Introduction
A pressure cooker is a hermetically sealed pot that utilizes the thermodynamic principles of steam pressure to cook foods faster than conventional cookers, and use energy more efficiently. Though pressure cookers have existed for many years, they have lately gained considerable popularity as they are considered to be among the beast means for healthy cooking in the market. Moreover, pressure cookers are very simple to use, and the resulting appearance and flavor of foods cooked in them make it difficult to believe how fast the cooking process is actually completed.
This paper will focus on the thermodynamic principles under which pressure cookers operate, as they do not seem to have gained similar popularity. Furthermore, this paper will enlighten the reader on the historical background of pressure cookers, design specifications as well as the advantages and disadvantages over traditional cooking appliances.
Background
The invention of the pressure cooker corresponds to a British physicist named Denis Papin; in 1679, he created the first model of the device, though it was first called a steam digester. According to Encyclopedia Britannia, the invention consisted of “a closed vessel covered with a tightly fitting lid that confines the steam until a high pressure is generated, raising the boiling point of the water considerably”. In order to prevent explosions, Papin added a safety valve for pressure release he invented himself. It was Papin’s vision that the steam digester would help the poorest individuals extract nutrients from bones; unfortunately, the early versions of the pressure cooker were not affordable to most, and were thus likely sold only to affluent households. Steam digesters also generated interest in the military, who utilized them to prepare meals quickly in camps.
In the 20th century, the term pressure cooker replaced steam digester, and they began to be present in kitchens of ordinary homes; however, they still had an industrial look to them, as they were composed of cumbersome gauges and valves. In the following years, their appearance progressed and began to resemble more common cookware. Subsequently, and due to the effects of World War Two, pressure cookers became widespread and recipe books exclusively devoted to pressure cooking were published. By the 1950s, approximately 40% of American households owned a pressure cooker, according to Amazing Food.
Basic Principle of Operation
Pressure cookers are sealed, and thus water within is heated to produce very hot steam, increasing the temperature inside the pot to up to 266 °F, which is higher than the temperature achieved by other types of cookware. As temperature rises and steam builds up inside the pot, pressure also increases. The increased temperature and pressure make food cook faster, with more concentrated flavors and do not reduce vitamin and mineral content, as explained by Rothkopf.
Thermodynamics of Pressure Cookers
Furthermore, pressure is directly related to temperature, according to Mows, the Gay-Lussac’s Law statesP1T1= P2T2. Moreover, the behavior of ideal gases is described by the equation of state PV=nRT, where n is the number of moles of material, R is a constant and P, V, and T stand for pressure, volume and temperature, respectively. This equation, however, does not embrace gases that become liquids. In this regard, the Van-Der Waals Gas Law, P+An2V2-V-nVo=nRT, explains that at given pressure and temperature levels, more than one volume can exist (liquid state and gas state). In this equation, P indicates fluid pressure, V is the total volume of the container, A represents the measure of attraction between the particles, Vo is the volume excluded by a mole of particles, n is the number of moles, R is the universal gas constant and T represents the absolute temperature. It is evident from both equations that as pressure increases in the pressure cookers, temperature must rise as well in order to balance the equation, as the number of moles remains constant.
The atmospheric boiling point of water is 100 °C or 212 °F at a pressure of 1 atm, which corresponds to the temperature under which the vapor pressure of the liquid is equal to the atmospheric pressure at sea level, and thus bubbles can form within the liquid. Above this point, water remains at a constant temperature until it has fully evaporated; further increasing the heat added to the system only increases the rate of evaporation, but not the temperature of the liquid itself. However, in pressure cookers, the lid prevents steam from escaping, thus building up steam inside the pot. Consequently, pressure increases and the boiling temperature of the water contained in the pressure increases as well, to approximately 250-266 °F.
The First Law of Thermodynamics, which is an adaptation of the Law of Conservation of Energy to thermodynamic systems, states that in an isolated system energy cannot be created nor destroyed. More specifically, according to Boundless, “any change in the internal energy (ΔE) of a system is given by the sum of the heat (q) that flows across its boundaries and the work (w) done on the system by its surroundings”. This law is expressed by the following equation: ∆E=q+w.
The pressure cooking process is isochoric, which means that volume is constant throughout the procedure and thus no work is done by the system. Therefore, the equation Q=∆E=32*nR∆T describes the system, where ΔT represents the temperature change, as explained by Mows.
Moreover, the Second Law of Thermodynamics states that in a closed system entropy either remains constant or increases (Zimmerman). Applied to the pressure cooker, this law explains why heat flows spontaneously from the hot air within the pot into the food, effectively cooking it. This transfer of heat occurs through convection (high-pressurized steam circling the food), conduction (through the heating of the pot’s metal walls, which is contact with the food), and radiation (from the electricity used to run the pressure cooker).
Pressure Cooker Design
Figure 1 shows a basic diagram of a pressure cooker, indicating its most important elements. According to National Presto Industries, the function of each element is:
Pressure regulator: Its function is to control and maintain the desired pressure level within the pot, typically at 15 psi. At this level, the regulator begins a gentle rocking motion and if it exceeds this level, it releases steam into the atmosphere to relieve pressure.
Vent pipe: It allows the release of excess pressure as it fits the pressure regulator.
Air vent/cover lock: Acts as a visual indicator of the pressure inside the pot, and automatically exhausts air from the cooker. It acts as a visual indicator given that if the lock is in the upward position, it indicates that the pressure cooker contains high pressure, whereas if it is in the downward position, it signifies there is no pressure in the unit. This allows users to determine whether it is or is not safe to open the pressure cooker.
Lock pin: It is located on the cover handle and prevents the lid from being lifted when there is pressure in the pot, as it functions jointly with air vent/cover lock.
Sealing ring: Its function is to act as a pressure-tight seal between the lid and the pot during the cooking process.
Overpressure plug: It is located at the lid or cover of the pressure cooker and it automatically releases steam in the event that the vent pipe cannot release pressure due to clogging.
Cooking rack: It is located at the bottom of the cooker, and holds food outside the liquid to allow for simultaneous cooking of different foods without mixing flavors.
Figure 1 – Basic parts of a pressure cooker
Risks Associated to Pressure Cookers and Safety Measures
As explained, pressure cookers are designed to withstand great pressures; if the pressure exceeds the permissible level, the regulator releases steam to relieve it. However, this mechanism can fail, due to clogging or other mechanical complication. As a result, the inner pressure may surpass the cooker’s capacity, which could lead to an explosion. These accidents were relatively common in the early years of pressure cooker development, as units did not possess the security features that characterize them today.
Pressure cookers have incorporated improved pressure relief valves, which can be in the form of rising valve stems or movable dials. Additionally, there are a series of secondary valve systems, which can also release excess pressure in case the primary regulator fails to do so. Moreover, a locking device does not allow for lid removal in unsafe conditions. A gauge acts as a visual indicator of the pressure level inside the cooker.
The possibility of a general system failure still exists, however, the potential for damage pressure cookers have is relatively low. In the eventuality that the lid latch fails, it would be violently separated from the unit, but at a speed that could only cause significant damage if a person is standing too close to it at the moment of the accident. Moreover, it is highly unlikely that the vessel itself fails explosively.
The main risks associated to current pressure cookers are not related to unit failure, but rather to human misuse or error. Silberman explains that placing their faces in front of the relief valves, which could lead to burnings due to the release of hot steam is a major risk. Moreover, people can open the cooker before the pressure inside has equalized, thus when lifting the lid boiling water and steam shoots out through the gap, potentially burning the user.
Harlan lists a series of safety tips to avoid pressure cooker accidents, among which are:
Checking the equipment before cooking: This involves making sure the sealing ring is not dried or cracked, and it is recommended to change it once a year.
Filling the pressure cookers up to two-thirds (2/3) of its capacity, as filling it more could block the steam releasing vents.
Use sufficient liquid for steam generation. Typically, at least half a cup of water should be used.
Constantly check on the unit while it is cooking for potential blockage of steam valves.
Do not use oil as it could melt parts of the unit.
Release pressure safely. This can be done by removing the pressure cooker from the heat source until pressure decreases naturally, running cold water over the closed unit or use the valve to release steam.
Benefits of Using a Pressure Cooker
The main advantage of pressure cookers over conventional saucepans is the reduced cooking time, which can be from three to ten times faster than traditional methods, consequently saving money and energy as well. Moreover, the super-heated steam in pressure cookers intensifies the food’s natural flavor, thus they require less salt, sugar, condiments or other additives.
Goldbeck and Goldbeck also explain that the reduced cooking time and airtight environment of pressure cookers results in maximum retention of nutrients, retaining more than other cooking methods, considering that because vitamins, minerals and other nutrients do not exit the unit throughout the process. Additionally, as foods are cooked by steam and the cooking rack can separate liquid fats contained in foods, pressure cookers allow the preparation of low-fat meals.
Conclusions
Pressure cookers, invented by Denis Papin in 1679, provide an alternate method for cooking based on the utilization of steam. Water is contained within a hermetically sealed unit, and heated until its boiling point. Steam is generated but it cannot exit the unit, therefore increasing pressure inside the cooker and, consequently, increasing the boiling temperature of the liquid as well up to 266 °F. Pressure cookers appear to be a great alternative for traditional saucepans, as they reduce cooking time, improve flavor and maximize nutrient retention. It is important for pressure cookers to incorporate safety valves, which can release steam to relieve excess pressure.
References
Amazing Food. "Pressure Cooker." 2014. Amazing Food - Made Easy. Web. 12 May 2016.
Berenstein, D. "Everyday physics: pressure cookers." 17 September 2008. Shores of the Dirac Sea. Web. 12 May 2016.
Boundless. "The Three Laws of Thermodynamics." n.d. Boundless. Web. 12 May 2016.
Encyclopedia Britannica. "Pressure cooker ." n.d. Encyclopedia Britannica. Web. 12 May 2016.
Harlan, Jessica. "Pressure Cooker Safety Tips." 31 May 2015. About Food. Web. 12 May 2016.
Mows, Michael. "The Kinetic Theory of Gases and Laws of Thermodynamics in a Pressure Cooker." 17 January 2012. Michael Mows Blog. Web. 12 May 2016.
National Presto Industries. "The History of Pressure Cooking." 2007. Discover Pressure Cooking. Web. 12 May 2016.
National Presto Industries, Inc. "School Program: Pressure Cooker Parts." n.d. PRESTO. Web. 12 May 2016.
Nikki, Goldbeck and David Goldbeck. "How to Use a Pressure Cooker." October 1990. Mother Earth News. Web. 12 May 2016.
Rothkopf, Marissa. "Gadget lust: Giving into the pressure cooker." 09 January 2014. Newsweek. Web. 12 May 2016.
Silberman, Gil. "Under what circumstance would a pressure cooker explode, and what kind of damage would it do?" 20 January 2015. Quora. Web. 12 May 2016.
