Pressure cooker

Pressure cooker . Cooking container with a seal that retains steam . The steam increases the pressure inside the pot, and makes the temperature rise to a level higher than the boiling point of the water (100 ° C at sea level), making it possible to cook food more quickly. It works with a safety valve that lets the steam escape when a certain pressure (working pressure) has been reached; With the improvement of manufacturing techniques, modern (super fast) pans operate at higher pressure than older ones. It can have a second safety mechanism to prevent excess pressure, if the valve fails, could burst the pot.

History and evolution

In 1679 , Anglo-French physicist and mathematician Denis Papin (1647-1712) invented a steam pot – which he called a digester – in which water boiled at a higher temperature than normal, so that meat and Other foods could be cooked in less time than in the conventional pots used in those days. He presented his invention at the “Royal Society” in London in the year 1681, but the novelty did not prosper and it remained as another scientific study. However, this work helped him to be admitted as a member of said scientific society. It would be necessary to wait until the 20th century for the invention to become a reality and pressure cookers to be manufactured.

Functional diagram of the safety valve. Under normal conditions the spring keeps the valve closed. As the pot heats up the internal pressure increases and pushes the valve until the tap is free, which allows the gases (air and water vapor) to escape to the outside and limits the pressure inside. The small size of the outlet hole is responsible for the characteristic hissing of old pots.

The first patent was granted in France , in 1948 , when M. Devedjian made a quick cook model which he called cocotte minute . In 1952 George Laverne perfected the invention with a new model of large capacity (300 liter) pots that had a special locking system. This model was very successful and was acquired by the French army.

The following year, in 1953 , the SEB (Societé d ?? Emboutissage de Bourgogne) supercocotte made by the Lescure brothers came to light. In 1954 the SEB patent was marketed in Spain under the name SEB-MAGEFESA and the pots were manufactured in a hardware workshop located in Algorta .

Since 1978, pressure cookers have become more and more complete and faster, perfecting the closing and safety mechanisms, thus providing greater pressure and faster cooking. What has evolved the most is the safety valve. The latest models have a system called Food Control that, in addition to indicating the pressure, evacuates the oxygen inside the pot. By indicating the absence of pressure, the person who handles the pot knows that he can open it safely, and by cooking without oxygen, the food keeps its properties and its natural color intact.

Functioning

Contrary to popular belief, water never boils inside a pressure cooker, speeding cooking by simply increasing the temperature of the water.

The pressure and temperature conditions inside the pressure cooker (1 → 2) prevent the liquid from boiling, except if the water vapor cools down quickly causing a rapid drop in pressure (2 → 3).

If the diagram in the figure is observed, boiling will be achieved as long as it is possible to cross the so-called “state change line”, which separates the liquid (L) and gas (G) zones in the thermodynamic diagram. However, when the pot lid is closed most of the gas inside will be air , not steam, so that at all times the pressure inside will be the sum of that due to water vapor, the amount of which increases due to evaporation as the temperature increases, and to the air, whose partial pressure is responsible that as the pot heats up the pressure inside it goes farther and farther from the saturation pressure, which prevents the boiling of the water inside the pot, that is, the line of change of state never cuts to line 1 → 2 that represents the evolution of pressure and temperature conditions inside the pot.

In the same way, once the maximum pressure determined by the valve (by its weight or by a spring) inside the pot has been reached, it cannot be modified, and keeping it on high heat does not accelerate cooking but simply increases the water evaporation and steam losses through the valve.

Exceptionally, boiling can occur from rapid cooling of the mixture of air and water vapor; for example, if you want to quickly open the pot and place it under running water. The water cools the walls of the container, causing condensation of the water vapor and a rapid drop in pressure inside the pot so that the state change line (2 → 3) is reached, and the sudden boiling of the water, with a virulence that can even cause the liquid to escape through the seal of the container or the valve itself. The same happens if the pot is opened while it is still under pressure, with the consequent risk of suffering burns from splashes of hot liquid or the steam itself.

The pot can take pressure on its own if, after cooling and opening, the contained steam escapes. If it closes again, as it continues to cool, the pressure drops and vaporization occurs at a lower temperature, with new steam production and movement of the exhaust valve.

Mountaineers also use pressure cookers for cooking, as otherwise cooking food becomes very difficult. At sea level the atmospheric pressure is 1 atmosphere and the boiling temperature in an open pan is 100 ° C. However, at higher altitudes the atmospheric pressure is lower and, therefore, the temperature at which the water boils decreases, as Charles Darwin described in the Voyage of the Beagle :

“In the place where we sleep the water necessarily boiled at a lower temperature than at a lower altitude due to the decrease in atmospheric pressure, the case being the opposite of that of the Papin pot. Thus, the potatoes after spending several hours cooking in water were almost as they were in the beginning. Even the potatoes were left on the fire overnight and when boiled again the next morning they were not yet cooked. I remembered this while listening to my colleagues discuss the cause of that; they had concluded that the damn potatoes were not for cooking. »

Curiously, the higher the altitude, the faster it cooks in a pressure cooker, since if the starting point (1) is lower, line 1 → 2 will reach a higher temperature at the working pressure of the pot.

 

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