Pressure Cooker Operation

The pressure and temperature conditions inside the pressure cooker (1→2) prevent boiling, except if the water steam cools quickly causing a quick descent in pressure.

Contrary to popular belief, water never reaches its boiling point inside the pressure cooker. The simple increase in temperature is what accelerates cooking.

The pressure and temperature conditions inside the pressure cooker (1→2) prevent boiling, except if the water steam cools quickly causing a quick descent in pressure (2→3). As shown in the diagram, boiling point will be reached as long as the "state change line" is crossed, separating the liquid (L) and gas (G) areas in the thermodynamic diagram. However, when the cooker lid is closed most of the gas inside will be air, not water steam, so the pressure inside is always the sum of the amount due to the water steam, whose amount increases by evaporation effect as the temperature increases, and the air, whose partial pressure is responsible of moving the pressure further away from saturation as the pressure cooker heats, which prevents water from boiling inside the cooker, i.e. the state change line never crossed the 1→2 line which represents the evolution of pressure and temperature conditions inside the cooker.

Likewise, once maximum pressure determined by the valve (through its weight or spring) is reached inside the cooker, it cannot be modified, and placing it over high flame does not accelerate cooking, it simply increases water evaporation and steam loss through the valve.

Boiling can exceptionally produce if a fast cooling takes place in the air and water steam mix; e.g. if the cooker is to be opened quickly and placed under a jet of water. This water jet cools the walls of the container, which causes condensation in the water steam and a fast descent of pressure inside de cooker so the state change line (2→3) is reached, and sudden water boiling is produced, with a virulence that may even cause the liquid to escape through the sealing gasket of the container or valve. Another thing happens if the cooker is opened while still pressurized, with the subsequent risk of suffering burns caused by hot liquid splashing or the steam itself.

Mountain residents also use pressure cookers, since otherwise cooking food becomes extremely difficult. At altitude at sea level, the atmospheric pressure is 1 atmosphere and the boiling point in an open pot is 100 ℃. However, atmospheric pressure is lower at more altitude and, therefore, boiling temperature for water decreases, as Charles Darwin described in the Beagle Journey:

Where we slept, water boiled necessarily at less temperature than in lower altitude as a result of a decrease in atmospheric pressure, being the opposite case compared to the Papin cooker. Therefore, after several hours of cooking potatoes, we would find that they were almost in the same state as at the beginning. Potatoes were even left over the fire during the entire night and when boiling them again the next morning, they were still not cooked. I remembered this while I listened to my peers discuss the cause of this; they had reached the conclusion that the bloody potatoes were not meant to be cooked. Oddly, the lower the altitude is, the faster they are cooked in a pressure cooker since if the starting point (1) is lower, the 1→2 line will reach higher temperature under the cooker’s working pressure.