Why does popcorn pop?
Quick answer
Each popcorn kernel holds a tiny drop of water (about 14% of its weight) inside a hard, airtight hull. When heated, the water turns to steam and the pressure builds. At around 180 °C (356 °F), the hull bursts, and the soft starch inside puffs into foam and sets, turning the kernel inside out.
Inside a kernel
| Part | Role |
|---|---|
| Hull (pericarp) | Hard, waterproof shell that traps steam |
| Endosperm | Starch that becomes the fluffy white part |
| Germ | The seed’s embryo |
| Water | Turns to steam and provides the “push” |
As the kernel heats, the starch softens into a hot gel. When the hull finally gives way, pressure drops suddenly. The gel expands up to about 40 times its volume, then cools into the crunchy foam we eat.
What makes the “pop” sound?
A 2015 study by French engineers filmed kernels with high-speed cameras. They found the sound isn’t the hull cracking or the kernel hitting the pan. It comes from steam escaping suddenly, like a cork leaving a bottle. The kernel can even jump a few centimeters in the air as a “leg” of starch pushes against the pan.
Why some kernels don’t pop
Unpopped kernels, nicknamed “old maids,” usually have:
- Too little water, often because they dried out in storage
- A cracked or weak hull that lets steam leak out slowly
Keeping popcorn in an airtight container helps more kernels pop.
Only certain corn pops
Not all corn pops. Popcorn (Zea mays everta) has an especially strong, airtight hull. Sweet corn or field corn usually just scorches.
An ancient snack
Archaeologists have found popped corn in Peru dating back about 6,700 years. In the US, popcorn became a cinema favorite during the Great Depression, because it was cheap.
Butterfly vs mushroom
Popcorn comes in two main shapes: butterfly (irregular “wings,” the usual cinema type) and mushroom (round, sturdy, used for caramel coatings).
For other kitchen chemistry, see why toast turns brown.
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