Editing Pressure cooker (section)

== Theory ==
{| class="wikitable floatright sortable"
|+ Pressure cooker pressures and temperatures
|-
! Gauge Pressure<br/>(rel. to sea level) !! Temp. !! Approximate<br/>Cooking time<br/>versus<br/> boiling<br/><ref group=note>Based on the chemistry rule-of-thumb that [[reaction rate]]s double for every {{cvt|10|C-change}} increase in temperature.</ref>
|-
| {{cvt|0|bar|psi}} || {{cvt|100|C}} || 100%
|-
| {{cvt|0.1|bar|psi}} || {{cvt|103|C}} || 80%
|-
| {{cvt|0.2|bar|psi}} || {{cvt|105|C}} || 70%
|-
| {{cvt|0.3|bar|psi}} || {{cvt|107|C}} || 61%
|-
| {{cvt|0.4|bar|psi}} || {{cvt|110|C}} || 50%
|-
| {{cvt|0.5|bar|psi}} || {{cvt|112|C}} || 43%
|-
| {{cvt|0.6|bar|psi}} || {{cvt|114|C}} || 38%
|-
| {{cvt|0.7|bar|psi}} || {{cvt|116|C}} || 33%
|-
| {{cvt|0.8|bar|psi}} || {{cvt|117|C}} || 31%
|-
| {{cvt|0.9|bar|psi}} || {{cvt|119|C}} || 27%
|-
| {{cvt|1.0|bar|psi}} || {{cvt|121|C}} || 23%
|}
[[File:Dampfdruckkurve.svg|thumb|right|The approximate vapor pressure of water as a function of temperature, or when viewed sideways, the boiling point of water as a function of pressure.]]
At [[Standard temperature and pressure|standard pressure]] the [[boiling point]] of water is {{convert|100|C|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 [[Evaporation|vaporize]] into [[steam]] efficiently carrying away heat keeping the food temperature at 100&nbsp;°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]]:<ref name=serious>[https://www.seriouseats.com/how-pressure-cookers-work How Pressure Cookers Actually Work By Tim Chin Jan. 28, 2022]</ref>

<math display="block">PV = nRT</math>
or<math display="block">T = PV/nR</math>
where <math>P</math>, <math>V</math> and <math>T</math> are the [[pressure]], [[volume]] and [[Thermodynamic temperature|temperature]]; <math>n</math> is the [[amount of substance]]; and <math>R</math> 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 (unit)|bar]] or {{convert|100|kPa|psi|0|abbr=on|lk=on}} [[Pressure measurement#Gauge|above]] the existing [[atmospheric pressure]], the water will have reached a temperature of approximately {{convert|120|C|F|0}} 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&nbsp;°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 (2.275&nbsp;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 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 {{convert|120|C|F|0}} 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&nbsp;°C per every 294 metres of altitude (see: [[High-altitude cooking]]), causing the boiling point of water to be significantly below the {{convert|100|C|F}} at [[Standard temperature and pressure|standard pressure]]. This is problematic because temperatures above roughly 90&nbsp;°C are necessary to cook many common vegetables in a reasonable time. For example, on the summit of Everest ({{cvt|8848|m|ft}}), the boiling point of water would be only {{convert|70|C|F}}. Without the use of a pressure cooker, many boiled foods may remain undercooked, as described in [[Charles Darwin]]'s ''[[The Voyage of the Beagle]]'' (chapter XV, March 20, 1835):

{{blockquote|Having crossed the Peuquenes [Piuquenes], we descended into a mountainous country, intermediate between the two main ranges, and then took up our quarters for the night. We were now in the [[Mendoza Province|republic of Mendoza]]. The elevation was probably not under {{convert|11000|ft|m}} [...]. 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 [[Steam digester|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.}}

When pressure cooking at high altitudes, cooking times need to be increased by approximately 5% for every {{convert|300|m|ft|abbr=on}} above {{convert|610|m|ft|abbr=on}} 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.{{citation needed|date=September 2019}}  Lightweight pressure cookers as small as {{convert|1.5|L|gal}} weighing {{convert|1.28|kg|lb}} are available for mountain climbers. [[Sherpa people|Sherpas]] often use pressure cookers in base camp.<ref>{{Cite journal|last=The Lancet|date=May 2003|title=A more challenging summit than Everest|journal=The Lancet|volume=361|issue=9372|pages=1837|doi=10.1016/s0140-6736(03)13535-0|s2cid=30840142|issn=0140-6736}}</ref>

=== Health benefits ===
Some food toxins can be reduced by pressure cooking. A Korean study of [[aflatoxin]]s in rice (associated with ''[[Aspergillus]]'' fungus) showed that pressure cooking was capable of reducing aflatoxin concentrations to 32% of the amount in the uncooked rice, compared to 77% from ordinary cooking.<ref>{{cite journal |last1=Park |first1=Je Won |last2=Kim |first2=Young-Bae |title=Effect of Pressure Cooking on Aflatoxin B in Rice |journal=Journal of Agricultural and Food Chemistry |date=March 2006 |volume=54 |issue=6 |pages=2431–2435 |doi=10.1021/jf053007e |pmid=16536630 }}</ref>