Pressure cooking is a method of cooking in a sealed vessel that does not permit air or liquids to escape below a preset pressure. A safety valve releases steam when the pressure exceeds the safety limit for the cooker. Because water's boiling point is affected by the atmospheric pressure, the pressure built up inside the cooker allows the liquid in the pot to rise to a temperature higher than 100 °C 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 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.
An early pressure cooker, called a 'steam digester', was invented by Denis Papin, a French physicist, in 1679.
