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Making water boil hotter

How Does a Pressure Cooker Cook Faster?

Trapping the steam raises the pressure, and higher pressure makes water boil above 100 degrees.

Ordinary pan Water boils at 100 °CPressure cooker Water boils at about120 °C
Twenty degrees hotter cuts the time by two thirds.

  1. Step 1 of 5

    Boiling point depends on the pressure above.

    Water about to boilMolecules trying to escapeAir pressing down on the surface

    Water boils when its molecules can push the air above it out of the way. Press harder from above and they need more energy — a higher temperature.

  2. Step 2 of 5

    A sealed lid traps the steam.

    1 Water boils2 Steam istrapped3 The pressurerises

    Steam takes up over a thousand times the space of the water it came from. With nowhere to go it builds up the pressure inside the pot.

  3. Step 3 of 5

    Higher pressure means hotter water.

    On a mountainboils at 90 °C —cooking is slowAt sea levelboils at 100 °CPressure cookerboils at 120 °C

    At about twice normal pressure water stays liquid up to 120 degrees. Everything in the pot is now cooking twenty degrees hotter than it could before.

  4. Step 4 of 5

    Reactions speed up sharply with heat.

    At 100 °C Three hoursAt 120 °C Under an hour

    Cooking chemistry roughly doubles in speed for every ten degrees, so twenty degrees turns three hours of stewing into under an hour.

  5. Step 5 of 5

    The valve is the whole safety system.

    1 Pressure rises2 The weightlifts and vents3 It can never gohigher

    A weighted valve lifts and lets steam out once the pressure reaches its limit, and a second plug blows if the first one ever blocks.

    This is also why a kettle at the top of a mountain makes bad tea — the water boils well below 100 degrees.

The short version

A sealed pot traps steam and raises the pressure, so water boils at about 120 °C and food cooks far faster.

Try it yourself

Compare the cooking times on a bag of dried beans for a pan and a pressure cooker. It is usually a third.

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