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 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.
Design
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.
Advantages
Foods are cooked much faster by pressure cooking than by other methods, (except for small quantities in microwave ovens) 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.
Several foods can be cooked together in the pressure cooker, either for the same amount of time or added later and timed accordingly. Manufacturers provide steamer baskets to allow more food to be cooked together inside the pressure cooker. However, the pressure cooker should never be filled with more than 2/3 its height with solid food or 1/2 full for foods that foam and froth, e.g., rice, dried beans, pasta, etc. A tablespoon of cooking oil can be added to minimise foaming.
The food is cooked at a temperature above the normal boiling point of water, killing most microorganisms. 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: The minimum quantity of water or liquid used in the recipe 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 it is healthier than other cooking methods. Using more liquid than necessary simply wastes energy and takes longer to reach boiling point, necessary to pressurise the cooker.
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.
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