Why does popcorn pop?

1. Popcorn is a special kind of corn. Its kernels are dried on the cob until just the right amount of water is left inside, about one seventh of its weight.
2. Inside the hard, shiny shell is starch, the food store for the seed, and a tiny bit of water. There's also the germ, the part that could grow into a new plant.
3. In a hot pot the kernels heat up. Above 100 degrees Celsius, the water inside starts to turn into steam, but the strong shell won't let it out.
4. Trapped steam pushes harder and harder on the inside of the shell, like air in a balloon. The hot starch turns soft and gooey.
5. POP! The shell bursts. The steam rushes out, and the soft starch puffs up into a white, fluffy foam many times bigger than the kernel. It cools and sets crunchy.
6. A few kernels never pop. They were too dry, or their shell had a tiny crack, so the steam leaked out instead of building up. Cooks call them old maids.

Tap each part of the kernel, then tap the popped flake to see what it became.

• Hull (shell). The hull is a hard, shiny shell. It is so tight that steam can't easily get out. That's why pressure builds up inside.
• Hard starch. Most of the kernel is starch packed hard and tight, like glass. It is food for the seed. This is what puffs up when the kernel pops.
• Soft starch. In the middle, the starch is softer and packed more loosely, with tiny gaps.
• Water droplets. Only about one seventh of the kernel is water, held in the starch. Heat turns it into steam, and steam takes up far more space than water.
• Germ. The germ is the living part of the seed. If you planted the kernel, it would grow into a new corn plant.
• Tip cap. The tip cap is where the kernel was joined to the cob. It's often the brown bit you find stuck in your teeth!
• Popped flake. When the kernel pops, the soft starch puffs into this white foam, full of tiny holes where steam bubbles were. It's about 40 times bigger than the kernel.

Change the heat under the pot. Each change heats a fresh handful of 40 kernels. Count how many pop, and watch one kernel through the magnifier.

Experiment: Does heating the pot slowly or quickly make more kernels pop?

1. Set the heat to low and watch the count of popped kernels.
2. Turn the heat up high and count again.
3. Watch the magnifier: where does the steam go when the heat is low?

What we found: With low heat, kernels warm up slowly and some steam leaks out through the shell before the pressure is high enough, so many don't pop. With higher heat they reach popping temperature quickly with their water still inside, so almost all pop.

Turn on JavaScript to try it yourself.

Choose your kernels and the heat, then take the pot off the heat at the right moment. Fill the bowl to the line without burning any.

Every popcorn kernel has three important things: a hard, tight shell called the hull, lots of starch inside, and a small amount of water, about one seventh of its weight. All three are needed for a pop.

When the kernel gets hot, the water inside turns into steam. Steam takes up much more space than water, so it pushes hard on the shell from inside. The shell is strong and tight, so the pressure keeps building, just like in a pressure cooker.

At about 180 degrees Celsius, the shell can't hold any longer and it bursts. The steam rushes out, and the hot, soft starch puffs up into a foam that is many times bigger than the kernel. It cools in a moment into a crunchy flake.

If the shell has a crack or the kernel has dried out, the steam leaks away and the kernel never pops. Heating too slowly gives the steam more time to leak too. That's why popcorn is cooked on fairly high heat and kept in a sealed jar.

Key words

Hull (pericarp)

The hard outer layer of the kernel that holds the steam in.

Steam

Water that has turned into a gas because it is hot.

Pressure

How hard something pushes on a surface.

Starch

The food store inside the kernel, made of long chains of sugar.

Foam

A material full of gas bubbles. A popped flake is a solid starch foam.

Old maid

A kernel that didn't pop.

Check your thinking

1. What builds up inside a kernel and makes it pop?

   Show the answer

   Steam pressure. Water turns into steam, which pushes on the shell until it bursts.

2. Why do some kernels never pop?

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   They were too dry or their shell was cracked. Without enough water, or with a leaky shell, steam can't build up enough pressure.

3. Why does popcorn pop better on high heat than low heat?

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   Less steam leaks out before it's hot enough to pop. Fast heating gets kernels to popping temperature before much steam escapes.

4. What is a popped flake made of?

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   Starch puffed into a foam. Steam blows the soft starch into a foam full of tiny holes.

5. About what pressure builds up inside a kernel before it pops?

   Show the answer

   About 9 atmospheres. Water's vapor pressure near 180 °C is close to 10 atmospheres, and kernels typically burst around 9.