Every popcorn kernel is a tiny pressure cooker that explodes at 180 degrees
Inside a popcorn kernel, a waterproof hull traps a little moisture. Heat it and the water turns to steam until the shell bursts at roughly 135 psi, blowing the starch into foam up to 50 times its original size. People in the Americas have been enjoying that trick for thousands of years.
The popping variety, Zea mays everta, is a kind of flint corn and one of six major corn types. What makes it special is a hull that is unusually strong and impervious to water, wrapped around starch that is almost all the hard kind, with 14 to 20 percent moisture. As heat builds, the starch gelatinises and pressure climbs to about 930 kPa at 180 °C. When the hull fails, the sudden pressure drop expands the steam, puffing the starch and protein into an airy structure that sets crisp as it cools.
Heating rate matters. Too fast, and the outer layers rupture before the centre softens, leaving hard cores. Too slow, and steam seeps out through the tip where the kernel joined the cob, so the pressure never peaks and the kernel stays unpopped. Vendors care about expansion because cinemas buy popcorn by weight and sell it by volume.
Corn was domesticated roughly 9,000 years ago in Mexico's Balsas River Valley. Cobs from the Paredones and Huaca Prieta sites on Peru's north coast point to popping corn as early as 4,700 BCE, and ears found in New Mexico's Bat Cave in 1948 and 1950 date back to about 3600 BCE.
Nineteenth-century Americans popped kernels by hand over flames. The Chicago candy maker Charles Cretors changed that with a patented steam-driven machine that popped corn in a mix of clarified butter, lard and salt, unveiled at the 1893 Columbian Exposition and later mounted on street carts. At 5 to 10 cents a bag, popcorn thrived through the Depression, and wartime sugar rationing tripled American consumption. Cinema owners first resisted it as a distraction before installing machines in their lobbies. General Mills won the first patent for a microwave popcorn bag in 1981.
Source: Popcorn
