How popcorn is made in slowmotion.

Popcorn: a matter of thermodynamics

Inside, a kernel of corn is soft because it contains the endosperm, which is made of starch and water, whereas the outer shell of the corn is very hard and, while it’s cooking, behaves like a pressure cooker. The heat, in fact, transforms a part of the water in hot steam, which increases the pressure between the kernel’s walls and dissolves the starch, turning it into a sticky jelly that is mixed with the remaining water. When the pressure inside the grain reaches more or less 9 atm (in a pressure cooker you get less than 2 atm), the outer shell explodes and all the water evaporates instantly, thus cooling the gluey starch which solidifies in the typical white foam.