Look a little closer
The power behind a popping kernel comes from a small amount of water trapped inside it. Popcorn has a hard, relatively moisture-resistant hull that holds in vapor until the rising pressure pushes the shell past its breaking point.
Inside the kernel are starch and water. Heating encourages the water to become vapor, but the tough hull prevents an easy escape. Pressure climbs, while the hot pressurized moisture softens the surrounding starch into a pliable mass.
Experiments found that most kernels begin to pop around 180 degrees Celsius. Near that temperature, internal pressure can exceed the strength of the hull and a tiny fracture rapidly opens. The sudden pressure drop drives the hot starch outward.
Water within the emerging starch expands as vapor, inflating the softened material into a foam. It then cools and sets into the pale, crisp structure we eat. The dramatic increase in size is essentially the kernel's interior unfolding and turning outward.
Ordinary corn does not perform equally well. Popcorn varieties have been selected for a particularly strong hull and a useful level of internal moisture. A cracked or overly dry kernel may leak vapor before enough pressure develops, leaving an unpopped piece behind.
The pop is not simply the sound of a kernel landing. Researchers connect its onset to the release of pressurized vapor and the acoustic response after the hull breaks. One small snack demonstrates a pressure vessel, a starch transformation and rapid expansion in a fraction of a second.
