Why Is Popcorn the Only Kind of Corn That Pops?
While popcorn is growing in the field, it looks much like any other kind of corn. But kernels of popcorn are extra hard and contain a lot of starch. The hard kernels hold moisture inside. When popcorn is heated, steam forms inside. The pressure of the steam bursts the kernels with a popping sound. Then the starchy inside puffs out.
Other kinds of corn contain soft starch. The kernels dry out when heated, and do not pop. Popcorn differs from other types of corn in that its hull has just the right thickness to allow it to burst open. Popcorn kernels expand many times their size when popped. Two tablespoons of kernels make a quart of popped corn.
Popcorn is the only kind of corn, one of the several types of corn that pops. Not all corn can pop. The other three familiar types—sweet, the kind we eat on the cob; dent, the kind used for animal feed and in processed foods; and flint, the decorative kind you see at Halloween and Thanksgiving—don’t have thick enough or strong enough hulls to contain the pressures necessary to liquefy the starch before bursting.
There are a few other grains that can pop, however. Amaranth is a poppy-seed-size grain that you can find at health food stores and that made up an important part of the ancient Aztec diet. It pops just like popcorn and is mixed into sweet syrup in a Mexican candy called alegría.
It’s also possible to pop certain kinds of rice: essentially a homemade version of Rice Krispies cereal (although real Rice Krispies are puffed instead of popped, a similar process that involves heating rice or another grain under high pressure in what is essentially a gun barrel, then shooting it out to rapidly release the pressure).