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Steam bursting out

Why Does Popcorn Pop?

Water sealed inside the kernel turns to steam, and the pressure finally bursts the shell.

1 A kernel withwater sealedinside2 Heat turns itto steam3 Pressurebursts theshell
A tiny steam explosion, every time.

  1. Step 1 of 5

    Each kernel has water sealed inside.

    A hard, unusuallywaterproof shellAbout 14% water,sealed insideDense starch packedaround it
    Three ordinary things. Only together do they make an explosion.

    Popcorn holds around 14 percent water inside a hard, unusually waterproof shell. That combination is what makes it pop.

  2. Step 2 of 5

    Heating turns that water into steam.

    As water Tiny volumeAs steam Over 1,000× the space

    Past 100°C the water wants to become vapour, which takes up over a thousand times more space. The shell will not let it.

  3. Step 3 of 5

    Pressure builds enormously.

    Inside the kernelabout 9×atmosphericpressureNormal air1 atmosphere

    Sealed in, the steam reaches around nine times atmospheric pressure and the inside heats to about 180°C before anything gives way.

  4. Step 4 of 5

    The shell fails all at once.

    1 The shellsplits2 Starch expandsinstantly3 It sets aswhite foam

    When it splits, pressure drops instantly. The hot starch inside expands violently and turns inside out into white foam.

  5. Step 5 of 5

    Other corn does not do this.

    Sweetcorn Porous shell — steamescapesPopcorn Sealed shell —pressure builds

    Sweetcorn's shell is porous, so steam leaks out gently and nothing builds up. Only popcorn's shell is sealed tightly enough.

    Kernels that fail to pop usually have a crack in the shell that let the moisture escape beforehand.

The short version

Popcorn pops because water sealed in a waterproof shell becomes high-pressure steam and bursts it open.

Try it yourself

Look at an unpopped kernel next to a popped one. The popped one is turned inside out — the shell is on the outside.

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