How does popcorn pop?

This question came from a reader submission (thanks Chendur!). Has a curious question stumped you lately? Feel free to submit your own question here:

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📓 The short answer

Popcorn kernels contain a small amount of water that turns into steam as it heats up. The steam gelatinizes the starch inside the kernel, and when the pressure causes the hull to burst, the starch rapidly cools and solidifies.

📚 The long answer

Few foods cook as ✨ dramatically ✨ as popcorn.

The flair! The pizzazz! It's a sight to behold. Here's a look at popcorn popping in ultra slow motion:

GIF of a popcorn popping in slow motion — Source: Warped Perception

How does popcorn pop?

The secret behind popcorn's popping is the design of its kernel. Popcorn is a whole grain made up of three main components: the germ (seed embryo), the endosperm (a combination of starch and water), and the pericarp (hull).

A simple diagram showing the parts of a popcorn kernel.

The hull and water content of popcorn make the kernel act like a pressure cooker when heated. Here's how popcorn pops:

Once the kernel reaches 212°F (100°C), the water inside turns to steam.
As the kernel heats further, the steam gelatinizes the hard starch into a superheated, soft substance.
When the pressure inside the kernel reaches 135 psi, the hull ruptures, causing the steam to rapidly expand and inflate the soft starch. The starch spills out of the kernel and cools rapidly.

Once all is said and done, the popped kernel is 8x less dense than before.

Why don't all popcorn kernels pop?

If you've ever popped a bag of popcorn, you know that there's always some unpopped kernels that didn't quite get the memo. What causes some popcorn kernels to pop and others not?

Reason #1: There is a crack in the hull.

If the hull has even a tiny crack, it can't build enough pressure inside the kernel. These kernels release steam before the starch can gelatinize, preventing the signature pop.

Reason #2: There is not enough water in the kernel.

Popcorn needs about 14% of its weight to be water to create enough steam to build pressure and gelatinize the starch. If there is not enough water, there is not enough steam, and the kernel will remain unpopped.

Reason #3: The heat is too slow, too low, or uneven.

If popcorn is heated too slowly or at too low a temperature, it might not create enough pressure to pop. Uneven heating can also cause one spot to rupture before the entire kernel is hot enough to expand fully.

🍿 Other interesting popcorn questions

Is all corn popcorn?

No, only popcorn pops. Popcorn's hull is right at the sweet spot of being thick enough to build pressure but thin enough to burst. Other whole grains, like millet, quinoa, and amaranth pop, but not to the same dramatic degree as popcorn.

Why is kettle corn a different shape than regular popcorn?

Popped mushroom and butterfly popcorn — "Mushroom and butterfly popcorn" by Bunchofgrapes is licensed under CC BY-SA 3.0 DEED.

While both kettle corn and regular popcorn are made from popcorn, they use different varieties of kernels.

Kettle corn is made from "mushroom popcorn," which produces a rounder, sturdier shape better at holding seasoning. Regular popcorn uses "butterfly popcorn" kernels, resembling the wings of a butterfly.

Why is popcorn associated with movies?

Today, popcorn is a popular movie theater treat, not only in the United States but also in many countries across the world.

But for a long time, movie theaters in the U.S. wanted nothing to do with popcorn. During the era of silent films, theaters aimed to appeal to wealthy customers and mimic the experience of going to a live theater. Popcorn might mess up the carpets or create a distracting sound in the theater.

However, two major events in the late 1920s expanded the moviegoing audience. First, the introduction of sound in movies in 1927 removed literacy as a barrier to enjoying motion pictures. Second, the Great Depression created a huge demand for cheap distractions from the hard times. Movies were now an inexpensive and accessible outing; by 1930, attendance to movie theaters had reached 90 million per week in the U.S.

So why popcorn? It became the perfect movie snack for several reasons. For one, it was cheap (5-10 cents per bag). It also could be made in mobile carts by street vendors who positioned themselves right outside theaters (eventually theaters couldn't ignore the financial appeal and started making their own concessions inside). Lastly, popcorn was unaffected by sugar shortages during World War II, unlike soda and candy bars.

By 1945, over half of the popcorn consumed in the U.S. was eaten at movie theaters, and it remains the top movie treat today.

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Caitlin

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Sources for this week's newsletter

P.S. Chendur (the popcorn enthusiast who submitted today's question) has a fantastic newsletter of his own where he breaks down how different everyday gadgets work. Definitely check it out!

How Do Gadgets Work?

Chendur Singaram

Understand the inner science, tech, and AI of your gadgets with me, a Carnegie Mellon alumnus. Join 335+ readers every other Wednesday to see the world differently in just 3 minutes—regardless of your background! 🚀

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📖 Book of the week

Book cover of "The Secret Life of Groceries"

The Secret Life of Groceries: The Dark Miracle of the American Supermarket by Benjamin Lorr

This book is an absolute treasure trove of interesting details around the history and players of the grocery business. I love a book that shows you the intricate behind-the-scenes of things we interact with on a daily basis. And the writer has such a great voice to make it all that much more interesting. It's a fascinating look at how products make it on to shelves, how Trader Joe's successfully differentiated itself from other stores, and how integral truckers are to basically everything dealing with physical goods.

Check out the full list of books I've recommended here.