The principle of a pressure cooker is cooking under increased pressure. It is well known that food gets cooked fast at high temperatures. Generally we cook food in water kept in open vessels. In these vessels, when the water is heated to 100 degrees Centigrade it begins to boil, becomes steam and escapes. Thus there is no possibility of heating the water beyond 100 degrees in open vessels. Hence it takes a lot of time to cook the food.
From physics, we know the boiling point increases with increase in pressure. Hence in pressure cookers, the steam is not allowed to escape but enclosed within the vessel. As more water is converted into gaseous steam, the pressure increases which in a feedback mechanism increases the boiling point to well beyond 100 degrees enabling fast cooking. Normally the temperature reaches about 120 degrees inside the pressure cooker.
The fundamental equation in physics that relates pressure (P), volume (V) and temperature (T) is given by Boyle’s law
P V = k T where k is constant.
According to the equation, if V is kept constant as in a pressure cooker and P or T is increased, the other parameter increases. In the cooker, both of them increase to enable fast cooking.
In order that the pressure does not reach very high values so as to cause an explosion, a weight and safety valve are provided to let out the excess steam. Also the body of the cooker is made of an alloy which can withstand high pressures.
