Why Does Popcorn Pop? A Pressure Vessel the Size of a Seed
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A kernel of popcorn is a grain of maize that has been engineered by thousands of years of selection into a small steam boiler. It contains just enough water, sealed inside a shell just strong enough, so that when it is heated the water turns to steam, the pressure climbs to about nine times that of the atmosphere, and the shell bursts and turns the kernel inside out in a few thousandths of a second. The result is a puff of starch foam thirty to fifty times the volume of the seed, and the sound, it turns out, is not the shell breaking.
What is inside a kernel
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.
The moment of popping
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.
Why only popcorn pops
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 ones that do not
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.
A very old snack
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.
The takeaway
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.