A pressure cooker is a special container used for cooking which has an air-tight lid and is made of steel or aluminum. Due to its design it is possible to build up a pressure of around 5 atmospheres in the container. Now as the pressure is increased, the boiling point of water rises. This makes steam reach a higher temperature of around 150 degree Celsius rather than 100 degrees Celsius which it has at normal room pressure. So the food is cooked at a much higher temperature than with the use of conventional open pans.
Also, steam is able to transfer heat by convection and conduction in a much better way than air or water. These effects, of steam being able to reach a higher temperature and the better ability to transfer heat that steam has, allow food to be cooked faster and this way of cooking requires much less energy than cooking in open pans.
The basic principle of a pressure cooker is that the boiling point of water increases with the pressure experienced by the water and the steam produced from the steam. Thus the pressure cooker is designed to control the escape of steam generated from the water inside the cooker. When the steam is not allowed to escape from the pressure cooker freely, its temperature as well as that of the water increases. Thus the temperature inside the pressure cooker is several degrees above the 100 degrees C., the boiling point of water at normal atmospheric pressure.
The higher temperature of the water in the food being cooked as well as the steam fills the entire pressure cooker completely, helps to heat and cook the food inside the pressure cooker evenly and quickly.
The pressure cookers are designed with systems of outlets for the steam which open only when the pressure of stem reaches a minimum level. This allows part of the steam in the pressure cooker to be released from time to time to prevent the pressure inside increasing to unsafe levels.