How Does Popcorn Pop? The Science of Popcorn

How Popcorn Pops
Popcorn pops because heat turns water in the corn seed into steam, bursting the kernel and releasing its contents as a foam.

People have snacked on popcorn since at least 3600 BC in Mexico. But, not all corn works as popcorn. Here’s a look at the science of how popcorn pops and how it’s different from other corn.

How Popcorn Pops

The quick explanation of how popcorn pops is that heat vaporizes water inside the kernel, which builds up pressure until it pops the skin of the corn kernel.

However, there’s more to soft, fluffy popcorn than just breaking a corn seed. The kernel contains protein, starch, oil, and water, with a hard hull called a pericarp. Quickly heating the kernel to a temperature of 180 °C (356 °F) vaporizes the water into steam and produces enough pressure (135 psi or 930 kPa) to rupture the hull. At the same time, the heat softens the starchy part of the seed, basically turning it into gelatin or natural plastic. When the corn kernel pops open, the protein and starch gel expands into a foam, which cools into a soft puff that is between 20 and 50 times bigger than the original seed.

Popcorn needs a water content between 14 and 15%. Too much moisture leads to chewy popcorn (at best) or moldy popcorn (the worst-case scenario). Too little moisture won’t produce the pressure needed for the corn to pop. Sometimes over-dried popcorn can be rehydrated to get it to pop.

Rapid heating is necessary. It’s easy to pop popcorn in hot oil, an air-popper, or the microwave. But, if you put a layer of popcorn on a baking sheet and slowly heat it, you’ll get roasted or burnt corn and not popped corn!

What Types of Corn Pop?

Good popped corn requires the right kind of the corn and the right amount of drying. Most corn used as popcorn is the cultivated strain Zea mays everta. Some heritage strains of corn and maize also pop. Popcorn you buy at the store usually is yellow or white pearl-type corn, but both pearl and rice-shaped popcorn is available, in white, yellow, purple, red, and multi-colors.

Freshly harvested popcorn pops, but the result is chewy, dense popcorn. To get fluffy kernels, the corn has to be dried until its moisture content is between 14% and 15%.

Popcorn vs Corn Nuts

If you dry field corn or sweet corn so that it has the right moisture content, at best only a few kernels will pop. The kernel that do pop won’t be as fluffy as regular popcorn, plus they’ll taste different. However, soaking dried kernels in water for three days and then frying them in oil produces a snack akin to Corn Nuts.

Other Grains That Pop

Corn isn’t the only grain that pops when heated. Sorghum, quinoa, millet, and amaranth grain all puff up when heated as the pressure from expanding steam breaks open the seed coat.


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  • Lusas, Edmund W.; Rooney, Lloyd W. (2001). Snack Foods Processing. CRC Press. ISBN 978-1-56676-932-7.
  • Smith, Andrew F. (1999). Popped Culture: The Social History of Popcorn in America. University of South Carolina Press. ISBN 978-1-57003-300-1.