by Ernest Capraro

Created on: February 01, 2009

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

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