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 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 with boiling liquid, such as water, inside the closed pressure cooker and the trapped steam increases the internal pressure and temperature, which is maintained throughout cooking time.

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 or "canning retorts".

Design

Pressure cookers are generally made from 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 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. Some pressure cookers have a non-stick interior.

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 heavier than normal pots of similar size.

Generations

There are three generations of pressure cookers:

First Generation

Also known as "old type," pressure cookers 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. Often, they only offer 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.

Second Generation

Also known as new or latest generation, pressure cookers operate with a spring-loaded valve which is often hidden from view in a proprietary mechanism. They do not release vapor during operation: only when the pan is opened, or as a safety mechanism if the heat source has not been reduced when the pan reaches pressure. Two or more pressure settings are also a mark of this generation.

Electric Pressure Cookers

Called "third generation" pressure cookers by their manufacturers, 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 vapor through the valve for foaming foods and liquids (beans, grains, milk, liquor, etc.).

Operation

The food to be cooked is placed in the pressure cooker, with a small amount of water or liquid required for the recipe. 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 a weight is used, the weight is placed on the steam vent when steam is being emitted, as this ensures the air inside has escaped) until the cooker reaches full pressure, then the heat is lowered to maintain pressure and timing the recipe begins at this point. As the internal temperature rises, the pressure also rises, until the pressure reaches the design gauge pressure. In some designs, a relief valve opens, releasing steam and preventing the pressure from rising any further. In others, the pressure regulator weight begins levitating on its nozzle, allowing excess steam to escape. The heat source does not need to be kept higher than necessary to maintain pressure. If the heat source is too high, it costs more in energy, and the life of the gasket/sealing ring is reduced by this extra heat. If the heat source is set too low e.g. minimum, food may be undercooked or pressure may be lost.

Most pressure cookers have a working pressure setting of 15 psi (approx. 107 kPa) 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 122 °C (252 °F) (refer to vapor pressure of water).

The higher temperature causes the food to cook faster; cooking times can typically be reduced to 1/3 of the time of conventional cooking methods. For example, when pressure cooking at 15 psi, cooking times are typically as follows: Shredded cabbage is cooked in one minute, fresh green beans in three minutes, potatoes cut to 1" cook in about six minutes (thicker potatoes will take longer) and a whole chicken takes only twenty minutes. Brown rice, lentils and beans can be cooked in ten minutes, instead of 45 minutes of simmering in an ordinary saucepan.

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 accordingly at lower pressures. This is often done by having different regulator weights or different pressure settings.

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—to activate the raising agents prior to cooking to achieve a light and fluffy texture; Christmas pudding also requires pre-steaming (note that pre-steaming is not required when reheating a Christmas pudding). The water must bubble gently when pre-steaming to ensure enough water is left over 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, 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 (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 that of deep-fried chicken, because moisture remains in the breading.

Pressure release methods

After cooking time, 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 cooking time for proper results—failure to follow the recommendation results in food that is either under-cooked or over-cooked.

Cold Water Quick Release

This method is the fastest way of releasing pressure with portable pressure cookers (unless the manufacturer advises against cold water release). This method involves running cold water from the cold tap 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 to open the pressure cooker. This method is NOT suitable for electric pressure cookers.

Manual, Normal, Regular, or Automatic Release

Also sometimes referred to as "Quick Release" but not to be confused with the cold water release, this method with varying names 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 when adding food later which cooks faster. For example, adding vegetables later when cooking meat, as the meat takes longer to cook than vegetables, so it is necessary to add the vegetables later to cook with the meat 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. The quick release method is not suitable for foods which foam and froth during cooking, as the hot contents could spray outwards. This release method takes about 2 minutes to open the pressure cooker.

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 it to cool down at room temperature and lower pressure without any further action. The pressure inside will lower as the pressure cooker cools down and 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.

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.^p. 262

The food is cooked at a temperature above the normal boiling point of water, killing most micro-organisms. 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.

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.

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.

Disadvantages

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 needs replacing when it is worn out. If the gasket cannot provide an airtight seal and steam leaks around the lid, the pressure cooker may not build up the pressure inside and the gasket needs replacing with a new one. The gasket should not be allowed to dry out as this will make it much harder to open and close the lid; however, the gasket can be smeared with cooking oil to alleviate this. Pressure cookers using non-rubber gaskets can have petroleum jelly applied to the rim of the cooker if the lid becomes hard to open or close as a result of a dry gasket. Any signs of wear on a gasket means a new one is required.

In order to inspect the progress of food cooked in a pressure cooker, the cooking process needs to be halted and the cooker opened. With a conventional saucepan, this can be done in a matter of seconds by removing the lid or just 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 a pressure cooker makes them unsuitable for applications where saving weight is a priority, such as camping.

Safety features

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 redundant safety valves, as well as some additional safety features, such as an interlock lid that prevents the user opening the lid while internal pressure exceeds atmospheric pressure. If any of the safety mechanisms are not in place correctly, the pressure cooker will not reach pressure. Pressure cookers should be operated only after reading the instructional manual, as each model differs slightly on the implementation of the safety mechanisms and should be regularly maintained in accordance with the manufacturer's instructions.

For first generation pressure cookers with 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.

Use at high altitudes

A pressure cooker is often used to compensate for the low atmospheric pressure at a very high elevation. 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):

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.

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 between 12 and 22% of the amount in the uncooked rice.

References

See also

Pressure frying

Rice cooker, many employ pressure cooking

Category:Cooking techniques Category:Pressure