Popping Popcorn

Contemplating air pop vs. oil pop? Either way, it’s actually water that pops your corn.

Corn, also known as maize, provides over 20% of the world’s nutrition. There are several types of corn, including the two shown in the photo below: sweet corn and popcorn. Each kernel of corn is actually a seed that, like most seeds, contains an embryo (a baby plant) and a seed coat for protection. Corn seeds are filled with a starchy endosperm that provides food for the baby plant (it's stained black in right-half of each kernel in the photo below).

This photo shows sweet corn seeds on the left and popcorn seeds on the right. On both sides, you'll see an intact kernel on top and the two halves of a kernel that's been split below. The right half of both split kernels has been stained so the starch inside appears black.

One major difference between the two types of corn seeds is the toughness of their seed coats and the makeup of their endosperms. We soaked both of the above seeds in water overnight and sliced them in half with a razor blade. The sweet corn seed was easy to split, while the popcorn seed required more pressure. Both contain starch and water, though in different amounts due to how they’re grown and harvested.

The combination of a hard seed coat, starch, and water are critical to how popcorn gets its pop. As a popcorn kernel is heated, water and oil inside the seed heat up and soften the surrounding starch. The surrounding shell is so tough that the water can’t escape when it initially boils into steam. As the steam gets even hotter, the water molecules move even faster and pressure builds up inside the seed.

When the seed coat bursts, steam cooks and fluffs the starch molecules as it's released.

At some point, the pressure gets so high (up to seven times normal pressure!) that the seed coat bursts. The steam rapidly expands through the starch, creating a puffy foam that gets cooked as the steam passes through. As the starch cools, you’re left with a crunchy, airy kernel of popcorn.

Starch molecules are dense in the unpopped kernel on the left and expanded in the popped kernel on the right.

Have some unpopped kernels at the bottom of the pot? Try slowing things down for a minute.

Want to take a closer look inside a seed? Check out this Science Snack to go inside a bean.