What if popcorn did not pop?

Popcorn seeds are small and hard. A grown-up heats them.

Pop, pop, pop! They puff up into fluffy white popcorn. What if they didn't pop?

Pour some popcorn kernels into your hand and they're small, hard and shiny, a bit like tiny stones. Heat them up, in a microwave or a popcorn machine run by a grown-up, and after a while they start to jump and pop, turning into big, fluffy, white popcorn.

So what if popcorn didn't pop? What would have to be different about the kernels? In this lab we follow three clues: the water hidden inside each kernel, the steam pressure that bursts it, and other grains that can pop too.

Popping corn is kitchen physics: a sealed container, a fluid heated past its boiling point, rising pressure and a sudden rupture. Researchers at École Polytechnique and other French institutions filmed it with high-speed cameras and showed that the whole pop lasts only about 100 milliseconds.

This thought experiment imagines popcorn that does not pop. To reason about it, we need to know what is inside a kernel, what conditions the pop requires, and why most other kinds of corn do not pop at all.

Each popcorn kernel has a hard shell on the outside.

Inside is soft starch with a tiny drop of water hidden in it.

A popcorn kernel has three parts: the germ; the endosperm, which is mostly starch; and the hard outer shell, called the hull or pericarp. The Popcorn Board explains that each kernel has a small drop of water stored inside a circle of soft starch.

The amount of water matters. Popcorn needs to be about 13.5 to 14 percent water to pop. And the hull matters too: it has to be just the right thickness to hold in the steam until it finally bursts.

A popcorn kernel is a whole grain made of the germ, the starchy endosperm and the pericarp, or hull. According to the Popcorn Board, each kernel stores a small amount of water within a circle of soft starch, surrounded by the hard outer hull, and popcorn needs a moisture content of about 13.5 to 14 percent to pop.

Of the four most common types of corn, sweet, dent, flint and popcorn, only popcorn pops. The Popcorn Board attributes this to its hull, which has just the right thickness to let pressure build until it eventually bursts open.

Imagine popcorn that never pops. You heat it, but it stays small and hard.

No fluffy popcorn at the movies! You'd have hot, crunchy seeds instead.

Now imagine popcorn that didn't pop. What could cause that? From what we know, a few things might: the hull might let the steam leak out instead of holding it in, the kernel might be too dry to make enough steam, or it might never get hot enough.

Without the pop, the starch inside would never puff up. Instead of light, crunchy popcorn that has swollen to 40 to 50 times its size, you'd have small, hard, hot kernels, which are no fun to eat. These are ideas, worked out from how popping works.

The pop depends on three conditions: enough water to make steam (about 13.5 to 14 percent moisture), a hull strong enough to contain the rising pressure, and heating to the critical temperature, about 180 °C in the high-speed study. Remove any one and the kernel might not pop: a leaky or cracked hull might vent steam gradually, too little water might never reach the bursting pressure, and too little heat would never get there.

Without popping, the starch would never expand into a foam, so the kernel would stay dense and hard. Which failure would dominate in practice is a reasoned guess, not a tested result.

When the kernel gets hot, the water inside turns into steam.

Steam needs lots of room. It pushes and pushes, until, pop! The shell bursts.

As a kernel heats up, the water inside turns to steam at around 100 °C. Steam takes up much more room than water, but the hull holds it in, so the pressure inside rises. The Popcorn Board says the kernel keeps heating to about 175 °C, until the pressure reaches about 135 pounds per square inch and bursts the hull.

Then the hot, soft starch puffs out, cools almost at once and sets into the fluffy shape we eat. The first bit of starch to burst out acts like a little leg, flipping the kernel into the air. That's why popcorn jumps!

The Popcorn Board describes the sequence: around 212 °F (100 °C) the water turns to steam and the starch becomes a superheated gelatinous mass; the kernel continues to about 347 °F (175 °C), when internal pressure reaches about 135 psi (about 9.3 bar) and bursts the hull. In high-speed films reported in Physics Today, Emmanuel Virot and Alexandre Ponomarenko found the hull ruptures at about 180 °C, when internal water vapor reaches a critical pressure.

The starch then expands into spongy flakes that act like a gymnast's legs, launching the kernel up to about a centimeter into the air. Surprisingly, the 'pop' sound comes after the hull ruptures; the researchers suggest pressure drops excite sound inside the kernel.

Most kinds of corn don't pop. Only popcorn does!

But a grain called sorghum can pop too. Its popped bits are tiny and white.

Only popcorn pops among the four most common kinds of corn: sweet corn, dent corn and flint corn don't. But corn isn't the only grain that can pop. Some kinds of sorghum, a grain grown around the world, pop into small, white, round puffs when heated.

Popped sorghum isn't new: it has been eaten in India and Africa for a long time. In 2021, researchers at Texas A&M University released a new sorghum that was bred especially for popping well. Its popped pieces are smaller than popcorn and have no hulls.

The American Society of Agronomy reported in October 2021 that Texas A&M University researchers had released a sorghum with excellent yield and superior popping quality, published in the Journal of Plant Registrations. Like popcorn, certain sorghum genotypes produce a white, roughly spherical piece of popped endosperm under high heat. Popped sorghum has long been used in India and Africa as food and animal feed, but the release notes that no sorghum hybrid had been developed before with popping as a primary selection trait.

The team selected for popping efficiency (the percentage of kernels that pop), expansion ratio and flake size. Popped sorghum is smaller than popcorn, more tender, and has no hulls.