How does puffing make wheat and rice grains expand?

Puffing wheat and rice grains is a process that relies on rapid pressure changes to expand the cereal structure.

The grains are first heated within a pressure chamber to a pressure ranging from 7 to 14 kilograms per square centimetre (100 to 200 pounds per square inch). When this pressure is instantaneously released by suddenly opening the chamber—often referred to as a puffing gun—the water vapour trapped inside the grain expands rapidly. This sudden expansion blows up the grains to several times their original size, typically 8-fold to 16-fold for wheat and 6-fold to 8-fold for rice. Following this expansion, the final product is toasted to a moisture content of approximately 3 percent to ensure the desired crispness.1