Pressure cooking-Pressure cooker cooking

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 water's boiling point increases as the pressure increases, the pressure built up inside the cooker allows the liquid in the pot to rise to a temperature higher than 100 °C (212 °F) 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 volume pressure cookers are often referred as pressure canners, due to their capacity to hold jars used in 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.

Operation
Since pressure cooking depends on the pressure of steam, the process cannot easily be used for methods of cooking that produce fairly little steam, such as roasting, pan-frying or deep-frying. Most pressure cookers sold in the U.S. have an internal pressure setting of 15 psi, the standard determined by the USDA in 1917. At this pressure water boils at 125 °C (257 °F). The higher temperature causes the food to cook faster. Cooking times can be reduced by a factor of 70 percent. For example, shredded cabbage is cooked in one minute, fresh green beans in three minutes, small to medium-sized potatoes cook in five minutes (depending on thickness and type), and a whole "roast" chicken takes only twenty minutes. It is often used to simulate the effects of long braising or simmering in shorter periods of time. Some pressure cookers do not reach the standard 15 psi pressure and cooking times may need to be increased slightly to compensate.<ref>http://missvickie.com/workshop/pressure.html</ref>

The materials used for making cookers are generally aluminium and stainless steel. The aluminium may be stamped and buffed or anodised, however aluminium pans should not be put in a dishwasher. Stainless steel pressure cookers range from the plain-jane variety at the low-end, to the more expensive modern units that are made like the best cookware with heavy, three-ply, or copper-clad bottoms for uniform heating. Most modern pressure cookers are dishwasher safe, although some manufacturers may recommend washing by hand. A gasket forms an airtight seal which does not allow air or steam to escape between the pan and the lid, the only way the steam can escape is through a regulator on the lid when the pressure has built up (or if the regulator is blocked, through a safety valve). Sometimes the gasket is referred to as a sealing ring.