The Physics of Popcorn

Popcorn is one of the few varieties of corn whose kernels can transform, almost instantly, from a hard, unremarkable seed into a light, fluffy puff. The secret lies in the kernel's structure. Each kernel has a small amount of water, roughly fourteen percent of its weight, sealed inside a dense, starchy interior and enclosed by an unusually hard and uniform outer hull called the pericarp. When a kernel is heated, the trapped water turns to steam, and because the hull is nearly airtight, pressure builds up inside rather than escaping. The pressure can climb to roughly nine times normal atmospheric pressure before the hull finally gives way. At that moment, the starch inside, which has been softening under the heat, gelatinizes almost instantly and expands violently as the pressure releases, turning the kernel inside out into the familiar white foam. Not every variety of corn can do this. Most corn kernels have hulls that are either too thin, too irregular, or too permeable to trap enough pressure before steam simply leaks out. Popcorn's hull is distinctive precisely because it is strong and evenly structured enough to hold that pressure until the explosive threshold is reached, which is why popcorn, alone among corn varieties, pops.