Why Does Popcorn Change Shape When Being Popped?

Heat and pressure cause popcorn to change shape.

A piece of fluffy white popcorn looks very different than a raw kernel of yellow popping corn. Popcorn changes shape when popped because cooking alters the nature of the molecules inside the corn.

The Facts

The corn kernel explodes during cooking.

Heating the corn creates a high level of pressure inside the kernel. According to the Cornell Center for Materials Research, this pressure grows so great that when the corn pops, the inside of the kernel "blasts through the outer shell."

Internal Change

The corn kernel has a hard outer shell and a soft center.

The corn kernel contains water and starch. Cooking turns the soft starch to jelly and the water into steam. These steam molecules expand and start to collide against the outer shell of the kernel.

External Change

The outer shell eventually bursts, releasing steam, pressure and starch. Once outside the kernel, the starch cools quickly, to form the familiar, fluffy shape of popped corn. As the United States Department for Agriculture states, popcorn changes shape because heat energy makes the kernel turn itself inside out.

References

• Cornell Center For Materials Research: Ask a Scientist
• NASA Education: Expansion and Contraction
• National Agricultural Library: Popcorn