Why does suddenly releasing pressure puff cereal grains?

The process of puffing cereal grains relies on the rapid expansion of water vapor trapped within the grain. During cereal processing, grains are heated inside a pressure chamber to between 7 and 14 kilograms per square centimeter.

When this pressure is instantaneously released by opening the chamber—often referred to as a puffing gun—the water vapor inside the grain expands violently. This sudden release of pressure forces the grain or cereal pellet to inflate to several times its original size, typically increasing by 6 to 16 times depending on the grain type. Following this expansion, the product is toasted to a low moisture content of approximately 3 percent to ensure the final texture is crisp.1