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The food to be cooked is placed in the pressure cooker along with a small amount of water or other liquid, e.g., stock required for the recipe. The lid is closed, the pressure setting selected and the pressure cooker is placed on a heat source, e.g., a stove, at the highest heat (if a weight is used, the weight is placed on the steam vent when steam is being emitted, as this ensures that the air inside has escaped) until the cooker reaches full pressure, after which the heat is lowered to maintain pressure; the timing specified in the recipe begins at this point. A common mistake is for the user to start timing the recipe when the pop-up indicator rises as soon as there is the slightest amount of pressure in the cooker, instead of waiting for the cooker to reached the selected pressure level before timing the cooking duration. Some pressure cookers have markers on the pop-up indicator which show the pressure level, but normally the pop-up indicator just shows that the cooker has pressure inside, which is not a reliable means of showing that the cooker has reached its selected pressure. This indicator also acts as an interlock to prevent the lid from being opened as long as there is any pressure inside.
As the internal temperature rises, the pressure also rises, until the pressure reaches the design gauge pressure. In some designs, a relief valve opens, releasing steam and preventing the pressure from rising any further. In others, the pressure regulator weight begins levitating on its nozzle, allowing excess steam to escape. The heat source does not need to be kept higher than necessary to maintain pressure. If the heat source is too high, it costs more in energy, and the life of the gasket/sealing ring is reduced by this extra heat. If the heat source is set too low, the food may be undercooked or pressure may be lost.
Most pressure cookers have a working pressure setting of 15 psi, which equates to 103 kPa or 1.03 bar over the existing atmospheric pressure, the standard determined by the United States Department of Agriculture in 1917. At this pressure boost relative to sea-level atmospheric pressure, water boils at 121 °C (250 °F) (refer to the vapour pressure of water page).
The higher temperature causes the food to cook faster; cooking times can typically be reduced to 1/3 of the time of conventional cooking methods. Note that the actual cooking time depends on: the pressure release method used (see next section), the thickness of the food - because thicker foods will take longer to cook. Meat - and some other foods like sponge puddings, Christmas puddings etc. - are timed according to their weight. Frozen foods need extra cooking time. When pressure cooking at 15 psi, cooking times are typically as follows: Shredded cabbage is cooked in one minute, fresh green beans in three minutes, potatoes cut to around 1 inch thick cook in six minutes (using the natural release method) and a whole chicken, which fits into the pressure cooker easily, takes about twenty minutes. Brown rice, dried lentils and dried beans can be cooked in ten minutes, instead of 45 minutes of simmering in an ordinary saucepan.
Some pressure cookers have a lower maximum pressure than the industry standard 15 psi, or can be adjusted to different maximum pressures; cooking times will increase accordingly at lower pressures. This is often done by having different regulator weights or different pressure settings. If the pressure cooker is not capable of cooking at the industry standard 15psi (the manufacturer's instruction booklet should state the maximum operating pressure) and the recipe is intended for cooking at 15psi, the cooking time will need to be extended, but this can overcook food. Increasing the heat too high during pressure cooking below 15psi will not make the food cook faster — the higher heat wastes energy, shortens the usable life of the gasket and other rubber parts. This is no substitute for pressure cookers which cannot operate at 15psi, which is the pressure setting that most recipes require and expect, since 15psi became the standard for pressure cookers in 1917.
Pressure cooking is often used to simulate the effects of long braising or simmering in shorter periods of time.
Recipes for foods using raising agents e.g. steamed puddings, call for gentle pre-steaming—without pressure—to activate the raising agents prior to cooking to achieve a light and fluffy texture. The water must bubble gently when pre-steaming to ensure enough water is left over for the entire pressure cooking time, otherwise the pan will boil dry. If pre-steaming is omitted, the result is a heavy and stodgy texture.
Since pressure cooking depends on the production of steam and always requires liquid, the process cannot easily be used for methods of cooking that produce little steam, such as roasting, pan frying or deep frying. However, the large chicken restaurant chain KFC uses a combination of pressure cooking and frying, using special pressure fryers made for this purpose (see the pressure frying article) where the chicken juices supply the water. Cooking time is reduced substantially, but the breading texture is much softer (less crispy) than that of deep-fried chicken, because moisture remains in the breading. Thick sauces will not create pressure and will just burn onto the interior base of the pressure cooker after prolonged heating, therefore sauces must be thickened after pressure cooking.