How Cooking Under Pressure Works
Pressure cookers function based on a very simple principle: In a tightly sealed pot, the boiling point of liquid is higher.
In a normal cooking vessel, water boils at 212 degrees Fahrenheit. When water boils, it turns to steam, and steam usually stays at 212 degrees. But everything changes in a closed environment. When water is brought to a boil in a pressure cooker, the water molecules can’t escape, increasing the pressure within. More energy is needed for the water to boil and steam, which in turn increases the temperature in the chamber. You can raise the pressure inside most pressure cookers to about 15 pounds above normal sea-level pressure, which in turn raises the boiling point of water to 250 degrees.
This means that you are cooking food with superheated steam that’s at a temperature up to 38 degrees higher than what’s possible in a normal pot. That translates to shorter cooking times, and since cooking requires much less liquid than usual, flavors concentrate. You also use less energy because you turn the heat down just enough to maintain the pressure level within the pot.
THE LANGUAGE OF THE PRESSURE COOKER
High and Low Pressure: Most pots have two pressure levels: high and low. The exact amount of pressure (measured in pounds per square inch, or psi) for each level varies slightly from pot to pot, but generally, high registers around 15 psi, while low pressure registers 5 psi. We use high pressure in most recipes since it is the most efficient, but we found low pressure produces slightly better results when cooking foods that need a more gentle hand, like some grains, rice, and beans. Since not all pressure cookers come with a low setting, we did develop options that rely on high pressure and deliver good results.
Natural and Quick Release: Once the cooking-under-pressure time specified in a recipe is up, you have to let the pressure out of the pot using one of two methods: natural release or quick release.
For a natural release, you turn off the heat and allow the pressure in the pot to naturally drop back down. This is the preferred method when you want to gently finish cooking the food through, since the food will continue to cook as the pressure drops. For instance, large cuts of meat benefit from a natural release by ensuring tender results. If you quickly release the pressure on a large cut of meat like our Sirloin Beef Roast with Mushroom Sauce, the meat will seize up and will have a tough texture come serving time.
A quick release is used when you want to stop the cooking immediately because the food can easily overcook (think chicken breasts). Or, if a gentle finish isn’t important, we simply opt for a quick release because it’s faster. Modern pressure cookers are equipped with quick-release valves that quickly and safely release the steam from the pot.
Don’t try to swap one release method for the other. When using a natural release, the food continues to cook as the pressure comes down. Therefore, if you switch from a natural to a quick release, you are going to undercook the food (and likely also cause problems in terms of final texture of the food). And if you switch from a quick to a natural release, the food will overcook.
Lock the Lid in Place: Before you are able to bring the pot up to pressure, modern pressure cookers have a safety mechanism that requires you “lock the lid in place.” The process is a little different on each model. You typically need to line up a marker on the lid with a marker on the pot, then twist the lid so that the handle on the lid aligns with the handle on the pot, thus creating an airtight seal. You may need to flip a locking switch, or make sure a locking indicator signals the pot is properly shut. If the lid is not properly locked on, today’s pots are designed so that pressure cannot build up.