Pop Goes the Kernel

 

Above is an animated gif of a popcorn kernel turning into the movie-time snack most of us enjoy. (If not, click here — sometimes animated gifs and email don’t mix well.) There’s evidence of people eating popcorn going back more than five or six thousand years, and its interesting texture and taste has allowed it to withstand the test of time.

But what makes popcorn pop?

There are a handful of different types of corn we eat: sweet corn, Indian (flint) corn, and field (dent) corn are three of the common types. Popcorn — Zea mays everta, if you want the scientific name of the relevant subspecies — is a sub-type of flint corn, specifically cultivated for popping. (That’s why, if you take a cob of corn — which is typically sweet corn — and throw it in the microwave or in a pan, it doesn’t turn into a soft but crunchy snack option.) It “works” because of the way the interplay between its kernel’s outer shell and its inner moisture.

Let’s start with that shell, called a hull. If you’ve ever had the misfortune of accidentally biting into an unpopped kernel, you know that these things are ridiculously hard. They’re good at keeping your teeth out — and, similarly, at keeping all the stuff inside in. That includes any moisture trapped inside — and there’s a good amount of moisture in there. Popcorn kernels are harvested when about 13.5% of its weight is provided by the internal moisture.

When we apply heat to the popcorn, that moisture goes to work. The heat turns it to steam which the hull keeps sealed inside. The steam continues to heat the starches inside the hull and, slowly but surely, turn it into a goo of sorts. That goo, a few minutes later, becomes the white popcorn we eat.

That happens because something else is going on while the steam liquefies the starches. The steam continues to build up, creating more and more pressure against the walls. If there’s enough moisture to begin with — that’s why the 13.5% number, above, is important — and if the hull doesn’t have any holes or cracks in it, the pressure will grow to about 135 pounds per square inch, per the Popcorn Board. At that point, the kernel reaches its breaking point and pops open. The hot, starchy goo solidifies nearly instantaneously after being launched into the air, as seen in the image above. And then it lands, nearly ready to eat. (Caution: contents may be hot.)

Bonus Fact: Popcorn gets its name from the popping sound it makes when the kernel opens up — but that’s not the sound of the hull splitting open. It’s due to the water vapor being suddenly released, similar to the popping sound a bottle of champagne makes when the cork flies out.

Double bonus!: Also per the Popcorn Board, popcorn can fly as high as three feet in the air when it pops. So you may not want to stand above it if cooking it in a pan.

From the Archives: Orville Redenbacher’s Worst Nightmare: The downside of (a lot!) of popcorn.

Related: A book of kitchen science experiments. It’s ostensibly for kids but, really, who doesn’t like kitchen science?