Pressure cooking

"Pressure cooker" redirects here. For other uses, see Pressure cooker (disambiguation).

A pressure cooker: the regulator is a weight on a nozzle next to the handle on the lid

Pressure cooking is the process of cooking food, using water or other cooking liquid, in a sealed vessel—known as a pressure cooker, which does not permit air or liquids to escape below a pre-set pressure. Pressure cookers are used for cooking food quicker than conventional cooking methods, which also saves energy.

Pressure cookers heat food quickly because the internal steam pressure from the boiling liquid causes saturated steam (or "wet steam") to bombard and permeate the food. Thus, higher temperature water vapour (i.e., increased energy), which transfers heat more rapidly compared to dry air, cooks food very quickly.

Pressure cooking allows food to be cooked with greater humidity and higher temperatures than possible with conventional boiling or steaming methods. In an ordinary non-pressurised cooking vessel, the boiling point of water is 100 °C (212 °F) at standard pressure; the temperature of food is limited by the boiling point of water because excess heat causes boiling water to vaporize into steam. In a sealed pressure cooker, the boiling point of water increases as the pressure rises, resulting in superheated water. At a pressure of 15 psi (103 kPa) above atmospheric pressure, water in a pressure cooker can reach a temperature of up to 121 °C (250 °F).

Pressure is created initially by boiling a liquid such as water or broth inside the closed pressure cooker. The trapped steam increases the internal pressure and temperature. After use, the pressure is slowly released so that the vessel can be safely opened.

Pressure cooking can be used to quickly simulate the effects of long braising or simmering.

Almost any food which can be cooked in steam or water-based liquids can be cooked in a pressure cooker.

History

A pressure cooker manufactured by Georg Gutbrod, Stuttgart, in about 1864

In 1679, French physicist Denis Papin, better known for his studies on steam, invented the steam digester in an attempt to reduce the cooking time of food. His airtight cooker used steam pressure to raise the water's boiling point, thus resulting in a much quicker cooking. In 1681, Papin presented his invention to the Royal Society of London, but his invention was treated as a scientific study. They granted him permission to become a member of the society afterwards.

In 1864, Georg Gutbrod of Stuttgart began manufacturing pressure cookers made of tinned cast iron.

In 1919, Spain granted a patent for the pressure cooker to Jose Alix Martínez from Zaragoza. Martínez named it the olla exprés (literally "express cooking pot") under patent number 71143 in the Boletín Oficial de la Propiedad Industrial.

In 1938, Alfred Vischler presented his invention, the Flex-Seal Speed Cooker, in New York City. Vischler's pressure cooker was the first one designed for home use, and its success led to competition among American and European manufacturers. At the 1939 New York World's Fair, National Presto Industries, known as the "National Pressure Cooker Company" at the time, introduced its own pressure cooker.

Variants

Large pressure cookers are often called pressure canners in the United States because of their capacity to hold jars used in canning. Pressure canners are specifically designed for home canning, whereas ordinary pressure cookers are not recommended for canning due to the risk of botulism poisoning, because pressure canners hold heat and pressure for much longer than ordinary pressure cookers and these factors are a critical part of the total processing time required to destroy harmful microorganisms. An autoclave is a type of pressure cooker used by laboratories and hospitals to sterilize equipment. Pressure fryers are used for deep fat frying under pressure — note that ordinary pressure cookers are not suitable for pressure frying. In the food industry, pressure cookers are often referred to as retorts or canning retorts.

Design

Parts

Portable pressure cookers consist of all or most of these basic component parts, depending on the manufacturer and model of pressure cooker:

Pan

• Metal pan body
• Pan handles, usually one each on opposite ends, for carrying the cooker with both hands

Lid

• Lid handle, usually with a locking device button or slider which "clicks" shut
• Gasket (also known as a "sealing ring") which seals the cooker airtight
• Steam vent with a pressure regulator on top (either a weight or spring device) which maintains the pressure level in the pan
• Pressure indicator pin, for showing the presence or absence of any pressure, however slight
• Safety devices on the lid

Accessories

• Steamer basket
• Trivet for keeping the steamer basket above liquid
• Metal divider, for separating different foods in the steamer basket e.g. vegetables

Pressure cookers are typically made of aluminum (aluminium) or stainless steel. Aluminum pressure cookers may be stamped, polished, or anodized, but aluminum cookers are unsuitable for the dishwasher. Aluminium pressure cookers are cheaper, but the aluminium is reactive to acidic foods (changes the flavour) and less durable than stainless steel pressure cookers.

Higher-quality stainless steel pressure cookers are made with heavy, three-layer, or copper-clad bottoms (heat spreader) for uniform heating because stainless steel has lower thermal conductivity. Most modern stainless steel cookers are dishwasher safe, although some manufacturers may recommend washing by hand. Some pressure cookers have a non-stick interior.

In most models, a gasket or sealing ring, made from either rubber or silicone, forms a gas-tight seal that does not allow air or steam to escape between the lid and pan; more expensive models feature a metal-to-metal seal. Normally, the only way steam can escape is through a regulator on the lid while the cooker is pressurized. If the regulator becomes blocked, a safety valve provides a backup escape route for steam. A loose-fitting rubber plug in the lid, held in place by steam pressure, provides a simple safety valve. If the pressure exceeds design limits, the plug pops out to depressurize the pot.

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 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 over-sized oval lids. With such cookers, one inserts the lid at an angle, then turns the lid to align it with the pot opening on top because the lid is larger than the opening. A spring arrangement straddles the top of the cooker and holds the lid in place. While cooking, the internal pressurized steam keeps the lid tightly in place, preventing accidental removal.

Because of the forces that pressure cookers must withstand, they are usually heavier than conventional pots of similar size.

Generations

There are three generations of pressure cookers:

First generation

Super cocotte décor SEB, 1973. Aluminium body, polyamide lacquered with an embossed aluminium lid and a stainless steel stirrup. On display at the Musée gallo-romain de Fourvière, Lyon. 18/10.

Also known as "old type" pressure cookers,^{[citation needed]} these operate with a weight-modified or "jiggly" valve, which releases pressure during operation. Some people might consider them loud or very loud because the weight-modified valve operates similarly to the piston in a steam engine. They typically offer only one pressure level—with the exception of some newer "old style" pressure cookers that allow the operator to change the weight of the weight-modified valve.

Second generation

Also known as "new" or "latest generation" pressure cookers,^{[citation needed]} these operate with a spring-loaded valve that is often hidden from view in a proprietary mechanism. This generation is characterized by two or more pressure settings. Some of these pressure cookers do not release any steam during operation (non-venting) and instead use a rising indicator with markings to show the pressure level. These only release steam when the pan is opened, or as a safety precaution if the heat source is not reduced enough when the pan reaches the required cooking pressure. Others use a dial that the operator can advance by a few clicks (which adjusts a spring underneath) to change the pressure setting or release pressure; these release steam during operation (venting).

Electric pressure cookers

Called "third generation" pressure cookers by their manufacturers,^{[citation needed]} these include an electric heat source that is automatically regulated to maintain the operating pressure. They also include a spring-loaded valve (as described above). Two or more pressure settings are available on this type of pressure cooker, along with features such as a timer and a setting to keep food warm. However, this pressure cooker type cannot be opened with a cold water quick-release method and should be operated with caution when releasing vapour through the valve, especially while cooking foamy foods and liquids (lentils, beans, grains, milk, gravy, etc.).

Pressure settings

Most pressure cookers have a working pressure setting of 15 pounds per square inch (psi), sometimes expressed as "lb" or "lbs" (pounds), which equates to 103 kPa, 1.03 bar, or 1 kg per square centimetre (kg/cm² or kgf/cm²) above atmospheric pressure. This standard was determined by the United States Department of Agriculture in 1917. At this pressure, water boils at 121 °C (250 °F) (described in vapour pressure of water article).

The higher temperature causes food to cook faster; cooking times can typically be reduced to one-third of the time for conventional cooking methods. The actual cooking time also depends on the pressure release method used after timing (see Pressure release methods for details) and the thickness and density of the food, since thicker (and denser) foods take longer to cook. Meat joints and some other foods like sponge puddings and Christmas puddings are typically timed according to their weight. Frozen foods need extra cooking time to allow for thawing.

When pressure cooking at 15 psi, approximate cooking times are one minute for shredded cabbage, ten minutes for potatoes cut into one-inch pieces (steamed or boiled) and three minutes for fresh green beans, assuming that the pressure is immediately released after timing and not left to drop naturally.

If the pressure is allowed to slowly drop after timing (see Pressure release methods for details), cooking times are even shorter. Examples of approximate cooking times using this method: potatoes cut into 1-inch pieces, seven minutes (potatoes suitable for boiling or steaming won't break up if the pressure is left to drop slowly after timing); a whole chicken, which easily fits into all but small pressure cookers, cooks in about twenty minutes. Stews can be cooked in under one hour, compared to several hours in a slow cooker or oven. Cooking times can be shortened if the food is cut into smaller pieces.

Some recipes may require cooking at lower than 15 psi and many pressure cookers have 2 or more selectable pressure settings or weights.

Non-standard pressure settings

Some pressure cookers have a lower maximum pressure than the industry standard 15 psi or can be adjusted to different maximum pressures for some recipes; cooking times will increase or decrease accordingly. This is typically done by having different regulator weights or different pressure settings. If the pressure cooker is not capable of cooking at the industry standard 15 psi and the recipe is intended for cooking at 15 psi, the cooking time will need to be extended (e.g. the cooking time for 15 psi multiplied by 1.3), but this can overcook food. The manufacturer's instruction booklet should state the maximum operating pressure, possibly using different pressure units instead of the traditional "psi" rating (these pressure units are mentioned above).

Below 15 psi, the alternative pressure units and values are: kPa below 100, bar below 1, kgcm² (or kgf/cm²) below 1. As an example, a pressure cooker with a maximum operating pressure of 0.8 bar equates to only 11.6 psi. Increasing the heat under pressure cookers designed for pressures below 15 psi will not make the food cook faster; it will only waste energy, possibly spoil food, increase the risk of boiling dry, and reduce the life of the gasket and other rubber parts. Virtually all electric pressure cookers operate below 15 psi.

Operation

Food is placed inside the pressure cooker with a small amount of water or other liquid (e.g., stock required for the recipe). Food is either cooked in the liquid or placed above the liquid to be steamed; the latter method prevents the transfer of flavours from the liquid. Steaming is done by using a suitable insert such as a metal steamer basket with a support trivet. The lid is closed, the pressure setting is selected, and the pressure cooker is then placed on a heat source such as a stove at the highest heat setting. For pressure cookers with a weight, the weight is placed over the steam vent pipe while steam is being emitted to ensure the air inside has escaped. Once the cooker reaches full pressure, the heat is lowered to maintain pressure; the timing specified in the recipe begins at this point. It takes several minutes for the pressure cooker to reach the selected pressure level. It can take as long as 10 minutes depending on: the quantity of food, the temperature of the food (cold or frozen food delays pressurisation), the amount of liquid, and the size of the pressure cooker.

Using boiling water from a kettle can quicken the heating process, unless the recipe calls for cold water. For example, boiled potatoes cook best when added to cold water because this prevents the potatoes from breaking up. Boiled potatoes also require the pressure to drop naturally after timing instead of being quickly released.

The newer generation pressure cookers, which have no weights, expel air from inside before reaching full pressure. A common mistake is for the user to start timing when the pop-up indicator rises, which happens when there is the slightest increase in pressure, instead of waiting for the cooker to reach the selected pressure level first.^{[citation needed]} Some pressure cookers have markers on the pop-up indicator that show the pressure level. However, the typical pop-up indicator only shows that the cooker has pressure inside, which does not reliably signal that the cooker has reached the selected pressure. This indicator often acts as an interlock to prevent the lid from being opened while there is internal pressure. Manufacturers may use their own terminology for this indicator, such as calling it a locking indicator.

As the internal temperature rises, the pressure also rises until it reaches the design gauge pressure. In some models, a relief valve subsequently opens, releasing steam to prevent the pressure from rising any further. In other models, the pressure regulator weight begins levitating above its nozzle, allowing excess steam to escape. At this stage, the heat source should be reduced because heat is only needed to maintain pressure. If the heat source is too high, then energy is wasted, too much liquid may be released to maintain the nominal pressure, and reduces the life of the gasket/sealing ring by wear. On the other hand, if the heat source is too low, the food may be undercooked or pressure may be lost. As a rough guide, medium heat is the highest setting required on any domestic stove to maintain the pressure level throughout cooking time.

Recipes for foods using raising agents (e.g., steamed puddings) require gentle pre-steaming, without pressure, in order to activate the raising agents prior to cooking and achieve a light, fluffy texture. The water must bubble gently while pre-steaming to ensure that enough water will be available for the entire pressure cooking time, otherwise the pan will boil dry. If pre-steaming is omitted, it can result in a heavy, stodgy texture.

Since pressure cooking depends on the production of steam from water, the process cannot easily be used for cooking methods that produce little steam such as roasting, pan frying, or deep frying. However, Kentucky Fried Chicken restaurants use a combination of pressure cooking and frying, with special pressure fryers in which chicken juices supply the water. Cooking time is reduced substantially, but the breading texture is much softer (less crispy) than deep-fried chicken since moisture remains in the breading. Thick sauces do not contain enough liquid to vaporize and create pressure, so they usually burn onto the interior base of the pressure cooker after prolonged heating. Sauces should normally be thickened after pressure cooking.

Food containers

Small containers such as plastic pudding containers, can be used in a pressure cooker, provided that the containers (and any covering used) can withstand temperatures of 130 °C (266 °F) and are not placed directly on the interior base. The containers can be used for cooking foods that are prone to burning on the base of the pressure cooker, which is directly in contact with the heat source. A lid for the container may be used, provided that the lid allows some steam to come into contact with the food and the lid is securely fitted (e.g., foil or greaseproof paper, pleated in the centre and tied securely with string). Cracked containers are not suitable. Cooking time is longer for covered containers because the food is not in direct contact with the steam. Since non-metal containers are poorer heat conductors, the type of container material stated in the recipe cannot be substituted without affecting the outcome. For example, if the recipe time is calculated using a stainless steel container and a plastic container is used instead, the food at the bottom of the container may be under-cooked, unless the cooking time is increased. Containers with thicker sides, e.g., oven-proof glass or ceramic containers, which are slower to conduct heat, will add about 10 minutes to the cooking time. Liquid can be added inside the container when pressure cooking foods such as rice, which need to absorb liquid in order to cook properly.

Pre-frying ingredients

The flavour of some foods, such as meat and onions, can be improved by gently cooking with a little pre-heated cooking oil, butter or other fat in the open pressure cooker over medium heat (unless the manufacturer advises against this) before pressure cooking. It is important not to overheat the empty pressure cooker and never heat the empty cooker with the lid and gasket in place. Overheating can cause warping and other damage. The pressure cooker should be allowed to cool briefly before adding liquid; otherwise some of the liquid will evaporate instantly, possibly leaving insufficient liquid for the entire pressure cooking time; if deglazing the pan, this has to be taken into account.

Pressure release methods

After cooking, there are three ways of releasing the pressure, either quickly or slowly, before the lid can be opened. Recipes for pressure cookers state which release method is required at the end of the cooking time for proper results. Failure to follow the recommendation may result in food that is under-cooked or over-cooked. Only one of these release methods is used after timing, as recommended in the recipe.

To avoid opening the pressure cooker too often while cooking different vegetables with varying cooking times, the vegetables that take longer to cook can be cut into smaller pieces and vegetables that cook faster can be cut into thicker pieces.

Cold water quick release

This method is the fastest way of releasing pressure with portable pressure cookers; it is recommended to read the manufacturer's instruction book, as some may advise against the cold water release or require it to be performed differently.

The cold water release method involves using slow running cold tap water, over the edge of the pressure cooker lid, being careful to avoid the steam vent or any other valves or outlets and never immersing the pressure cooker under water. It is most suitable for foods with short cooking times. It takes about 20 seconds for the cooker to cool down enough to lower the pressure so that it can be safely opened. This method is not suitable for electric pressure cookers.

Manual, normal, regular, or automatic release

This method is sometimes called a quick release, not to be confused with the cold water release. It involves the quick release of vapor by lifting (or removing) the valve, pushing a button, or turning a dial. It is most suitable to interrupt cooking to add food that cooks faster than what is already in the cooker. For example, since meat takes longer to cook than vegetables, it is necessary to add vegetables to stew later so that it will cook only for the last few minutes. Unlike the cold water release method, this release method does not cool down the pressure cooker. The user must release the steam with caution to avoid scalding injury. This release method is not suitable for foods that foam and froth while cooking; the hot contents might spray outwards due to the pressure released from the steam vent. This release method takes about two minutes to release the pressure before the lid can be opened.

Natural release

The natural release method allows the pressure to drop slowly; this is achieved by removing the pressure cooker from the heat source and allowing the pressure to lower without action. It takes approximately 10 to 15 minutes (possibly longer) for the pressure to disappear before the lid can be opened. On many pressure cookers, a coloured indicator pin will drop when the pressure has gone. This natural release method is recommended for foods that foam and froth during cooking, such as rice, legumes, or recipes with raising agents such as steamed puddings. The texture and tenderness of meat cooked in a pressure cooker can be improved by using the natural release method. The natural release method finishes cooking foods or recipes that have longer cooking times because the inside of the pressure cooker stays hot. The natural release method can be used with potatoes with skins to keep the skins intact if they have been deeply pierced before cooking. This method is not recommended for foods that require very short cooking times because the food will be overcooked. When using the natural release method for foods such as potatoes, the cooking time on the heat source is shorter than that of the other release methods. On the other hand, if the natural release method is not used, the cooking time will need to be extended by a few minutes to compensate.

Advantages

Foods cook much faster with pressure cooking than with other methods (except for small quantities in microwave ovens). Pressure cooking requires much less water than conventional boiling, so food can be ready sooner. Less energy is required than that of boiling, steaming, or oven cooking. Since less water or liquid has to be heated, the food reaches its cooking temperature faster. Using more liquid than necessary wastes energy because it takes longer to heat up; the liquid quantity is stated in the recipe. Pressure cookers can use much less liquid than the amount required for boiling or steaming in an ordinary saucepan. 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, as they would be if food were boiled in large amounts of water. Due to the shorter cooking time, vitamins are preserved relatively well during pressure cooking.

Several foods can be cooked together in the pressure cooker, either for the same amount of time or added later for different times. Manufacturers provide steamer baskets to allow more foods to be cooked together inside the pressure cooker.

Food is cooked at a temperature above the normal boiling point of water, killing most micro-organisms. A pressure cooker can be used as an effective sterilizer for jam pots, glass baby bottles, or for water while camping.

The pressure cooker speeds cooking considerably at high altitudes, where the lower atmospheric pressure reduces the boiling point of water. Lower water temperature reduces water's effectiveness for cooking or preparing hot drinks. The increased temperatures due to pressure cooking are also used to promote the Maillard reaction to develop more desirable flavor profiles that would not be obtainable using temperatures typical of boiling. The flavours are more concentrated in the higher temperature and sealed environment of the pressure cooker, so less seasoning is required.

Disadvantages

Pressure cookers are considerably more expensive than conventional saucepans of the same size. The additional gasket (sealing ring) requires special care when cleaning (e.g., not washed with kitchen knives), unlike a standard lid for a saucepan. Food debris must be cleaned from the gasket after every use to ensure a good seal. The gasket/sealing ring must be replaced if there are any signs of wear (e.g., a small split). If the gasket cannot provide an airtight seal and steam leaks around the lid, even when the gasket is fitted properly, the pressure cooker may not build up pressure inside. This can be caused by a defective gasket or if the rim of the pressure cooker is damaged in any way. The gasket should not be allowed to dry out because this makes it harder to open and close the lid; however, the gasket can be smeared lightly with vegetable oil to help alleviate this problem (using too much lubrication can make the gasket swell and it may not fit properly).

In order to inspect the food, the pressure cooker needs to be opened, which halts the cooking process. With a conventional saucepan, this can be done in a matter of seconds by visually inspecting the food. As a result, accurate timing is essential to the recipe and the recommended cooking time for the food must be monitored e.g. with an audible timer.

The increased weight of conventional pressure cookers makes them unsuitable for applications in which saving weight is a priority, such as camping. However, small, lightweight pressure cookers are available for mountain climbers (see Use at high altitudes).

A minimum quantity of liquid is required to create and maintain pressure, as indicated in the manufacturer's instruction manual. More liquid is required for longer cooking times. This is not desirable for food requiring much less liquid, but recipes and books for pressure cookers take this into account.

Safety features

Early pressure cookers equipped with only a primary safety valve risked explosion from food blocking the release valve. On modern pressure cookers, food residues blocking the steam vent or the liquid boiling dry would trigger additional safety devices. Modern pressure cookers sold from reputable manufacturers have sufficient safety features to prevent the pressure cooker itself from exploding. When excess pressure is released by a safety mechanism, debris of the food being cooked may also be ejected with the steam — sometimes forcefully with some noise. This can be avoided if the pressure cooker is regularly cleaned and maintained in accordance with the manufacturer's instructions and never overfilled with food and/or liquid. A pressure cooker should never be filled to more than two-thirds of its interior height with solid food, half full for liquids and foods that foam and froth (e.g., rice, pasta), and no more than one-thirds full for pulses (e.g., lentils). A tablespoon of cooking oil can be added to minimise foaming.^p. 262

Modern pressure cookers typically have two or three redundant safety valves and some additional safety features, such as an interlock lid that prevents the user from opening the lid when the internal pressure exceeds atmospheric pressure, preventing accidents from a sudden release of hot liquid, steam and food. If safety mechanisms are not correctly in place, the cooker will not pressurize the contents. Pressure cookers should be operated only after reading the instruction manual because each model has slightly different safety mechanisms.

For first generation pressure cookers with a weighted valve or "jiggler", the primary safety valve or regulator is usually a weighted stopper, commonly called "the rocker" or "vent weight". This weighted stopper is lifted by the steam pressure, allowing excess pressure to be released. There is a backup pressure release mechanism that releases pressure quickly if the primary pressure release mechanism fails (e.g., food jams the steam discharge path). One such method is a hole in the lid that is blocked by a low melting point alloy plug and 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 to release pressure. If the pressure continues to increase, the grommet itself will blow out to release pressure. These safety devices usually require replacement if they are activated by excess pressure. Newer pressure cookers may have a self-resettable spring device, fixed onto the lid, that releases excess pressure.

For second generation pressure cookers with selectable pressure and (often hidden) spring valve operation, a common safety feature is the gasket, which expands and releases excess pressure downward between the lid and the pot. This release of excess pressure is forceful and sufficient to extinguish the flame of a gas stove.

Pressure cookers sold in the European Union (EU) must comply with the Pressure Equipment Directive.

Use at high altitudes

A pressure cooker can be used to compensate for lower atmospheric pressure at high elevations. The boiling point of water drops by approximately 1°C per every 294 metres of altitude (1°F per every 540 feet of altitude), causing the boiling point of water to be significantly below the 100 °C (212 °F) at standard pressure. Without the use of a pressure cooker, boiled foods may be undercooked, as described in Charles Darwin's The Voyage of the Beagle (chapter XV, March 20, 1835):

Having crossed the Peuquenes [Piuquenes], we descended into a mountainous country, intermediate between the two main ranges, and then took up our quarters for the night. We were now in the republic of Mendoza. The elevation was probably not under 11,000 feet [...]. 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.

At higher altitudes, the boiling point of liquid in the pressure cooker will be slightly lower than it would be at sea level. When pressure cooking at high altitudes, cooking times need to be increased by approximately 5% for every 300 metres (1,000 ft) above 610 metres (2,000 ft) elevation.

Lightweight pressure cookers as small as 1.5 litres (0.40 US gal) weighing 1.28 kilograms (2.8 lb) are available for mountain climbers. Sherpas often use pressure cookers in base camp.

Science of pressure cooking

The boiling point of water (and water-based liquids) is determined by the ambient pressure. Pressure cookers always require liquid in order to cook food under pressure. At atmospheric pressure and at sea level, the boiling point of water is 100 °C (212 °F) and excess heat only increases the rate at which water evaporates into steam vapour; more heat does not increase the temperature of the water. At higher altitudes above sea level, the atmospheric pressure is lower and thus the boiling point of water is lower, because the lower pressure pushing on the water makes it easier for the water molecules to escape to the surface compared to higher pressure. Inside a pressure cooker, once the water (liquid) is boiling and the steam is trapped, the pressure from the trapped steam increases and this pushes on the liquid, which increases its boiling point, because it becomes harder for the water molecules to escape from the surface as the pressure increases on it. The heat applied to the liquid by the heat source continues to create more steam pressure and the extra heat also raises the temperature of the liquid under this increased pressure. Both the liquid and steam are at the same temperature. Once the selected pressure level is reached, the pressure regulator on the lid indicates this and now the heat source can be lowered to maintain that pressure level and save energy, since extra heat will not increase the temperature of the liquid if the pressure is not allowed to rise — excess pressure will only escape as fast-flowing steam from the lid.

Steam and liquids transfer heat more rapidly than dry air. As an example, the hot air inside an oven at, say 200 °C (392 °F), won't immediately burn your skin, but the wet steam from a boiling kettle at 100 °C (212 °F) will scald your skin almost instantly and 'feel' hotter, despite the steam (and water) in the kettle being at a lower temperature than the air inside a hot oven. Since steam and liquids heat substances faster, the pressure cooker can cook food quicker — under pressure—compared to ordinary cooking methods.

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 12–22% of the amount in the uncooked rice. Pressure cookers are not guaranteed to destroy all harmful microorganisms in food, especially when used for short periods of time.

Foods unsuitable for pressure cooking

Some foods are not recommended for pressure cooking in liquid. Such foods could expand too much, froth, and sputter, which can result in blocking the steam vent. Examples include macaroni, cranberries, and cereals such as oatmeal.

Use in terrorism

Main article: Pressure cooker bomb

The appliance has been adapted as a crude type of bomb, which has been used in terrorist attacks.

• 2006 Mumbai train bombings
• 2010 Stockholm bombings (failed to explode)
• 2010 Times Square car bombing attempt (failed to explode)
• 2013 Boston Marathon bombings

See also

• Pressure frying
• Rice cooker, many employ pressure cooking

External links

• hip pressure cooking – Recipes, tips and cooking times for pressure cooking
• Science of Pressure Cooking