Not all corn can be popcorn. There are six types of corn commonly grown, and while some wild types will pop, only the popcorn subspecies, Zea mays averta, pops on command. This popping corn is a special type of maize that puffs up when it is heated in oil or dry heat. While the strain was first developed by pre-Columbian Native Americans, special varieties have been developed by agriscientists that improve popping yield.

As with all cereal grains, each kernel of popcorn contains a certain amount of moisture in its starchy endosperm. Unlike most other grains, the outer hull of the popcorn kernel is thick and impervious to moisture, enabling pressure to build up inside.

• Moisture Becomes Steam. As the kernel is heated past the boiling point, water in the kernel begins to turn to steam, generating an internal pressure of about 9 atm. In kernels of other grains, this steam escapes through the hull as fast as it forms, but in the tightly sealed popcorn kernel, the steam is held in by the hull. The pressure starts to build until the hull suddenly ruptures, causing a small explosion. The starch in the endosperm (the core) bursts to about 40 times its original size, turning the kernel inside out. The average popping temperature for popcorn is around 347°F.

Endosperm Turns To Foam. Because the moisture is evenly distributed throughout the starchy endosperm, the sudden expansion turns the endosperm into an airy foam which gives popcorn its unique texture. “Old Maids” Don’t Pop. Kernels which do not pop, known as “old maids,” are believed to have not enough moisture to create enough steam for an explosion. Or, they might have have a leaky hull.

The heat-generated pressure turns the kernel inside out, Photo by Leonardo Morales | SXC.

The ideal popcorn kernel contains approximately 14% moisture. Without enough water, the kernel will not pop at all, which is why old kernels don’t pop. That’s why popcorn kernels should be kept in tightly sealed containers to prevent them from drying out.