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.
Pressure cookers are generally made from aluminium or stainless steel. The former may be stamped and buffed or anodized, but this metal is unsuitable for the dishwasher. Higher quality stainless steel pressure cookers are made with heavy, three-ply, or copper-clad bottom (heat spreader) for uniform heating, since stainless steel has lower thermal conductivity. Most modern units are dishwasher safe, although some manufacturers may recommend washing by hand.
A gasket or sealing ring forms a 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.
The food to be cooked is placed in the pressure cooker, with a small amount of water. The lid is closed, the pressure setting selected (or if a weight is used, the weight is placed on the steam vent) and the pressure cooker is placed on a heat source, e.g., a stove, at the highest heat until the cooker reaches full pressure, then the heat is turned down and timing the recipe begins at this point. 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.
Most pressure cookers have a working pressure setting of 15 psi (approx. 107 kPa) 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 122 °C (252 °F) (refer vapor pressure of water).
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 than 15 psi, or can be adjusted to different maximum pressures; cooking times will vary accordingly. This is often done by having different regulator weights or different pressure settings.
Since pressure cooking depends on the production of steam, 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 (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.
Foods are cooked much faster by pressure cooking than by other methods, (except for small quantities in microwaves) and with much less water used than boiling, so dishes can be ready sooner. Less energy is required than when boiling, steaming or oven cooking. Since less water is necessary, the foods come to cooking temperature faster.
The food is cooked at a temperature above the normal 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, and thus is healthier than other cooking methods.
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.
Pressure cookers have a reputation as a dangerous method of cooking with the risk of explosion. Early pressure cookers equipped with only a primary safety valve were at risk of explosion if poorly maintained, allowing food residues to contaminate the release valve. Modern pressure cookers typically have two or three independent safety valves, as well as some additional safety features, such as an interlock to prevent opening the lid while internal pressure exceeds atmospheric pressure. However there is still a risk of explosion, especially if cookers are not thoroughly and regularly maintained.
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 upwards.
A pressure cooker is often used by mountain climbers to compensate for the low atmospheric pressure at a very high elevation. Under these circumstances water boils at temperatures significantly below 100 °C (212 °F) and, without the use of a pressure cooker, may leave boiled foods undercooked, as described in Charles Darwin's The Voyage of the Beagle:
At the place where we slept water necessarily boiled, from the diminished pressure of the atmosphere, at a lower temperature than it does in a less lofty country; the case being the converse of that of a Papin's digester. Hence the potatoes, after remaining for some hours in the boiling water, were nearly as hard as ever. The pot was left on the fire all night, and next morning it was boiled again, but yet the potatoes were not cooked."
Lightweight camping/mountaineering pressure cookers are available.
Some food toxins can be reduced by pressure cooking. A Korean study of aflatoxins in rice (associated with Aspergillus fungus) showed that pressure cooking was capable of reducing aflatoxin concentrations to between 12 and 22 % of the amount in the uncooked rice.