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.
(republished on 07/24/06)