Workings of a Pressure Cooker – Part 1

Pressure Cooker

Pressure cooking saves a great deal of energy compared with conventional cooking methods. Pressure cookers have their place in almost every household. As the name implies this form of cooking relies on pressure. But how is this pressure generated? Secondly how does pressure cooking save energy? In order to answer these questions one should have a lucid idea of what boiling point is, how it affects cooking, and other nuances. These articles are dedicated to answer these questions in the simplest way possible.

As mentioned during our Physics tuition classes, boiling point of water is defined as the temperature at which the pressure of the water vapour generated is equal to atmospheric pressure. Generally this happens at hundred degree centigrade. But the atmospheric pressure is not a constant. It changes with altitude; atmospheric pressure goes down with an increase in altitude. This definitely affects the boiling point. But what does it have to do with cooking? Cooking is a process in which heat is transferred to raw food material. Water is used to a medium to transfer heat in a controlled fashion. Our cooking woes at high altitude begin here. To understand it precisely lets discuss what happens at boiling point temperature.

When a system, like a bowl of water, is heated the energy is transferred to water and the temperature of water increases. As the temperatures goes up reaches the boiling point, the extra energy delivered is not used for increasing the motion of the particles present in the liquid. The extra energy plays a role in increasing the entropy of the system and causing a change of state. In our case when excess energy is pumped into water, the temperature of the water remains 100 degree centigrade but the amount of liquid water decreases and finally the vessel turns empty. No matter how much extra energy is used the temperature of liquid water does not increase beyond 100 degree centigrade under atmospheric pressure condition. In order to cook food early one has to supply large amounts of heat to the raw material. And the heat transfer depends on the temperature difference. So to attain high temperature difference one has to heat water to high temperatures.

By definition, boiling point is the temperature at which the pressure of water vapour is equal to the atmospheric pressure. So in order to vary boiling point one can vary the pressure of the surroundings. This exactly is the trick used in a pressure cooker and also the one comes to play in high altitude cooking. Let’s discuss the rest in the next article.