You can pop corn with any heat source, but to puff rice you also need a pressure chamber. Popcorn is the only grain that can be ‘puffed’ at home, because it has the right moisture content (14 per cent) and a fragile hull. When the temperature of the kernel reaches 100 degrees Celsius (212 degrees Fahrenheit), water starts to boil inside the starchy cells of the endosperm, causing them to expand. At 175 degrees Celsius (347 degrees Fahrenheit), the superheated starchy material explodes through the hull of the kernel and instantly cools to form a chewy bubble. Grains of rice don’t contain enough moisture to pop under normal conditions. Instead, the rice must be heated in a pressure chamber at up to 14 kilograms per square centimetre (200 pounds per square inch). When the chamber is opened, the sudden change in pressure and volume causes the hot starch cells in the rice to expand rapidly, bursting through and puffing up like popcorn.
Answered by Dave Roos