How a Pressure Cooker Works
1On busy weeknights, my dad makes dinner in a pressure cooker. Hard dried chickpeas that can take hours to cook in a regular pot turn soft in much less time. I wanted to know how this heavy pot works, so I asked my dad questions and read the booklet that came with it. Here is what I learned.
2A pressure cooker looks like a big pot, but its lid is special. The lid locks onto the pot with a twist. Around the inside edge of the lid is a rubber ring called a gasket. When the lid is locked, the gasket seals the pot so that air and steam cannot leak out around the edge.
3Cooking begins when the pot heats the water inside it. As the water boils, it turns into steam. In a regular pot, the steam escapes into the air. In a pressure cooker, the steam is trapped. More and more steam builds up, and it pushes on the food and on the walls of the pot. This push is called pressure.
4High pressure changes the way water acts. In an open pot, water boils at its usual boiling point, and it does not get any hotter while it boils. Under high pressure, the boiling point rises. The water and steam inside the cooker become hotter than boiling water in an open pot. The extra heat moves into the food, so beans, rice and tough meats cook faster.
5Too much pressure would be dangerous, so a pressure cooker has a safety valve. A valve is a small part that can open and close. If the pressure climbs too high, the valve opens and releases some steam with a loud hiss. When dinner is done, my dad turns off the heat and lets the pressure drop. He never opens the lid until all the pressure has been released. Then he unlocks the lid, and the smell of warm chickpeas fills the kitchen.