Food is placed inside the pressure cooker with a small amount of water or other liquid (e.g., stock required for the recipe). Food is either cooked in the liquid or placed above the liquid to be steamed; the latter method prevents the transfer of flavours from the liquid. Steaming is done by using a suitable insert such as a metal steamer basket with a support trivet. The lid is closed, the pressure setting is selected, and the pressure cooker is then placed on a heat source such as a stove at the highest heat setting. For pressure cookers with a weight, the weight is placed over the steam vent pipe while steam is being emitted to ensure the air inside has escaped. Once the cooker reaches full pressure, the heat is lowered to maintain pressure; the timing specified in the recipe begins at this point. It takes several minutes for the pressure cooker to reach the selected pressure level. It can take as long as 10 minutes depending on: the quantity of food, the temperature of the food (cold or frozen food delays pressurisation), the amount of liquid, and the size of the pressure cooker.
Using boiling water from a kettle can quicken the heating process, unless the recipe calls for cold water. For example, boiled potatoes cook best when added to cold water because this prevents the potatoes from breaking up. Boiled potatoes also require the pressure to drop naturally after timing instead of being quickly released.
The newer generation pressure cookers, which have no weights, expel air from inside before reaching full pressure. A common mistake is for the user to start timing when the pop-up indicator rises, which happens when there is the slightest increase in pressure, instead of waiting for the cooker to reach the selected pressure level first. Some pressure cookers have markers on the pop-up indicator that show the pressure level. However, the typical pop-up indicator only shows that the cooker has pressure inside, which does not reliably signal that the cooker has reached the selected pressure. This indicator often acts as an interlock to prevent the lid from being opened while there is internal pressure. Manufacturers may use their own terminology for this indicator, such as calling it a locking indicator.
As the internal temperature rises, the pressure also rises until it reaches the design gauge pressure. In some models, a relief valve subsequently opens, releasing steam to prevent the pressure from rising any further. In other models, the pressure regulator weight begins levitating above its nozzle, allowing excess steam to escape. At this stage, the heat source should be reduced because heat is only needed to maintain pressure. If the heat source is too high, then energy is wasted, too much liquid may be released to maintain the nominal pressure, and reduces the life of the gasket/sealing ring by wear. On the other hand, if the heat source is too low, the food may be undercooked or pressure may be lost. As a rough guide, medium heat is the highest setting required on any domestic stove to maintain the pressure level throughout cooking time.
Recipes for foods using raising agents (e.g., steamed puddings) require gentle pre-steaming, without pressure, in order to activate the raising agents prior to cooking and achieve a light, fluffy texture. The water must bubble gently while pre-steaming to ensure that enough water will be available for the entire pressure cooking time, otherwise the pan will boil dry. If pre-steaming is omitted, it can result in a heavy, stodgy texture.
Since pressure cooking depends on the production of steam from water, the process cannot easily be used for cooking methods that produce little steam such as roasting, pan frying, or deep frying. However, Kentucky Fried Chicken restaurants use a combination of pressure cooking and frying, with special pressure fryers in which chicken juices supply the water. Cooking time is reduced substantially, but the breading texture is much softer (less crispy) than deep-fried chicken since moisture remains in the breading. Thick sauces do not contain enough liquid to vaporize and create pressure, so they usually burn onto the interior base of the pressure cooker after prolonged heating. Sauces should normally be thickened after pressure cooking.
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