The pressure inside a pressure cooker typically ranges from 15 to 20 psi (pounds per square inch) above atmospheric pressure, which allows water to boil at temperatures between approximately 250°F (121°C) and 260°F (127°C). This elevated pressure raises the boiling point of water, enabling food to cook faster. The exact pressure depends on the specific model of the pressure cooker and the desired cooking temperature.
An increase in pressure allows an increase in the boiling temperature of the liquid in the cooker. At 15 psi (the highest rated home kitchen pressure cooker) the boiling temperature of water is 250 degrees F.
An increase in pressure allows an increase in the boiling temperature of the liquid in the cooker. At 15 psi (the highest rated home kitchen pressure cooker) the boiling temperature of water is 250 degrees F.
Vapour rom the boiling water is not allowed to escape and so the pressure above the water increases. This raises the temperature at which the water inside the cooker boils.
On the food
A pressure cooker minimizes the escape of fluids or air. The build-up of air and fluid increases the pressure inside the cooker, which also increases the boiling point. An open kettle allows the fluid and air to escape, heat is lost thus cooking time is longer.
The principle involved in a pressure cooker is that increasing the pressure inside the cooker raises the boiling point of water, allowing food to cook at higher temperatures and reducing cooking time. This results in faster cooking and tenderizing of tough cuts of meat.
Water boils at 120°C in a pressure cooker because the increased pressure inside the cooker raises the boiling point of water. Normally, water boils at 100°C at standard atmospheric pressure, but the pressure cooker traps steam, increasing the pressure and allowing water to reach higher temperatures before boiling. This higher temperature cooks food faster and more efficiently.
The higher pressure raises the boiling point of water.
The universal gas law states that (pressure * volume/temperature) of a gas is a constant. Hence, if the volume stays the same (as in a pressure cooker), the air in the cooker can increase in temperature beyond the boiling point of water as pressure builds up over the normal atmospheric pressure. A higher temperature means faster cooking. The above is a very simplistic view of the physics involved.
Boiling point = temperature where the vapour pressure is equal to the pressure of the atmosphere above the liquid. When you have a pressure cooker, the pressure above the water is higher than atmospheric pressure, therefore the boiling point of water is elevated to above the boiling point in an open pot. The water temperature exceeds 100 degrees Celsius, and thus the cooking process is accelerated.
You can change the boiling point of a liquid by adjusting the pressure on the liquid. Increasing the pressure raises the boiling point, while decreasing the pressure lowers it. This is why water boils at a lower temperature at higher altitudes where the atmospheric pressure is lower.
If you are referring to cooking with a pressure cooker, the higher pressure attained in this type of cooker allows the water to reach a higher temperature before boiling. This will cook food more quickly due to the higher temperature, which in turn is due to the higher pressure. Keep in mind that once water reaches the boiling point, the temperature remains constant; the only way to attain a higher temperature, then, is to increase the pressure.