Pressure 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 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 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
The food to be made is placed in the pressure cooker, along with some amount of water. The vessel is then sealed and placed on a heat source (e.g. a stove). When the water reaches the boiling point at atmospheric pressure it begins to boil, but since the produced steam cannot escape the pressure rises, consequently raising the internal boiling point. Once the pressure increases to the designed amount above air pressure a relief valve opens, releasing steam and preventing the pressure from rising any further. Since pressure cooking depends on the production 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 about 100 kPa (15 psi) over atmospheric pressure, the standard determined by the USDA in 1917. At around 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; e.g., cooking times can be reduced by 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. 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, or can be adjusted to different maximum pressures; cooking times will vary accordingly.

Pressure cookers are generally made from aluminium or stainless steel. The former may be stamped and buffed or anodised, but this metal is unsuitable for the dishwasher. Expensive 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 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.

The pressure cooker presents several advantages. Foods are cooked much faster than other methods, (except perhaps the microwave), so dishes can be ready sooner. Also, less energy is required than when boiling, steaming or oven cooking, particularly if multiple foods are cooked at once. This also reduces washing up - a pressure cooker is also easy to clean after cooking.

A pressure cooker is more hygienic compared to open cooking in a pot or utensil. The kitchen is kept cleaner because, compared to traditional, open boiling, almost no steam and oils escape to the atmosphere - to end up deposited on the walls. Also, the food is cooked above the boiling point of water, killing more germs. (But note that some toxins are thermostable, and will not be neutralised. Most foods must be cooked for at least 15 minutes to be sure that reasonable sterilisation has been achieved). The pressure cooker can also be used as an ad hoc steriliser - for jam pots, for example.

Some claim that the pressure cooker is easy to cook with in comparison to other modern gadgets - it is certainly versatile. Pressure cookers can be used to prepare a wide variety of different recipes, covering most cooking styles and foods. The pressure cooker pan can be used as an ordinary saucepan for cooking larger quantities of food, reducing the number of utensils required. Due to its tight lid, it can be used on board boats and other moving vehicles, and to store cooked food for short times

Finally, the pressure cooker allows cooking 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 beverages.

Safety features
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 mechanisms, as well as some additional safety features required for UL approval or the equivalent in other countries, such as an interlock to prevent opening the lid while internal pressure exceeds atmospheric pressure.

Modern pressure cookers employ several safety features, such as a lid interlock and a gauge to indicate when the cooker is pressurized. The pressure cannot build up unless the lid is properly closed and locked in place.

A regulator releases steam when the pressure exceeds the designed pressure for the cooker; this 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 escape. There is usually a backup pressure release mechanism (a safety valve of sorts) that may employ any of a number of 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. If the internal temperature (and hence pressure) gets too high, the metal plug will melt, resulting in release of pressure. Another technique takes the form of a rubber grommet with a metal insert at the center. At a sufficiently high pressure, the grommet will distort and blow out of its mounting hole, rapidly releasing the pressure. A common safety feature is the design of the gasket, which expands and releases excess pressure downwards between the lid and the pan (if the pressure cooker is heated on a gas burner, this downward release of pressure can be strong enough to extinguish the gas flames). In some pressure cookers, excess pressure forces the pressure indicator above its housing which releases the pressure vertically 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 altitude. Under these circumstances water boils at temperatures significantly below 100 °C and without the use of a pressure cooker, may leave boiled foods undercooked, as described in Charles Darwin's 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.  I found out this, by overhearing my two companions discussing the cause, they had come to the simple conclusion, "that the cursed pot [which was a new one] did not choose to boil potatoes.""

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 12% to 22% of the level in the uncooked rice.