Our last CSA share of the year contained a lovely surprise – three ears of popcorn, still on the cob. My kids were excited, because although our CSA also gives us a bountiful share of sweet corn in August, popcorn is much less like a vegetable. After all, most vegetables don’t explode.
Superficially, sweet corn and popcorn kernels look the same. But even if I’d taken the time to dry out some of that pile of sweet corn in my kitchen back in August, we’d only get a bunch of funny-smelling toasted kernels if we tried to pop it. Popcorn can explode because it has two things that sweet corn does not: a lot of hard starch in its center and a thick outer covering.
We can eat sweet corn kernels without breaking or grinding them first because they have a thin hull – it’s thick enough to provide a little snap when you bite through it, but not thick enough to break your teeth. You wouldn’t want to try that with popcorn. At least, my dentist doesn’t recommend it. But that tooth-cracking hull makes a dandy pressure cooker.
Put a bunch of popcorn into hot oil or air, and the starch and water inside the kernels heat up. As the temperature passes 100°C, the water starts to boil and turn to steam, but the hull keeps the steam from expanding and the pressure inside the kernel starts to climb. As superheated steam permeates starches in the kernel they soften and expand, raising the internal pressure even higher. By the time the temperature inside a kernel reachs 177°C, the liquids inside it are seething at pressures nine times higher than atmospheric pressure. If that weren’t bad enough, the heat also starts to melt and weaken the kernel wall. Soon, the kernel can’t hold back the maelstrom – its wall ruptures, releasing a tiny cloud of superheated steam and starch.
The amount of moisture in the kernel is critical: too little, and there’s not enough steam to break open the kernel. Too much, and the kernel wall melts and ruptures before the pressure climbs high enough to puff out the starch. (Those unpopped kernels at the bottom of the bowl? Probably lost too much moisture in storage.)
Once the kernel is open, the pressure of the starch cloud drops, and the rapidly expanding steam carries a film of starch outward. The type of starch inside the kernel determines how far the puff expands: hard starches stretch farther than soft ones. As the cloud expands, it cools until the starch sets into the shape of a miniature explosion – a puff that can soak up butter without collapsing or hold up a layer of hot sugar. Just the thing for a cold winter’s night.
Graph from Gökmen, 2004.
Gökmen, Sabri 2004. Effects of moisture content and popping method on popping characteristics of popcorn. Journal of Food Engineering 65: 357–362
Hoseneya, R. C., K. Zeleznaka and A. Abdelrahmana 1983. Mechanism of popcorn popping. Journal of Cereal Science 1(1): 43-52.
McGee, H. 2004. On Food and Cooking, 2nd edition. Scribner: New York.