Why Does Popcorn Pop? A Pressure Vessel the Size of a Seed

A popcorn kernel has three parts. The outer hull, the pericarp, is thin but very strong and, in popcorn, unusually impermeable. Inside it is the endosperm, the store of starch the seed would have used to feed the growing plant, which in popcorn is mostly the hard, glassy kind rather than the soft floury kind. And within the endosperm is water, around fourteen percent of the kernel's weight, dispersed through the starch. A kernel that has dried out below about eleven percent will not pop, and neither will one that has absorbed too much moisture, which is why popcorn is sold in sealed bags and why a jar left open for a year gives disappointing results.

As the kernel heats, the water inside turns to steam but cannot escape through the hull, so the pressure rises. At around 180 degrees Celsius the pressure reaches about 135 pounds per square inch and the starch, softened by the heat and moisture, has turned into a hot gel. The hull then fails, the superheated steam expands violently, and the gelatinised starch is blown outward into a foam of thin walls and bubbles, which cools in the air within a fraction of a second and sets into the crisp, white shape. The kernel has turned inside out, and the small brown fragment at the base of a popped piece is the hull that held it all in.

High-speed film of the event, made by two French physicists in 2015, showed that the pop is quicker than expected and that the sound comes not from the hull cracking but from the sudden release of steam, in the same way that a champagne cork's bang is the gas, not the cork. The film also showed the kernel jumping: a leg of starch shoots out first and pushes against the pan, flipping the kernel into the air like a gymnast.

All maize has starch and water, and all of it will scorch, but only one variety pops, because only it combines the necessary features:

• •A hull strong enough and sealed enough to hold the pressure until it is high, rather than leaking steam as sweetcorn does
• •A mostly hard, glassy endosperm that turns into a gel and then a foam, rather than the soft starch of flour corn, which merely bakes
• •The right moisture content, close to fourteen percent, to make enough steam
• •A small, round kernel that heats evenly through

The unpopped kernels at the bottom of the bowl, known in the trade as old maids, failed for one of a few reasons. Most often the hull had a small leak, a crack from handling or drying, that let the steam out gradually. Some had dried below the critical moisture, and some simply sat in a cool spot of the pan and never reached temperature. Research at Purdue University in 2005 found that the best-popping varieties have hulls with the most orderly crystalline cellulose, which makes them a better seal, and that hull structure rather than starch is what separates the good popcorns from the bad. The practical advice follows: store the kernels sealed, heat the pan evenly, and do not blame the recipe.

Popcorn is among the oldest prepared foods in the Americas. Kernels and cobs found in caves in New Mexico were dated to about 4,000 BC, and popcorn remains from Peru go back to at least 4,700 BC, before pottery was in use there, which suggests the kernels were popped directly on hot stones or embers. Aztec ceremonies used it as decoration and offering. It became a cinema food in the 1930s, when Depression-era Americans could afford a five-cent bag when they could afford little else, and the microwave bag, patented in 1981, made it a home food again. The kernel, meanwhile, is unchanged: a seed that has been bred to explode.

Not all popcorn pops into the same shape. Most kernels sold for home and cinema use are butterfly types, which burst into irregular pieces with several wings sticking out. The wings give a light, tender texture and plenty of surface for butter and salt, but they break easily. Mushroom types pop into dense, rounded balls that survive tumbling in a pan, which is why they are preferred for caramel corn and other coated snacks. The shape depends on the variety and on how the hull splits, and both kinds can come from the same bag.

Maize is not the only grain that can be puffed. Sorghum pops into small white puffs and is eaten this way in parts of India, and amaranth seeds pop on a hot pan into tiny grains used in the Mexican sweet called alegria. Grains without a sealed, pressure-holding hull can be puffed artificially instead. Industrial puffing machines heat rice or wheat in a sealed chamber and then release the pressure suddenly, which blows the grain up in the same way a hull does for popcorn. This is how puffed rice cereals and many rice cakes are made.

A microwave oven heats food by making water molecules twist back and forth in a rapidly alternating field, so in principle it suits popcorn, since the water is exactly what needs heating. In practice dry kernels heat unevenly, and the oil and flavourings would never get hot enough to sizzle. Microwave popcorn bags solve this with a susceptor, a patch of plastic film coated with a microscopically thin layer of metal, usually aluminium, built into one face of the bag. The metal absorbs microwave energy and becomes very hot, acting as a small frying pan under the kernels and fat.

That is why the bag must go in the right way up, with the side marked for the purpose facing down, so the hot patch sits beneath the kernels. It also explains the standard advice to stop when the pops slow to a couple of seconds apart rather than waiting for silence. Kernels pop over a spread of times because they differ slightly in moisture and hull strength, so a few stragglers always lag behind. Meanwhile the popped pieces, now dry and resting on a very hot surface, start to scorch, and a handful of unpopped kernels is a better result than a burnt bag.

Popcorn was not always a cinema snack. Early picture houses in the United States tried to look like grand theatres and discouraged noisy, messy food, and street vendors sold popcorn outside instead. The Great Depression of the 1930s changed that. Popcorn was cheap to make, a small bag cost very little, and struggling cinema owners discovered that selling it inside could earn more than the films themselves.

Cinemas installed their own popping machines, and popcorn sales held up even when ticket sales fell. During the Second World War, when sugar rationing limited sweets, popcorn became even more popular. The habit spread with Hollywood films around the world, and today concession sales, with popcorn at their centre, remain a major part of how cinemas make money.

Popcorn pops because each kernel is a sealed hull around hard starch and about fourteen percent water; heated to around 180 degrees, the water becomes steam at nine atmospheres, the hull bursts, and the starch gel is blown outward into a foam that sets in an instant. Only popcorn has the impermeable hull and glassy starch to manage it, the sound is the steam rather than the shell, and the kernels that fail had leaked, dried out or never got hot enough.