Pressure cookers

Last updated: October 12, 2008.

Want to cook your dinner in a fraction of the time? You could use a microwave to zap it with energetic waves. But another popular solution is to seal it in a pressure cooker: a kind of saucepan that cooks things quicker by boiling them at a higher temperature than usual. Although considered old-fashioned by some, pressure cookers are still a convenient and economical way to prepare food. Let's take a closer look at how they work!

Photo: A pressure cooker in action. Notice the small valve on the top (just to the right of the central handle) through which steam escapes. Photo copyright © and courtesy of Simon Law 2005, published on Flickr under a Creative Commons License.

How pressure and temperature affect boiling

Suppose you have a saucepan full of potatoes that you want to cook. You fill the pan with water, put it on a hot stove, and wait for the water to boil. Now you probably think the water will boil "when it's hot enough"—and that's true, but only half true. The water will actually boil when the molecules it contains have enough energy to escape from the liquid and form water vapor (steam) above it. The hotter the water is, the more energetic the molecules are and the more easily they can escape. So temperature plays an important part in making things boil.

But pressure is important too. The higher the pressure of the air above the water, the harder it is for the molecules to break free; the lower the pressure, the easier it is. If you've ever tried making a cup of tea on a mountain with a portable camping stove, you'll know the water boils at a lower temperature at high altitudes. That's because air pressure falls the higher up you go. At the top of Mount Everest, air pressure is about a third of what it would be at sea level, so water boils at roughly 70°C or 158°F (see why on this MadSci forum posting). Mountain-top tea tastes pretty disgusting because the water boils at too low a temperature: even though it's boiling, the water is too cold to "cook" the tea leaves properly.

Photo: Civilisation is a cup of tea—but only when you make it at sea-level. That's when water boils at what we consider its "proper" temperature (100°C or 212°F).

Read more about pressure, temperature, and how molecules behave when liquids boil.

How pressure cookers work

Let's forget the tea and return to our potatoes. Suppose we could somehow arrange things so that the air above our saucepan was actually at a much higher pressure than usual. That would make the water boil at a significantly hotter temperature, which would make the potatoes cook more quickly.

This is the basic idea behind pressure cookers. A pressure cooker is a big steel saucepan with a tight-fitting lid. The outer edge of the lid has a thick circle of rubber called a gasket that fits between the bottom of the lid and the top of the pan to make a really tight seal.

When you fill the pan with water and place it on the stove, the water heats up and some of its molecules escape to form steam up above it. With a normal pan, the steam would just drift off into your kitchen and disappear. But with a pressure cooker, the gasket and lid stop the steam escaping so the pressure soon builds up. Although the water inside the pan boils, the higher pressure means it boils at a higher temperature than normal that cooks your food more quickly. A special valve on the top of the lid allows a small amount of steam to escape, keeping the pressure higher than normal but not so high that the cooker explodes. If the pressure inside the pan builds up too much, the valve pops right out, rapidly lowering the pressure to a safe level again.