The Pop in Popcorn Isn't the Kernel Breaking

Feed the Reader

5 hours ago

4 min read

Put a handful of kernels in a hot pan, put the lid on and wait. For a while, nothing. Then one goes. Pop. Then another, and another, and soon the pan is rattling with little explosions and the lid is jumping.

It seems obvious what's happening. The kernel bursts open, and the bursting makes the noise. The force of the burst sends it flying.

Both of those turn out to be wrong. Or at least, not quite right.

A pressure cooker the size of a pea

First, the bit that is as simple as it seems.

Inside every kernel is a tiny drop of water, held in a ring of soft starch, and the whole lot is wrapped in a tough outer shell called the hull. For popping to work, the kernel needs to hold about 14 per cent moisture.

Heat the kernel and that water turns to steam. But steam takes up much more room than water, and the hull won't let it out. So the pressure inside keeps climbing. Meanwhile, the hot, wet starch softens into a kind of molten paste.

In other words, the kernel is a pressure cooker the size of a pea.

Eventually, the hull can't hold it any longer. It gives way at a pressure of about 135 pounds per square inch, or roughly 930 kilopascals, and a temperature of around 180 degrees Celsius. In one study, only about a third of the kernels had popped at 170 degrees, but by 180 degrees almost all of them had.

When the hull bursts, the sudden release of steam puffs the soft starch and proteins up into an airy foam, which sets into the crisp, fluffy shape you know as it cools. A kernel can end up between twenty and fifty times its original size.

Why popcorn pops

That also explains why you can't pop just any grain. Popcorn's hull is unusual among grains: it's tough, and it keeps the moisture locked in. That's the whole trick. Without a hull that can hold the pressure, the steam just leaks away and nothing dramatic happens.

It's also why old popcorn can be disappointing. Popcorn tends to dry out as it ages, and the kernels that don't pop at the bottom of the bowl, sometimes called "old maids", just don't have enough water left inside to make enough steam.

The silent crack

Now for the surprise.

In 2015, a pair of French researchers filmed popping kernels with a high-speed camera and recorded the sound at the same time, so they could line up exactly what was happening with exactly what could be heard.

What they found was that the pop doesn't happen when the hull breaks. The pop typically comes about a hundred milliseconds after the kernel cracks open. A tenth of a second later. The breaking itself is, to our ears, silent.

So where does the noise come from? The researchers' explanation is that it comes from the water vapour. When a kernel pops, it releases around twenty milligrams of water vapour that has been locked up inside. As that vapour escapes, it leaves behind a sudden drop in pressure, and the resonance from that pressure drop is the pop you hear. It's the same kind of physics as the pop of a champagne cork.

It's worth being clear that this is the researchers' best explanation for the sound, rather than something proven beyond all doubt. As one account put it, they suspect the pockets of heated water vapour cause the noise.

The leg that kicks

The other surprise is how a kernel jumps.

You might picture it as a tiny rocket, with steam blasting out of the crack and shoving it off the pan. That's not what the high-speed footage showed.

Instead, as the kernel bursts, a piece of starchy foam pushes out of the hull and forms a sort of leg. That leg presses against the bottom of the pan, gets squashed like a spring and then springs back, launching the kernel up into the air. It doesn't just jump, either. It flips, turning a somersault on the way up.

So popcorn doesn't fly like a rocket. It flies because it kicks.

Putting it together

Not every kernel pops at exactly the same moment, of course, which is why the pan goes from silence, to a few lonely pops, to a full drum roll and back down again. But the basic picture holds up. Water becomes steam, pressure builds, the hull gives way at a fairly well-defined temperature, and the starch puffs into foam. Then a little leg kicks the kernel off the pan, and the sound you hear comes a fraction of a second after the crack.

What you hear isn't what you think

What's lovely about this is how familiar the experience is, and how wrong our instincts were about it. Most of us have heard popcorn popping hundreds of times. It seems obvious that the pop is the sound of something breaking.

But it isn't. The breaking is silent. The jump isn't a jet of steam. The thing you hear isn't quite the thing you think is happening.

Which is a nice reminder that there's real physics hiding in even the most ordinary places. Including the bottom of the snack bowl.