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Pressure cooking is a method of cooking in a sealed vessel that does not permit air or liquids to escape below a preset pressure. Because water's boiling point increases as the pressure increases, the pressure built up inside the cooker allows the liquid in the pot to rise to a temperature higher than 100 °C (212 °F) before boiling. The higher temperature causes the food to cook faster. Cooking times can be reduced by a factor of three or four. For example, shredded cabbage is cooked in one minute, fresh green beans take about five, small to medium-sized potatoes (up to 200 g) may be ready in five minutes or so and a whole chicken takes no more than twenty-five minutes. It is often used to simulate the effects of long braising or simmering in shorter periods of time.
A safety valve releases steam when the pressure exceeds the safety limit for the cooker; usually the steam pressure lifts a weighted stopper allowing excess pressure to escape. There is usually a backup pressure release mechanism, in the form of a hole in the lid blocked by a plug of low melting-point alloy. If internal temperature (and hence pressure) gets too high, the metal plug will melt, resulting in a release of the pressure.
An early pressure cooker, called a 'steam digester', was invented by Denis Papin, a French physicist, in 1679.
A pressure cooker is often used by mountain climbers to compensate for the low atmospheric pressure at high altitude. Without it, water boils off before reaching 100 °C, leaving the food improperly cooked, as described in Charles Darwin's Voyage of the Beagle:
At the place where we slept water necessarily boiled, from the diminished pressure of the atmosphere, at a lower temperature than it does in a less lofty country; the case being the converse of that of a Papin's digester. Hence the potatoes, after remaining for some hours in the boiling water, were nearly as hard as ever. The pot was left on the fire all night, and next morning it was boiled again, but yet the potatoes were not cooked. I found out this, by overhearing my two companions discussing the cause, they had come to the simple conclusion, "that the cursed pot [which was a new one] did not choose to boil potatoes."
A larger scale version of a pressure cooker, used by laboratories and hospitals to sterilise biological waste materials, surgical instruments etc. is known as an autoclave.