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Q & A: pressure cooker

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Most recent answer: 10/22/2007
Q:
Why do vegetables cook more quickly in a pressure cooker than if they were being cooked in a pan of boiling water?
- Luke (age 11)
Notre Dame School, Sheffield, England
A:
When you coook food in ordinary boiling water, the water stays at 100 deg C (212 deg F). As more heat goes into it, the heat just boils off the water, rather than heating it further.

In a pressure cooker the same thing happens but at higher temperature, because at higher pressure the boiling point goes up. The boiling point is the temperatue at which just as amny water molecules leave the liquid going into a little steam bubble as leave the bubble going back into the liquid. When the temperature is just barely higher than that the bubbles just grow, so you see boiling.

When the pressure is higher, the liquid itself isn't much changed, but there's a higher density of water molecules in the steam. (That's why it has higher pressure.) That means that water molecules are leaving the steam more rapidly. So to get to the boiling point, where the water molecules leaving the liquid keep up with that, you need to go to higher temperature.

Of course things cook faster at higher temperature.

Mike W.

(published on 10/22/2007)

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