Why popcorn pops

Written by James Schnable

Popped corn Photo: D3 San Francisco, flickr (click to see photo in original context

Popping corn, or anything else, all comes down to pressure. Pop-corn has a particularly impermeable pericarp (the corn kernel’s shell), so as it is heated, the water inside the kernel vaporizes into steam and the starch turns into something close to a liquid. Eventually the heat creates enough pressure to split the pericarp and the starch of the corn kernel bursts out, resolidifying into the distinctive shape of popcorn. If there is even the smallest hole in the pericarp, the steam can escape from the kernel as it’s generated so the pressure never builds up enough to explode the pericarp — the reason some kernels will fail to pop in every batch. The explosive build up of steam is also the reason tea kettles need to be able to release steam while they’re used to boil water. The alternative would be exploding tea kettles which are a lot more dangerous (and a lot less tasty) than exploding corn kernels.

Un-popped popcorn photo: MissTessmacher, flickr (click to see photo in its original context)

It was this reason (along with my discovery of the website on April 1st) that I was so suspicious of the idea of popped sorghum a few days ago. Thanks to Party Cactus and Jeremy, I now know that sorghum does indeed pop like corn (there’s even a variety called “Tarahumara Popping”) and, in fact, thanks to the link Jeremy provided, I’ve discovered that most grains and even some other things (including cowpeas!) can be popped using the proper equipment.

By using a machine that is in some ways similar a pressure cooker, even grains without hard impermeable pericarps can be popped. The popping machine lets pressure rise equally inside and outside of whatever grain is being popped. When the outside pressure is released, the grains/kernels/seeds instantly pop.

It doesn’t sound as visually satisfying at the pop-corn popper I remember from my childhood, where half the fun was watching in anticipation for the first kernels to leap into the air, but a very cool invention never the less.

I’ll certainly be keeping an eye open for popped sorghum to show up out here in the bay area.

A. Normal popcorn kernel. Applying heat makes the pressure build up inside the kernel until the kernel pops open. B. A popcorn seed with a hole in the pericarp. Heat creates pressure, but it escapes through the hole so it never builds up enough to pop open the kernel. C. Even in grains with permiable pericarps, the pressure can build up inside, if the pressure outside is also high. Dropping the outside pressure suddenly still caused the grain to pop.

Written by Guest Expert

James Schnable is an assistant professor and the co-founder of two start ups. His academic lab works on comparative and functional genomics as well as high throughput phenotyping of maize, sorghum, and related orphan grain crops and wild grass species. He’s interested in plants, farming, and saving the world through agriculture, the usual. James blogs at James and the Giant Corn.


  1. Apparently popped sorghum was also discussed/sampled in an old episode of Bizarre Foods. I’m trying to either track down a copy or at least find out whether the flavor was commented on in the show.

  2. Needs a better name. Popped sorgh? Popped ghum? Somebody call the marketing people.
    Actually, you know, I’ll bet you $5 if you call a vendor they’ll send you a sample. Tell ’em you want to discuss it on a blog. Of course, you’ll need to disclose that you aren’t a shill of big sorghum. But they’ll probably send it.
    Test it out on your lab at lab meeting for a larger sample size.

  3. I just remembered that we have a gap in our non-transgenic field that my lab was wanting to fill with something yummy. I suggested sorghum, but they were like, “why?.” I knew it was edible but could find little online besides sorghum syrup. Now I know why! Any thoughts on which variety would be best for popping?

  4. In the book “Lost Crops of the Incas” they discuss popping Quinoa (with the aid of a machine as described above) and “Popping Beans” (P. vulgaris).
    There was a Nova episode “Seeds of Tomorrow” that had Noel Vietmeyer enjoying popped Quinoa (and amaranth?)
    I have the book and used to have a transcript of this now-obscure episode. Is there a way to get to it online?

  5. Oh, and Indian cooking involves casting mustard seeds into hot oil as a start. The resulting excitement can be a pain to clean-up. (That’s why good invented splatter-guards!)
    Tossing cranberries into a simmering delicious cocoanut/yellow curry sauce (to add colour and tartness) can be exciting too, depending on their temperature and degree of hydration.
    I once read of a new orchardist trying to figure-out how to get rid of piles of fruit pits: no good as compost (too sprout-y), gravel (too roll-y or sharp edge-y), nor as fuel (too explode-y).

  6. Is there any biological utility to the plant provided by its cobs popping? Would it help put out a fire, for instance? (It seems like it would help a fire to me) Or is this popping phenomenon of no use to the plant?

  7. I don’t think there would be any advantage to having your seeds pop in a fire, as by the time you have pop-able seeds, the parent plant would be dead or dying, and the survival of the seeds is much more important than protecting the plant. It could be that the particular moisture, starch, and pericarp (shell) properties of popcorn help the seeds adapt to some environmental conditions, but has the side effect of making the kernels pop when you heat them up.

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