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 the boiling point of water 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 is created at the beginning by boiling a liquid, such as water or broth, inside the closed pressure cooker and then trapping the steam. This increases the internal pressure and temperature, which is maintained throughout the cooking time. Afterward, the pressure is slowly released so that the vessel can be safely opened.
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 because of their capacity to hold jars used in canning. A version of a pressure cooker used by laboratories and hospitals to sterilize objects is known as an autoclave. In the food industry, pressure cookers are often referred to as retorts or "canning retorts".
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The first experiment to cook under pressure was done by French physicist Denis Papin in 1679, better known for his studies on steam, when he invented the digesteur in an attempt to reduce the cooking time of food. This hermetic cooker used steam pressure to increase the water boiling point, thus resulting in a much quicker cooking. In 1681 Denis Papin presented his invention to the Royal Society of London, but the feature didn´t go forward and was left as another scientific study. It granted him permission to become a member of the society on his own right. But it wasn´t up until the 20th century, where the invention was made a reality and the very first pressure cookings were made.
In 1919, the first patent ever was granted in Spain to Jose Alix Martínez from Zaragoza, of what was called olla exprés or pressure cooker. Under the 71143 patent number of the Boletín Oficial de la Propiedad Industrial in the 798 issue on the 16 of November of 1919, page 1480.
Alfred Vischler waited until 1938 to present his invention called Flex-Seal Speed Cooker in New York. Enven though Vischler´s idea was a good one, the product that catched the attetion of housewives was the one called Presto, that appeared in 1939 in World Trade Fair in New York.
Pressure cookers are generally made of aluminum 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 bottoms (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. Some pressure cookers have a non-stick interior.
In most models, 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; other more expensive models feature a metal-to-metal seal. Normally, the only way the steam can escape is through a regulator on the lid when the pressure has built up. If the regulator becomes 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, over-sized 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.
Because of the forces they have to withstand, pressure cookers are usually heavier than normal pots of similar size.
There are three generations of pressure cookers:
Also known as "old type" pressure cookers, these operate with a weight-modified or "jiggly" valve which release pressure during operation. They are very loud because the weight-modified valve operates under the same principle as a piston in a steam engine. They often offer only one pressure level - with the exception of some newer "old style" pressure cookers which allow the operator to change the weight of the weight-modified valve.
Also known as "new" or "latest generation" pressure cookers, these operate with a spring-loaded valve which is often hidden from view in a proprietary mechanism. Some of these pressure cookers do not release any steam vapour during operation, but only when the pan is opened, or as a safety precaution if the heat source has not been reduced enough when the pan reaches the required cooking pressure. Two or more pressure settings are also a mark of this generation. Some second generation pressure cookers use a rising indicator with marks to show the pressure level; others use a dial which the operator can advance by a few clicks (which adjusts a spring underneath) in order to change the pressure setting or release the pressure; steam vapour is released during operation.
Called "third generation" pressure cookers by their manufacturers, these include an electric heat source which is automatically regulated to maintain the operating pressure. They also include a spring-loaded valve as described above. Two or more pressure settings are also available on this type of pressure cooker. However, this pressure cooker type cannot be opened with the cold water quick-release method and should be operated with caution when releasing vapour through the valve, especially when cooking foaming foods and liquids (lentils, beans, grains, milk, gravy, etc.).
The food to be cooked is placed in the pressure cooker along 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 cooked by steam - this is done by using a suitable insert such as a steamer basket with a support trivet. The lid is closed, the pressure setting selected and the pressure cooker is placed on a heat source, e.g., a stove, at the highest heat (if using a pressure cooker with a weight, the weight is placed over the steam vent pipe when steam is being emitted, as this ensures the air inside has escaped) until the cooker reaches full pressure, after which the heat is lowered to maintain pressure; the timing specified in the recipe begins at this point.
The newer generation pressure cookers, without weights, expel the air inside before reaching full pressure. A common mistake is for the user to start timing when the pop-up indicator rises as soon as there is the slightest amount of pressure in the cooker, instead of waiting for the cooker to first reach the selected pressure level. Some pressure cookers have markers on the pop-up indicator which show the pressure level, but normally the pop-up indicator just shows that the cooker has pressure inside, which is not a reliable means of showing that the cooker has reached its selected pressure. This indicator also acts as an interlock to prevent the lid from being opened as long as there is any pressure inside.
As the internal temperature rises, the pressure also rises until it reaches the design gauge pressure. In some models, a relief valve then opens, releasing steam in order to prevent the pressure from rising any further. In other models, the pressure regulator weight begins levitating on its nozzle, allowing excess steam to escape. At this stage, the heat source should be reduced, as now only enough heat is required to maintain pressure. If the heat source is too high, costly energy is wasted, too much of the liquid may be released in order to maintain the nominal pressure, and the life of the gasket/sealing ring may be reduced by wear caused by this extra heat. On the other hand, if the heat source is set too low, the food may be undercooked or pressure may be lost.
Most pressure cookers have a working pressure setting of 15 psi, which equates to 103 kPa or 1.03 bar[1] 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 121 °C (250 °F) (refer to the vapour pressure of water page).
The higher temperature causes the food to cook faster; cooking times can typically be reduced to 1/3 of the time for conventional cooking methods. The actual cooking time also depends on the pressure release method used after timing (see next section), and on the thickness of the food, since thicker foods take longer to cook. Meat - and some other foods like sponge puddings, Christmas puddings etc. - are timed according to their weight. Frozen foods need extra cooking time to allow for thawing. When pressure cooking at 15 psi, cooking times are typically as follows: shredded cabbage, one minute; fresh green beans, three minutes; potatoes cut to around 1 inch thick, eight minutes (normal release) or six minutes (using the natural release method, for keeping unpeeled skins intact); a whole chicken, which easily fits into all but small pressure cookers, about twenty minutes.
Some pressure cookers have a lower maximum pressure than the industry standard 15 psi, or can be adjusted to different maximum pressures; cooking times will increase or decrease accordingly. This is often 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 (the manufacturer's instruction booklet should state the maximum operating pressure) and the recipe is intended for cooking at 15 psi, the cooking time will need to be extended, but this can overcook food. Increasing the heat under pressure cookers designed for pressures below 15 psi will not make the food cook faster — it will only waste energy and shorten the usable life of the gasket and other rubber parts. This is no substitute for pressure cookers which cannot operate at 15 psi, which is the pressure setting that most recipes require and assume, since 15 psi became the standard for pressure cookers in 1917.
Pressure cooking is often used to simulate the effects of long braising or simmering in shorter periods of time.
Recipes for foods using raising agents, e.g. steamed puddings, call for gentle pre-steaming without pressure in order to activate the raising agents prior to cooking to achieve a light and fluffy texture. The water must bubble gently when 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, the result is a heavy and stodgy texture.
Since pressure cooking depends on the production of steam and always requires liquid, 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, using special pressure fryers made for this purpose (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 with deep-fried chicken since moisture remains in the breading. Thick sauces will not contain enough liquid to vaporize and create pressure, so they will likely burn onto the interior base of the pressure cooker after prolonged heating; therefore, sauces should be thickened after pressure cooking.
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 results in food that is either under-cooked or over-cooked.
To avoid opening the pressure cooker too often when cooking vegetables with varying cooking times, those which take longer to cook can be cut into smaller pieces and those which cook faster can be cut into thicker pieces.
This method is the fastest way of releasing pressure with portable pressure cookers (unless the manufacturer advises against cold water release). It involves running cold tap water over the pressure cooker, avoiding the steam vent or any other valves. It is most suitable for foods with short cooking times and takes only about 20 seconds for the cooker to cool down enough so that the pressure falls and it can be safely opened. This method is not suitable for electric pressure cookers.
Sometimes called "Quick Release", but not to be confused with the cold water release, this method involves the evacuation of vapor quickly by lifting (or removing) the valve, pushing a button or turning a dial. It is most suitable for interrupted cooking to add food which cooks faster than what is already in the cooker. For example, since meat takes longer to cook than vegetables, for stews it is necessary to add the vegetables later so that they will cook only for the last few minutes. This release method does not cool down the pressure cooker like the cold water release method. Care must be taken when releasing the steam to avoid scalding injury. This release method is not suitable for foods which foam and froth during cooking, as the hot contents could spray outwards from the pressure being released from the steam vent. This release method takes about two minutes to release the pressure before the lid can be opened.
The natural release method allows the pressure to drop slowly; this is achieved by removing the pressure cooker from the heat source and the pressure lowers without any further action. It takes about 10 to 15 minutes before all the pressure has gone. This method is recommended for foods which foam and froth during cooking, such as rice or pasta, legumes or for recipes such as steamed puddings with raising agents. Meat cooked in a pressure cooker requires the natural release method, otherwise its texture will be tough instead of tender. The natural release method finishes cooking foods e.g. potatoes because the inside of the pressure cooker stays hot. This method is not recommended for foods which require very short cooking times, as the food will be overcooked.
Foods cook much faster with pressure cooking than with other methods (except for small quantities in microwave ovens), and require much less water than boiling, so dishes can be ready sooner. Less energy is required than when boiling, steaming, or oven cooking. Since less water or liquid has to be heated, the foods reaches its 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 foods to be cooked together inside the pressure cooker. However, the pressure cooker should never be filled to more than 2/3 its interior 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.[2]p. 262
The food is cooked at a temperature above the normal boiling point of water, killing most micro-organisms. A pressure cooker can also be used as an effective sterilizer, for jam pots and glass baby bottles, for example, or for water while camping.
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.[3]
The pressure cooker speeds cooking considerably at high altitudes, where the low atmospheric pressure otherwise reduces the boiling point of water, which reduces water's effectiveness for cooking or preparing hot drinks.
Pressure cookers are considerably more expensive than conventional saucepans of the same size. The additional gasket requires special care when cleaning, unlike a standard lid for a saucepan. The gasket/sealing ring must be replaced if there are any signs of wear. If the gasket cannot provide an airtight seal and steam leaks around the lid, even when fitted properly, the pressure cooker will not build up pressure inside. The gasket should not be allowed to dry out as this makes it harder to open and close the lid; however, the gasket can be smeared with cooking oil to alleviate this.
In order to inspect the progress of cooking, the cooking process needs to be halted and the cooker opened, which takes time. With a conventional saucepan, this can be done in a matter of seconds by removing the lid, or by visual inspection if cooking without the lid on.
The risk of scalding from escaping steam is greater with a pressure cooker, as the temperature is above the 100 degrees Celsius at which water boils at sea level in conventional cooking.
The increased weight of conventional pressure cookers makes them unsuitable for applications where 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, with more liquid required for longer cooking times. This is not desirable for food requiring less liquid. However, recipes and books for pressure cookers take this into account.
Early pressure cookers equipped with only a primary safety valve were at risk of explosion if poorly maintained, such as if food residues contaminate the release valve. Modern pressure cookers typically have two or three redundant safety valves as well as some additional safety features, such as an interlock lid that prevents the user from opening the lid as long as the internal pressure exceeds atmospheric pressure. If any of the 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, as each model differs slightly as to how the safety mechanisms are implementated, and they should be regularly maintained in accordance with the manufacturer's instructions.
For first generation pressure cookers with a weighted valve or "jiggler", 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 continues to increase, the grommet itself will blow out to release pressure.
For second generation pressure cookers with selectable pressure and, often hidden, spring valve operation, a common safety feature is the design of the gasket, which expands and releases excess pressure downward between the lid and the pot.
A pressure cooker is often used to compensate for the low atmospheric pressure at high elevations. Under these circumstances water boils at temperatures significantly below 100 °C (212 °F) and, without the use of a pressure cooker, may leave boiled foods 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.
Lightweight pressure cookers as small as 1.5 litre weighing 1.28 kg are available for mountain climbers. Sherpas often use pressure cookers in base camp.
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.
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