Rattle-tattle-spat-spat-spat. Mmmm, Mom’s making pot roast, I’d better stick around for dinner tonight.
Growing up, I couldn’t help but be fascinated by the pressure cooker. It was unpredictably noisy, and never failed to produce delicious meals. I’m sure that Mom explained it to me more than once (she was a science teacher in her turn too), so it’s a pleasure to share my understanding with you as well.
A few basic ideas make the pressure cooker possible. Beginning with the obvious, boiling water produces steam, which is hot. Steam can be used to cook food, as it transfers the heat to whatever it comes in contact with. To cook faster, you want hotter steam, and a lot of it.
When you boil water in a regular pan – a sauce pan for instance – most of the heat from the stove is used to heat the water to boiling, and to slowly turn the water into steam. That steam then floats off into the room, playing no part in the cooking. In a covered pan, the steam is trapped up to a point, but the lid does not have a tight seal, so steam escapes, usually making a mess if you let things get too hot. Worse, if you do have a tight seal, the steam is trapped inside until it builds up enough pressure to blow the top off the pan. Whoa!
Where does that pressure come from? In physics, there is a familiar equation called the “ideal gas law” that relates volume (an enclosed space), pressure (defined as force divided by area), temperature (in Kelvin, but that won’t matter for our discussion), the gas constant (again, won’t affect us), and the amount of gas (steam in this case) inside. Students often refer to the equation as “pivnert”, because it is most commonly seen in the form PV=nRT. Where this matters to us, is that since the volume of a covered pan doesn’t change, any changes in the amount of steam (n) or the temperature of the steam (T) cause the pressure (P) to change in the same way. Therefore: more steam = more pressure, and higher temperature = higher pressure.
The pressure cooker is designed to safely take advantage of higher temperatures and pressures for effecient cooking. For starters, it is fitted with a sealed, locking lid, so steam doesn’t spray out the sides, and the lid doesn’t go flying off. To avoid turning your sealed pan into a bomb, there is also a small pressure relief valve built in.
Temperature is controlled by the stove burner. The small amount of water inside the pan is soon heated to boiling. All heat beyond that is used to convert water to steam, and to heat the steam to higher temperatures. As more steam is produced, and as the temperature increases, the pressure inside the pan also increases. An increase in pressure also raises the boiling point of water, so more heat is required to produce more steam. With the stove burner at a constant temperature, an equilibrium can eventually be reached, where temperature and pressure and amount of steam all stay pretty much constant. However, if that equilibrium is not reached, the pressure keeps on building as more steam is produced and heated further. To regulate the pressure inside, a small valve (basically a narrow metal tube) allows steam to escape from the pan. This escape is controlled by a weighted cover that fits over the valve. Remember that pressure is force divided by area. When the steam inside the pan builds up enough pressure, it can exert enough force on the bottom of the weight to lift it slightly. When the weight lifts, the path is clear for steam to escape. As soon as a little steam has escaped, the pressure lessens, and the weight falls back down. This generally occurs fairly quickly in a few successive bursts, causing the familiar rattling sound of the pressure cooker, as the weight bounces around under the exertion of the escaping steam.
Inside the pan, the hot, pressurized steam works to cook the food. Each water molecule (steam is still water, remember) carries a good deal of heat, thanks to the high temperature. When it comes in contact with the meat (or potatoes, or carrots…) it transfers some of that heat to the food. Multiplied by the millions (actually many times more than that) of steam molecules colliding with the food, that heat adds up, and cooks the food. The higher temperature and pressure speed the cooking process by ensuring that there are more collisions (because there is more steam present) and that more heat is transferred (higher temperature) with each collision. Before long, the meal is ready – hot, moist, and just falling apart with the flavors we all love.
Good physics, good cooking, and good eating. With a pressure cooker, it’s all yours.
