Pressure cooker
A pressure cooker is a sealed vessel for cooking food with the use of high pressure steam and water or a water-based liquid, a process called pressure cooking. The high pressure limits boiling and creates higher temperatures not possible at lower pressures, allowing food to be cooked faster than at normal pressure.
The prototype of the modern pressure cooker was the steam digester invented in the seventeenth century by the physicist Denis Papin. It works by expelling air from the vessel and trapping steam produced from the boiling liquid. This is used to raise the internal pressure up to one atmosphere above ambient and gives higher cooking temperatures between. Together with high thermal heat transfer from steam it permits cooking in between a half and a quarter the time of conventional boiling as well as saving considerable energy.
Almost any food that can be cooked in steam or water-based liquids can be cooked in a pressure cooker. Modern pressure cookers have many safety features to prevent the pressure cooker from reaching a pressure that could cause an explosion. After cooking, the steam pressure is lowered back to ambient atmospheric pressure so that the vessel can be opened. On all modern devices, a safety lock prevents opening while under pressure.
According to the New York Times Magazine, 37% of U.S. households owned at least one pressure cooker in 1950. By 2011, that rate dropped to only 20%. Part of the decline has been attributed to fear of explosion along with competition from other fast cooking devices such as the microwave oven. However, third-generation pressure cookers have many more safety features and digital temperature control, do not vent steam during cooking, and are quieter and more efficient, and these conveniences have helped make pressure cooking more popular. While pressure cookers are generally considered safe when used properly, an estimated 1700 injuries occur each year in the US.
History
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 cooking food more quickly. In 1681 Papin presented his invention to the Royal Society of London as a scientific study; he was later elected as a member.In 1864, Georg Gutbrod of Stuttgart began manufacturing pressure cookers made of tinned cast iron.
Although the concept of cooking with pressurized steam had been known for two centuries, the term "pressure cooker" was not commonly used until the early 20th century. The earliest citation of the phrase given in the Oxford English Dictionary is from a Lincoln, Nebraska, newspaper in 1914. However, the Dictionary editors apparently overlooked or did not have access to certain Colorado newspapers from a few years earlier than that. As early as 1910, the inventor Zeno E. Crook founded a business called "The Pressure Cooker Company" in Denver, Colorado. Crook had developed an aluminum cooker of a size practical for home use, and soon began marketing it to communities in the high country of Colorado, where the device proved to be well suited for use in high-altitude cooking. In many of these communities, Crook's pressure cooker was hailed as a marvelous new invention, until 1918, when Popular Science Monthly broke the news that this "invention" was actually more than 200 years old.
In 1918, Spain granted a patent for the pressure cooker to José 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 1924, the first pressure cooking pot recipe book was published, written by José Alix and titled "360 fórmulas de cocina Para guisar con la 'olla expres'", or 360 recipes for cooking with a pressure cooker.
In 1935, the Automa pressure cooker was introduced. Mountaineers attempting to climb Mount Everest took it along with them to cook in higher altitudes.
In 1938, Alfred Vischer presented his invention, the Flex-Seal Speed Cooker, in New York City. Vischer's pressure cooker quickly gained popularity, and its success led to competition among American and European manufacturers. At the 1939 New York World's Fair, the National Pressure Cooker Company, later renamed National Presto Industries, introduced its own pressure cooker.
First generation
Also known as "old type" pressure cookers, these operate with a weight-modified or "jiggler" valve, which releases pressure during operation. Some people consider them loud because the valve rattles as excess steam is released. Older pressure cookers typically offered only one pressure level, but from the 1960s onwards some allow the operator to change the weight of the valve, thus changing the pressure.Today most pressure cookers are variations on the first-generation cookers, with the addition of new safety features such as a mechanism that prevents the cooker from being opened until it is entirely depressurized.
Second generation
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 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 to change the pressure setting or release pressure; these release steam during operation.Third generation "electric pressure cookers"
After the stove-top pressure cookers came the electric pressure cookers in 1991, called the "third generation" pressure cookers.These include an electric heat source that is automatically regulated to maintain the operating temperature and pressure. They also include a spring-loaded valve and are typically non venting during cooking.
An electric pressure cooker integrates a timer. Depending on cooking control capability, there are three generations of electric pressure cookers:
- First-generation electric, with mechanical timer. There is no delayed cooking capability.
- Second-generation electric, with digital controller. Delayed cooking becomes possible and the controller shows a countdown timer when working pressure is reached.
- Third-generation electric, with smart programming, which includes pre-set cooking times and settings based on heating intensity, temperature, pressure and duration.
Theory
At standard pressure the boiling point of water is. With any food containing or cooked with water, once the temperature reaches the boiling point, any excess heat causes some of the water to vaporize into steam efficiently carrying away heat keeping the food temperature at 100 °C.In a sealed pressure cooker, as the water boils, the steam is trapped in the cooker which raises the pressure. However, the boiling point of water increases with pressure resulting in superheated water.
The equation for the pressure, temperature and volume of the steam is given by the ideal gas law:
or
where, and are the pressure, volume and temperature; is the amount of substance; and is the ideal gas constant.
In a sealed pressure cooker the volume and amount of steam is fixed, so the temperature can be controlled either directly or by setting the pressure, such as with a pressure release valve.
For example, if the pressure reaches 1 bar or above the existing atmospheric pressure, the water will have reached a temperature of approximately which cooks the food much faster.
Pressure cookers also use steam and water to rapidly transfer the heat to the food and all parts of the vessel. While compared to an oven, a pressure cooker's 120 °C is not particularly high, ovens contain air which is subject to thermal boundary layer effects which greatly slows heating, whereas pressure cookers flush air from the cooking vessel during warm up and replace it with hot steam. For items not placed within the liquid, as this steam condenses on the food it transfers water's latent heat of vaporization, which is extremely large, to the surface, rapidly bringing the surface of the food up to cooking temperature. Because the steam condenses and drips away, no significant boundary layer forms and heat transfer is exceptionally efficient, and food heats much faster and more evenly.
However some recipes require browning to develop flavors as during roasting or frying. Higher temperatures are attainable with conventional cooking where the surface of the food can dry out. Such browning occurs via the Maillard reaction, at temperatures higher than the roughly achieved in pressure cooking. Because those temperatures are not reached in pressure cooking, foods are generally browned by searing them, either in the open pressure cooker or another pan beforehand.
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, causing the boiling point of water to be significantly below the at standard pressure. This is problematic because temperatures above roughly 90 °C are necessary to cook many common vegetables in a reasonable time. For example, on the summit of Everest, the boiling point of water would be only. Without the use of a pressure cooker, many boiled foods may remain undercooked, as described in Charles Darwin's The Voyage of the Beagle :When pressure cooking at high altitudes, cooking times need to be increased by approximately 5% for every above elevation. Since the regulators work off the pressure differential between interior and ambient pressure, the absolute pressure in the interior of a pressure cooker will always be lower at higher altitudes.
Weight is a concern with backpackers, so mountaineering pressure cookers are designed to operate at a lower differential pressure than stove-top units. This enables them to use thinner, and therefore lighter materials. Generally, the goal is to raise the cooking temperature enough to make cooking possible and to conserve fuel by reducing heat lost through boiling. Lightweight pressure cookers as small as weighing are available for mountain climbers. Sherpas often use pressure cookers in base camp.