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Post by Adam Cole, The Salt at NPR Food (2/27/2014)

Listen to the Story on Morning Edition [audio src="http://pd.npr.org/anon.npr-mp3/npr/me/2014/02/20140227_me_popcorn_science.mp3"]

Popcorn is a truly ancient snack. Archaeologists have uncovered popcorn kernels that are 4,000 years old. They were so well-preserved, they could still pop.

Dolores Piperno, a paleobotanist with the Smithsonian's Tropical Research Insitute, says corn, and specifically popcorn, helped lay the foundations for the Aztec empire.

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"When you have a very highly productive crop like corn, that makes the rise of high civilizations possible," she says.

Piperno grows the wild, great-grandaddy of modern corn — a weird grain called teosinte. It has just a few kernels on each stalk, and they're too hard to eat or to grind into flour. But teosinte has a special property that almost makes up for these shortcomings: It can pop.

"All early corns were popcorns," Piperno says. "They were around for millennia before these other forms of corn."

After a couple of thousand years, the Mesoamericans managed to cultivate varieties of corn that were good for flour, but they still ate popcorn. The Aztec language even has a word for the sound of many kernels popping at once — totopoca.

After the Spanish invaded, popcorn spread around the world, and soon people began to figure out how popcorn actually works.

It turns out that rock-hard kernel — the thing that makes teosinte and popcorn impossible to eat raw — is the key.

"It acts as a pressure cooker," says David Jackson, a food scientist at the University of Nebraska. He says the durable kernel keeps water and starch sealed inside. When a kernel is heated, the starch liquefies and the pressure builds until the seed coat breaks.

"The pressure cooker essentially fails, and it explodes outward into a popped kernel," Jackson says.

The liquefied starch froths outward, cooling and solidifying in a fraction of a second. If you look at a popped kernel under the microscope, you can actually see the bubbles that were formed by the expanding steam. That's why popcorn is so light and fluffy — it's made of bubbles.

There's something to think about next time you're stuck watching a bad movie.

Copyright 2014 NPR.

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From Aztecs to Oscars: Popcorn's Beautiful, Explosive Journey