Pressure cooking is a method of cooking in a sealed vessel that does not permit air or liquids to escape below a preset pressure. Because the boiling point of water increases as the pressure increases, the pressure built up inside the cooker allows the liquid in the pot to rise to a higher temperature before boiling.
Pressure cookers may be referred to by several other names. An early pressure cooker, called a steam digester, was invented by Denis Papin, a French physicist, in 1679. Large pressure cookers are often called pressure canners in the United States, due to their capacity to hold jars used in home canning. A version of a pressure cooker used by laboratories and hospitals to sterilize materials is known as an autoclave. In the food industry, pressure cookers are often referred to as retorts.
A gasket or sealing ring forms an gas-tight seal which does not allow air or steam to escape between the pot and the lid; normally, the only way the steam can escape is through a regulator on the lid when the pressure has built up. In case the regulator is blocked, a safety valve is provided as a backup escape route for steam. The simplest safety valve is a loose-fitting rubber plug in the lid, held in place by steam pressure. If the pressure exceeds design limits, the plug pops out of its seat.
To seal the gasket, some pressure cookers have a lid lock with flanges, similar to a bayonet-style lens mount, that works by placing the lid on the pot and twisting it about 30° to lock it in place. Contemporary designs of this style of cooker also have a pressure-activated interlock mechanism that prevents the lid from being removed while the cooker is pressurized.
Other cookers, particularly the larger types used for home canning, have oval, oversized lids. With these, since the lid is larger than the opening in the top of the pressure cooker, one inserts the lid at an angle, then turns the lid to align it with pot opening. A spring arrangement straddles the top of the cooker and holds the lid in place. When cooking, the pressurized steam inside keeps the lid tightly in place, preventing accidental removal.
Pressure cookers are usually heavy, because they need to be strong. However, some pressure cookers are manufactured for camping, and can be as light as 1.2 kg for a four-litre pot.
Most pressure cookers have a working pressure setting of about one atmosphere (100 kPa or 15 psi gauge 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 125 °C (257 °F).
The higher temperature causes the food to cook faster; cooking times can typically be reduced by about 70 percent. For example, shredded cabbage is cooked in one minute, fresh green beans in three minutes, small to medium-sized potatoes cook in about eight minutes (depending on thickness and type), and a whole chicken takes only twenty minutes. Brown rice and lentils and beans can be cooked in ten minutes instead of 45.
Some pressure cookers have a lower maximum pressure, or can be adjusted to different maximum pressures; cooking times will vary accordingly. This is often done by having different regulator weights.
The food is cooked above the boiling point of water, killing bacteria and viruses. The pressure cooker can also be used as an effective sterilizer, for jam pots and glass baby bottles for example, or for water while camping.
With pressure cooking, heat is very evenly, deeply, and quickly distributed.
It is not necessary to immerse food in water: Enough water to keep the pressure cooker filled with steam is sufficient. Because of this, vitamins and minerals are not leached (dissolved) away by water. Since steam surrounds the food, foods are not oxidized by air exposure at heat, so asparagus, broccoli, and so on retain their bright green colors and vitamins.
The pressure cooker speeds cooking considerably at high altitudes, where the low atmospheric pressure otherwise reduces the boiling point of water and hence reduces water's effectiveness for cooking or preparing hot drinks.
The primary safety valve or regulator usually takes the form of a weighted stopper, commonly called "the rocker," or "vent weight". This weighted stopper is lifted by the steam pressure, allowing excess pressure to be relieved. There is a backup pressure release mechanism that may employ any of several different techniques to release pressure quickly if the primary pressure release mechanism fails (for example, if food jams the steam discharge path). One such method is in the form of a hole in the lid blocked by a plug of low melting point alloy; another is a rubber grommet with a metal insert at the center. At a sufficiently high pressure, the grommet will distort and the insert will blow out of its mounting hole, relieving the pressure. If the pressure gets still higher, the grommet itself will blow out. A common safety feature is the design of the gasket, which expands and releases excess pressure downward between the lid and the pot.
In some pressure cookers, excess pressure forces the pressure indicator above its housing which releases the pressure vertically upwards.
There is a lightweight camping/mountaineering pressure cooker which weighs little more than a standard camping pot.