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 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 home 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.DesignPressure 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.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.[1]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. However, some pressure cookers are manufactured for camping, and can be as light as 1.2 kg for a four-litre pot.OperationThe food to be cooked is placed in the pressure cooker, with a small amount of water. The vessel is then sealed and placed on a heat source, e.g., a stove. 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.Most pressure cookers have a working pressure setting of 15 psi (100 kPa) over the existing atmospheric pressure, the standard determined by the United States Department of Agriculture in 1917.[2] At this pressure boost relative to sea-level atmospheric pressure, water boils at 125C (257F).The higher temperature causes the food to cook faster; cooking times can typically be reduced by about 70 percent. For example, shredded cabbage is cooked in one minute, fresh green beans in three minutes, small to medium-sized potatoes cook in about eight minutes (depending on thickness and type), and a whole chicken takes only twenty minutes. Brown rice and lentils and beans can be cooked in ten minutes instead of 45.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. This is often done by having different regulator weights.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 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.AdvantagesFoods are cooked much faster by pressure cooking than by other methods, (except perhaps microwaves), and with 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 is necessary, the foods come to cooking temperature faster.The food is cooked above the boiling point of water, killing bacteria and viruses. 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: Enough water to keep the pressure cooker filled with steam is sufficient. Because of this, vitamins and minerals are not leached (dissolved) away by water. Since steam surrounds the food, foods are not oxidized by air exposure at heat, so asparagus, broccoli, and so on retain their bright green colors and vitamins.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.Safety featuresPressure 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 valves, as well as some additional safety features, such as an interlock to prevent opening the lid while internal pressure exceeds atmospheric pressure.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 gets still higher, the grommet itself will blow out. A common safety feature is the design of the gasket, which expands and releases excess pressure downward between the lid and the pot.In some pressure cookers, excess pressure forces the pressure indicator above its housing which releases the pressure vertically upwards.Use at high altitudesA pressure cooker is often used by mountain climbers to compensate for the low atmospheric pressure at a very high elevation. Under these circumstances water boils at temperatures significantly below 100C 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."There is a lightweight camping/mountaineering pressure cooker which weighs little more than a standard camping pot.Use in food detoxificationSome 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 from 12% to 22% of the level in the uncooked rice.Attach the lid securely, quickly reducing steam pressure, and opening the pot when cooking is completed. The rubber gasket seal and the steam vent tube are the critical parts of this appliance; make sure that the gaskets are solid, clean, not ripped or torn; and that the vent tube is clean and clear, not clogged with food. Use the cleaning implement that comes with your pressure cooker or a pipe cleaner to keep that vent tube clear. Many crockpot recipes convert easily to the pressure cooker. Cheaper cuts of meat, like brisket and chuck, cook to perfection in the pressure cooker just like they do in the crockpot, only the cooking time is really reduced. A recipe that cooks for 8-10 hours on low in a crockpot usually cooks for about 1 hour in a pressure cooker. One caveat: watch out for foods that foam! Dried beans, pasta, and some fruits (especially apples) can create foam when they cook; small particles can ride up on that foam and clog the steam vent. Only fill the pressure cooker half full when cooking these foods (my pressure cooker instructions say to only fill one-third full), and add a tablespoon of oil to keep the foam to a minimum. Foods that cause the most problems with foaming include split peas and beans, oatmeal, apples, cranberries, and pearl barley. If you are using an old-fashioned pressure cooker (older than 5 years), for quick release, the steam has to be released by placing the cooker in the sink and running cold water over it. (Make sure to keep the water out of the steam valve!) This reduces the temperature and therefore reduces the steam pressure. You can let the cooker cool down naturally until the pressure is released; this takes anywhere from 10-30 minutes. Timing is critical when you're pressure cooking. Foods like large pieces of meat have a bit of 'wiggle room' in timing, but fresh vegetables and fruits can be quickly overcooked. If your pressure cooker doesn't have a built-in timer, make sure you have a reliable, accurate timer that you use every time you pressure cook. Most pressure cooker recipes use foods that have the same cooking times; beef and potatoes, or chicken with parsnips. You can use an interrupted cooking method, as in this recipe for Crockpot Chicken Alfredo, releasing the lid and adding other ingredients as the cooking time reaches a few minutes. When you use this method, be sure to write down the times that the more fragile ingredients are added, and carry that timer with you if you leave the kitchen.Make sure that you use the amount of ingredients called for in the recipe; liquid amounts are particularly important. You need a certain amount of liquid to build up steam so the food cooks at the correct temperature in the proper time frame. Do not use your pressure cooker as a deep fryer (i.e. fill it full of cooking oil) unless it is specifically marketed as a fryer!!Don't store your pressure cooker closed with the lid on; that will just allow aromas to stay in the cooker; molds and off flavors can develop. Store the lid separately from the base. Also do not store the rubber gasket in the base. Sprinkle a bit of baking soda inside the cooker when you store it to prevent these problems.Think twice about leaving a pressure cooker alone while it's cooking. Never let children or pets play around the appliance when it's cooking. Be very careful when you release pressure; you can burn yourself even with the new cookers that have safety releases. NEVER try to force the lid open. And finally, again, read that instruction booklet from cover to cover!To open the lid, you must release the steam from the pressure cooker first. Steam and heat combine under pressure to cook foods at about 250 degrees; hotter than the 212 degrees (temperature of boiling water) that is attained on the stovetop or in the oven.If you use the quick release features, make sure to only release steam in very short bursts (like the pulse feature on your food processor). If you hold the release button open, hot liquid may spurt through the valve and burn you.It's best to undercook food until you're sure of the pressure cooker and recipes; you can always put the appliance back together and cook again for 1-3 more minutes to finish cooking. And keep an eye on the pressure indicator rod. When the pressure cooker is cooking, the rod should be in the raised position (or jiggling, on older models). If it is not, there may not be enough liquid in the cooker to covert to steam and the food can burn. Stop the cooker according to the manufacturer's instructions and when the pressure goes down, lift the lid and add liquid. You can brown foods first in most pressure cookers. For an old-fashioned type, place the cooker on medium-high heat and brown the foods; then add liquids and remaining ingredients, cover, bring pressure up, and cook. For a newer cooker, most have a brown function; see manufacturer's instructions. If you're adapting a recipe to pressure cooking, set the timer to 1/3 of the cooking time of the original recipe. If a recipe cooks on the stovetop for 1 hour, cook for 20 minutes in the pressure cooker, release the steam, following directions, and test for doneness. You can refasten the lid and cook for 1-3 minutes longer if the food needs it. Different recipes also have different cooling methods. Old-fashioned cookers may use the cold water method as described on the previous page. Newer cookers have a steam release valve that is pressed in short bursts until the pressure is reduced. Or you can just remove the heat or electric source and let the cooker cool naturally for 10-30 minutes (this method is best for longer-cooking foods like large cuts of meat). Using your pressure cookerPressure cookers adapt best to recipes that normally use a moist cooking method such as soups, stews, tough cuts of meats, artichokes, steamed puddings, etc. Add vegetables at the very end to avoid a mushy result. Generally, cooks in high altitudes over 3,500 feet should increase cooking times by about ten percent. Liquid is an important part of pressure-cooking. The amount of liquid necessary will vary depending on the manufacturer. Some will require as little as one-half cup of liquid and others up to two cups. Pressure cooker storageWhen storing your pressure cooker, be sure to store it with the lid completely detached and to the side of the pot. If you store it closed, you will trap smells and odors inside the pot to greet you on your next usage. Thoroughly wash the rubber seal and rub it with mineral oil after each use to preserve it. The rubber seal should last through about 150 meals. Store the valve and rubber seal (if not attached) inside the cooker. Pressure cooker tips and hintsPressure-cooking is really fairly simple, but here are a few important tips to remember:

Most importantly, be familiar with the owner's manual for your particular cooker. Read it thoroughly and follow all manufacturer's recommendations. You can use more liquid than recommended, but never use less. Read and understand the recipe before you begin. Be sure the lid is properly closed and locked into position before developing pressure. Use that timer! Timing as important as developing pressure. Once you have reduced pressure according to directions, shake the pot before opening the seal to readjust the inner temperature. Cut same foods into pieces of uniform size to promote even cooking. When mixing foods, cut those that cook more quickly into larger pieces and those that cook more slowly into smaller pieces. If your recipe calls for browning or searing as a preliminary step, be sure to scrape up the brown bits clinging to the bottom so they are loose when you add the liquid. This will discourage scorching. Since flavors are more concentrated with this cooking method, you may want to reduce herbs and seasonings when converting conventional recipes. Choose fresh herbs over dried herbs. If you end up with too much liquid, simply cook in the uncovered pot until the liquids are reduced to your satisfaction. If you desire the flavors of foods to mix, let them come into contact with the liquid which transmits flavors. If you do not wish flavors to mix, place individual foods on a rack above the liquid. Steam does not transmit or mix flavors. When cooking dried beans, cereal grains, lentils, split peas, rice or other foods that absorb liquid and expand, do not fill the cooker more than half full to avoid the potential of clogging the vent during the cooking process. The normal fill line is two-thirds. In general, bring the recipe to the boil before closing the lid to facilitate quicker pressurization and reduce the chances of burning the food.Pressure cooker historyThe term pressure cooker first appeared in print in 1915. The first commercial pressure cooker debuted in the United States at the New York World's Fair in 1939, made by National Presto Industries. It's defined as a saucepan with a locking lid that creates within intense steam heat to cook foods in a shorter period of time than a conventional saucepan. This process not only tenderizes tough foods, but also keeps flavor and nutrients within, producing a tastier, healthier meal. Today's pressure cookers are hi-tech wonders, with some even designed specifically for microwave use. Pressure canners are also very popular.How to choose a pressure cookerToday's pressure cookers give you a choice of a quick-release option, taking even less time to finish that meal and without the need for wasted water to cool things down. When purchasing your pressure cooker, keep in mind the 6-quart size is the standard for most recipes.

Select a cooker with a detachable pressure regulator that can adjust the pressure to low (5 pounds=220 degrees F./100 degrees C.), medium (10 pounds=235 degrees F./115 degrees C.), or high (15 pounds=250 degrees F./120 degrees C.). The higher the pressure, the higher the internal temperature and the less cooking time you will need. The safety valve will automatically vent the steam.

Choose a pot with heat-resistant handles, a locking lid that is easy to maneuver, and a heavy bottom of stainless steel construction so you can use the same pot to do preparatory sauting.Pressure cooker accessories One of the most important accessories you will need is a timer. A couple of extra minutes of cooking probably won't harm a pot roast, but it could ruin a more delicate dish or any accompanying veggies. A rack or trivet is a common requirement and often comes with the cooker. A steamer basket to keep foods above the liquid is also desirable for cooking some vegetables. For cooking desserts, you will need a 5-cup heatproof souffl dish (that fits in side your pressure cooker), 1/2-cup heatproof ramekins for puddings, custards and timbales, and a 7- or 8-inch spring form pan (to fit in your cooker) for cheesecakes. You should also have a heat-diffuser, which prevents direct contact between the heat source and the bottom of the cooker. It will be needed when preparing rice, pasta or bean dishes to prevent sticking and scorching.