We may not have always known what makes popcorn pop, but humans have been enjoying popcorn for thousands of years. Ancients attributed popcorn's magic to tiny gods trapped within the kernel that would burst with anger when heated, but we now know that the real reason why popcorn pops is a fascinating mix of chemistry and physics.
The Kernel - A Closer Look
To understand why a kernel of popcorn bursts when heated, we must first understand the construction and composition of a corn kernel.
The outer, light yellow, transparent hull of the corn is the secret to popping corn. When intact, the hull is impervious to moisture and key to creating the pressure needed to pop the corn.
Inside of the hull is the endosperm. The endosperm is comprised of a hard starch and a small amount of moisture (approximately 14%). As anyone who has bitten into an unpopped kernel knows, before popping the starchy center is rock hard.
The Perfect Pop
When the kernel is heated, the small amount of moisture trapped in the kernel turns to steam, which is pushed past the boiling point by the pressurized environment created by the hull. The pressurized heat gelatinizes the hard starch, turning it from rock solid to a very malleable form.
As the kernel continues to heat, the pressure eventually exceeds the strength of the hull, at which point the hull ruptures. The pressurized steam within the kernel immediately expands and causes the gelatinized starch to puff in the process.
As the steam escapes, the temperature of the starch quickly drops and allows it to once again take solid form. The result is a perfectly fluffy, popped kernel.
In every batch of popcorn, there are always a few kernels that never pop or simply crack open without popping. There are several possible causes, but a few missing links in the popping formula are usually the cause.
A faulty hull is the most common cause of a mis-pop. If the hull has a small crack or otherwise compromised area, pressure will not build within the kernel. As the moisture in the kernel heats and turns to steam, it slowly leaks out of the kernel. These kernels may stay completely intact or will split open before the starch gelatinizes, causing an open but compact kernel.
Low moisture content in the kernel is also a common cause of failure to pop. Without enough steam in the kernel, pressure can not build and the starch can not gelatinize. Popcorn manufacturers carefully dry the corn to achieve kernels with the perfect amount of moisture for optimum popping.
Improper heating can also cause failure to pop. Corn pops best in dry heat produced by air or oil. This type of heat surrounds the kernel and heats it evenly and quickly. Heating the corn too slowly or at temperatures that are too low may not create enough pressure within the kernel for it to pop. Heating the corn unevenly or too slowly may cause the hull to rupture in one spot before the entire kernel is hot enough to expand, leaving an unpopped and probably burned kernel.