A pressure cooker is, essentially, a sealed container with an escape valve that allows food to be cooked at pressures above atmospheric pressure. This allows increased temperatures in the vessel, meaning that the food cooks faster.

The first practical pressure cooker actually pre-dates the description of the physical laws that govern volume, pressure and temperature. 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.

The ideal gas law, also called the general gas equation, is the equation of state of a hypothetical ideal gas. It is a good approximation of the behaviour of many gases under many conditions, although it has several limitations. It was first stated by Benoît Paul Émile Clapeyron in 1834 as a combination of the empirical Boyle's law, Charles's law, Avogadro's law, and Gay-Lussac's law.[1] The ideal gas law is often written in an empirical form:

𝑝𝑉 = 𝑛𝑅𝑇

where 𝑝, 𝑉 and 𝑇 are the pressure, volume and temperature respectively; 𝑛 represents the amount of substance; and 𝑅 is the ideal gas constant.

This historical information about Pressure cookers and pressure cooking is given below and below that you will find all the Pressure cooking-related recipes on this site.

Pressure Cookers, A History:

Origins and development of Pressure Cookers

In 1864, Georg Gutbrod of Stuttgart began manufacturing pressure cookers made of tinned cast iron. In 1938, Alfred Vischer presented his invention, the Flex-Seal Speed Cooker, in New York City. Vischer's pressure cooker was the first designed for home use, 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.[10] 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.

Second generation These operate with a spring-loaded valve that is often hidden from view in a proprietary mechanism.[10] 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 alters the spring tension) to change the pressure setting or release pressure;[10] these release steam during operation (venting).

Third generation 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 (as described above) and are typically non venting during cooking.

An electric pressure cooker integrates a timer. Many of the latest generation of pressure cookers are multifunctional (multicookers): pressure cooker, saute/browning, slow cooker, rice cooker, egg cooker, yogurt maker, steamer, sous vide, canner, and stockpot warmer that can also be used to keep cooked food warm. Since Since 2018, many manufacturers have come out with their own pressure cookers that can air fry, which are now known as air fryer pressure cookers. Air fryer pressure cookers generally have two separate lids, one for pressure cooking, and one for air frying.

The Physics of Pressure Cooking

At standard pressure (ie at sea level) the boiling point of water is 100°C (212°F). With any food containing or cooked with water, once the temperature reaches the boiling point, any excess heat causes some of the water to vaporise 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 and, as 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 (see above), which can also be expressed as:
𝑇 = 𝑃𝑉/𝑛𝑅

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 100 kPa above the existing atmospheric pressure, the water will have reached a temperature of approximately 120°C 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, but it must be remembered that 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 vaporisation, which is extremely large (2.275 kJ/g), 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 flavours as are seen during roasting or frying. Such browning occurs via the Maillard reaction, at temperatures higher than the roughly 120°C (248°F) achieved in pressure cooking. Because those temperatures are not reached in pressure cooking, many recipe call for food to be generally browned by searing them, either by frying in the open pressure cooker or by using a pan beforehand (this is why electric pressure cookers tend to have a sauté function to allow frying before pressure cooking).

Pressure Cooking

Pressure cookers are available in different capacities for cooking larger or smaller amounts, with 6 litres' capacity being common. The maximum capacity of food is less than the advertised capacity because pressure cookers can only be filled up to 2/3 full, depending on ingredients and liquid used.

Modern pressure cookers typically have two or three redundant safety valves and 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 pressurise the contents. Pressure cookers should be operated only after reading the instruction manual, to ensure correct usage. Pressure cooker failure is dangerous: a large quantity of scalding steam and water will be forcefully ejected and if the lid separates it may be propelled with considerable force. Some cookers with an internally fitted lid may be particularly dangerous upon failure as the lid fits tighter with increasing pressure, preventing the lid from deforming and venting around the edges. Due to these dangers pressure cookers are generally over-engineered in a safety regard and some countries even have regulations to prevent the sale of non-compliant cookers.

Pressure cooking always requires a water-based liquid to generate the steam to raise the pressure within the cooker. Pressure cooking cannot be used for cooking methods that produce little steam such as roasting, pan frying, or deep frying. A minimum quantity of liquid is required to create and maintain pressure, as indicated in the manufacturer's instruction manual. For venting cookers 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.

Pressure Release

Manual, normal, regular, or automatic release

This method is sometimes called a quick release, not to be confused with the cold water release (mentioned below). It involves the quick release of vapour by gradually 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. Releasing the steam with care avoids the risk of being scalded by the rapid release of hot steam. 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.

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.

Cold water quick release

This method is the fastest way of releasing pressure with portable pressure cookers, but can be dangerous if performed incorrectly. Hence it is safer to release pressure by using the other methods. The manufacturer's instruction book 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, otherwise steam can be ejected from under the lid, which could cause scalding injury to the user; also the pressure cooker lid can be permanently damaged by an internal vacuum if water gets sucked into the pressure cooker, since the incoming water blocks the inrush of air.

The cold water release 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, as they are not immersible. This type of pressure cooker cannot be opened with a cold water quick-release method.

Foods unsuitable for pressure cooking

Some foods are not recommended for pressure cooking. Foods such as noodles, pasta, cranberries, cereals and oatmeal can expand too much, froth and sputter, potentially blocking the steam vent and creating an unsafe condition.