, glossy outer wall (the hull) around a starchy centre. Good popping kernels hold roughly 13 to 14.5 percent water, packed inside the starch. Put the kernel in a hot pan and that water warms up like any other water. At 100 °C it would normally boil and drift away as steam.
Here it can't. The hull is sealed tight, so as the water tries to turn to steam, the steam has nowhere to go and its pressure climbs. This is exactly how a pressure cooker lets food get hotter than 100 °C. Meanwhile the starch inside is softening into a hot, gooey paste.
Below is a pan of 48 kernels. Drag along the chart (or use the slider) to turn up the heat. The orange curve is how hard steam pushes at each temperature. Each kernel has a hull that holds up to a certain pressure, shown as the warm band. Kernels glow red as they near their limit, and pop when the steam wins.
Steam pressure rises steeply with temperature. At 100 °C it equals the air around us, 1 atmosphere. At 140 °C it is about 3.5. At 180 °C it reaches roughly 10. Researchers at the École Polytechnique in France, Emmanuel Virot and Alexandre Ponomarenko, filmed popping kernels with high-speed cameras in 2015. They estimated the hull gives way at around 9 to 10 atmospheres, and found the critical temperature is about 180 °C.
The change is sharp. In their tests only about a third of kernels popped at 170 °C, while at 180 °C almost all did. Kernels vary slightly in hull thickness and water content, so each has its own tipping point, but they all sit close together. That is why popping in a pan starts with a few scattered pops, then builds to a rattle, then fades.
When the hull finally ruptures, the hot, pressurised water vapour rushes out, and the sudden release is the sound you hear. The softened starch is released too, foaming up as the pressure around it drops, and sets into the light white flake with its odd branches.
The jump has a neat trick behind it. The 2015 team saw that a popping kernel often pushes a thin starchy "leg" out against the surface, and that leg acts like a spring, flipping the kernel into the air. Jumps ranged from a few millimetres to several centimetres, and the kernels spin as they go, a bit like a gymnast tucking in a somersault.
Every bag ends with a few stubborn kernels. They are usually kernels that never reached the critical pressure: too dry to build enough steam, or with a hull that lets the steam leak out before it can burst. That is also why fresh, properly stored popcorn pops better. Dry kernels simply have less water to work with.
So next time you stand listening to the rattle, you are hearing thousands of little hulls reaching the same limit, one at a time: the point where trapped steam finally outweighs a seed's last bit of stubbornness.
Sources: Virot and Ponomarenko, "The nature of the popping of popcorn", Journal of the Royal Society Interface, 2015. Steam pressure values from standard water saturation data.