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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 1.2kg 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 122C (252F) (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. Pressure cooking is often used to simulate the effects of long braising or simmering in shorter periods of time. 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. 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. Use at high altitudes 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 100C (212F) 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.[citation needed] Use in food detoxification 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. Rice cooker, many employ pressure cooking If the lid could be removed while the contents are boiling under pressure, it would result in a dangerous frothing and explosion of superheated liquid, likely to severely scald any person in the vicinity of the cooker. All About Pressure Settings Food for Thought: Putting the Pressure on Poisons, Science News Online, April 15, 2006[dead link] Media related to Pressure cookers at Wikimedia Commons Wikibooks Cookbook has a recipe/module on Dry roasting Grilling (la Plancha) Hot salt frying Hot sand frying Searing Baking Grill-roasting Grill-baking Roasting (modern) Smoking Charbroiling Broiling Grilling (gridiron) Toasting Roasting (traditional) Rotisserie Blanching Boiling Decoction Parboiling Coddling Creaming Infusion Poaching Simmering Slow cooker Steeping Stewing Bain-marie Double boiling Double steaming Steaming Sous-vide Browning Blackening Frying (Deep frying Pan frying Stir frying (bao) Shallow frying) Sauting Caramelizing Gentle frying Sweating Barbecuing Braising Griddling Stir frying (chao) Clay pot cooking Earth oven Microwaving Pressure cooking Pressure frying Roman oven/Tandoor Vacuum flask/Haybox cooking Pickling Souring Fermentation Curing Food preparation Food preservation Food safety Categories: Cooking techniquesHidden categories: All articles with dead external links Articles with dead external links from June 2009 Articles needing additional references from September 2008 All articles needing additional references All articles with unsourced statements Articles with unsourced statements from January 2010 We are high quality suppliers, our products such as China Yag Laser Machine , Freckle Removal Manufacturer for oversee buyer. 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