When potatoes are boiled in a pot, they usually take 30 minutes to cook. But boil them in a pressure cooker and they will be ready in less than 10 minutes. Why?
When heat is applied to a pot of water it begins to turn into steam. Bubbles are formed by the steam rising from the bottom to the surface. A temperature of 212 degrees Fahrenheit or 100 degrees Celsius is normally given as the boiling point of water but is only correct if cooking at sea level. As you go to a higher elevation the atmospheric pressure (14.7 psi) falls, causing the boiling point of water to also fall. Extra cooking time is therefore needed. Once water reaches its boiling point in a vessel it does not matter how much heat you add it can never get any hotter. It just turns into steam and that why potatoes take longer to cook in an open top pot.
But a pressure cooker has a sealed lid, so steam produced when water boils builds up inside. As the pressure rises, so does the boiling point of water. The cooking temperature is therefore increased, which reduces the cooking time needed to cook the food. A typical modern pressure cooker operates at an extra pressure of about 15 psi or about twice atmospheric pressure. So water inside boils at about 252 degrees Fahrenheit and that is why food cooks faster
The pressure cooker has a pot like base and a domed lid. A rubber gasket between the two provides a pressure tight seal. In the center of the domed lid is a vent in which a weighted pressure regulator is placed. The weight seals the vent, but lifts to control the steam pressure inside at 15 psi. A gentle rocking of the regulator indicates that proper cooking pressure is being maintained by the heat source. Usually high heat is applied to start the water inside the pressure cooker boiling then reduced to maintain the gentle rocking motion. Cook for the length of time indicated in a recipe or cooking chart. For most produce like potatoes and carrots cooking time is about 4-8 minutes.
Due to the high operating pressure and high heat it is very important to observe a few safety precautions. Always add liquid to a pressure cooker to avoid excessive over heating and melting of the cooker bottom. Always lock the lid in place. Always make sure that the cover lock operates when heat is applied. Never try to open a lid under pressure. A safety relief plug releases the pressure if the regulator fails to rise.
