Why does popcorn pop?
Each kernel is a tiny pressure cooker that holds out until about 180 °C, then explodes into a foam up to 50 times its size.
▶ Start the storyPopcorn pops because each kernel is a sealed pressure cooker. Inside its strong hull sits hard starch holding 14–20% moisture. As the kernel heats, that water turns to steam, but unlike most other grains, popcorn's hull is impervious to moisture, so the steam can't escape. The pressure keeps rising until the hull finally gives way at around 930 kPa of pressure and 180 °C. The pressure drops instantly, the steam expands, and it blows the softened starch and proteins into an airy foam that sets into a crispy puff as it cools. The kernel ends up 20 to 50 times bigger than it started.
Step 1: Sealed kernel
Hard starch with 14–20% moisture inside a moisture-proof hull.
Step 2: Steam builds
Water turns to steam; the starch softens.
Step 3: Hull bursts
At about 930 kPa and 180 °C.
Step 4: Foam sets
Steam puffs the starch into a foam that crisps as it cools.
That also explains the duds at the bottom of the bowl. The industry calls them 'old maids': kernels that simply didn't have enough moisture to make enough steam. Heat matters too. Too slow, and steam leaks out through the tip where the kernel was attached to the cob; too fast, and the hull bursts before the center has softened.
Humans have enjoyed this little explosion for a very long time: evidence of popping corn from about 4,700 BCE has been found on the coast of Peru.
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Recap
Water, a sealed hull and heat: steam builds, the hull bursts, the starch foams and sets.
Surprising fact · The hull gives way at about 930 kPa and 180 °C, and the kernel ends up 20 to 50 times its original size.
Sources (1)
No source, no claim. Every fact in this lesson (16 claims) cites at least one of these.