Why does a pressure cooker cook so fast? Boiling point and pressure

Seven in the morning. Chickpeas soaked overnight have been drained and tipped into the pressure cooker, and the lid is locked. After a while a familiar whistle fills the kitchen. In an open pan the same chickpeas would need an hour, sometimes more; in a pressure cooker they usually soften in a fraction of that time. How does that small combination of a sealed lid and a whistling valve cut the cooking time so dramatically?

The answer lies in a sentence most of us learnt by heart at school without ever questioning it: “Water boils at 100 °C.” That sentence is incomplete. The full version is this: water boils at 100 °C at sea level, under normal atmospheric pressure. When the pressure changes, the boiling temperature changes too, and that is the whole secret of the pressure cooker.

What exactly is boiling?

Leave a glass of water on the table and within a few days you will see that the level has dropped. Water evaporates a little at every temperature: the fastest molecules at the surface break away from the liquid and escape into the air. In a closed container this vapour builds up a pressure, called the water’s vapour pressure. As the temperature rises, the molecules move faster and the vapour pressure climbs steeply: from about 2.3 kPa at 20 °C to about 47 kPa at 80 °C, about 101 kPa at 100 °C and about 199 kPa at 120 °C.

Boiling is evaporation that happens not only at the surface but inside the water as well. Imagine a bubble of vapour forming at the bottom of the pan. For that bubble to grow without being crushed, the pressure of the vapour inside it must equal the pressure pushing in on it from outside. So:

Water boils when its vapour pressure equals the pressure outside.

At sea level the air presses down with an average pressure of about 101.3 kPa (1 atmosphere). The vapour pressure of water reaches this value at 100 °C. So 100 °C is not a universal constant; it is a number that belongs to this particular pressure.

Why does water boil at a lower temperature in the mountains?

As you climb higher, the column of air above your head gets shorter and the air pressure falls. To match a lower pressure, the water does not need to get as hot, so the boiling temperature drops. Here are approximate values calculated with the standard atmosphere model:

As a rough rule, the boiling temperature drops by about 1 °C for every 300 metres you climb. Real air pressure also shifts a little with the weather, so actual measurements can differ by a few tenths of a degree.

This difference has a very practical consequence in the kitchen: cooking depends on temperature, not on bubbles. In Erzurum a pan bubbles away just as eagerly as one in Izmir, but the water inside is 6–7 °C cooler. Dried pulses soften, eggs set and the connective tissue in meat turns into gelatin more slowly. That is why, up on the high plateaus, food never seems to finish cooking.

The pressure cooker’s trick

The pressure cooker plays this game the other way round: instead of lowering the pressure, it raises it. The rubber sealing ring in the lid closes the pan airtight. As the water heats up, the steam it produces cannot escape, and the pressure inside rises. A higher pressure means the water needs a higher vapour pressure to boil, in other words a higher temperature.

The pressure does not rise forever. The valve in the lid (a whistle or a weighted pin) opens when the pressure inside reaches a set value and lets out the excess steam. The whistle that fills the kitchen is the sound of this controlled release. Most home pressure cookers work at about 0.5–1 bar (50–100 kPa, or roughly 7–15 psi) above the outside air pressure. At sea level, that means water boils at about 110–120 °C (230–248 °F).

A subtle detail: the valve controls the difference between the pressure inside and the pressure outside. In Ankara the outside pressure is already lower, so the temperature inside the same cooker stays about 2 °C below what it reaches at the coast. That is where the advice to add a little extra cooking time at altitude, even in a pressure cooker, comes from.

Why do twenty degrees make such a difference?

The gap between 100 °C and 120 °C looks small, but it has a big effect in two separate ways.

First, heat flows into the food faster. The rate of heat flow is proportional to the temperature difference: between a chickpea at 20 °C and water at 120 °C the difference is 100 degrees, a quarter more than the 80-degree difference with water at 100 °C.

Second, the changes that soften food speed up with temperature. In pulses, substances such as pectin that glue the cells together dissolve; in meat, collagen turns into gelatin; starch takes up water and swells. All of these speed up markedly as the temperature rises. As a result, for long-cooking foods such as chickpeas, dried beans or meat on the bone, the cooking time is often cut by half or more. Exactly how much shorter it gets depends on the food, the size of the pieces and the cooker.

What a pressure cooker cannot give you is a crisp, browned crust. The Maillard reaction behind browning speeds up above about 140 °C, while the inside of the cooker is full of water and sits at around 120 °C.

A short history: Papin’s “bone softener”

The pressure cooker’s ancestor goes back to the 17th century. In 1679, in London, the French physicist Denis Papin showed the Royal Society a tightly sealed vessel in which the steam pressure built up until it could soften even bones, and he called it a “digester”. In a booklet published in 1681 he described how the vessel could be used for cooking. Papin also designed a safety valve that let out excess steam so that the vessel would not burst; the valve on today’s cookers goes back to his idea. Pressure cookers for the home became widespread in the 20th century.

Safe use: the physics behind the rules

A pressure cooker is a vessel holding water above 100 °C and steam under pressure. The rules for using it are not arbitrary:

1. Do not open the lid until the pressure has dropped. The water inside stays liquid only because of the high pressure. If the pressure falls suddenly, this superheated water boils instantly, and steam and food can spray out. Release the pressure the way your cooker’s manual describes, and never force the lid before the indicator or pin has dropped.
2. Keep the valve and sealing ring clean. A blocked valve cannot let out excess steam. Before every use, check that the valve is clear and that the sealing ring is not cracked; replace a worn ring.
3. Respect the fill limit. As a general rule, fill the cooker no more than two-thirds full, and no more than half full for foods that foam and swell, such as chickpeas, dried beans, rice and bulgur wheat. Foam and skins can block the valve.
4. Never use it dry. There must always be enough liquid in the cooker to make steam.
5. Turn the heat down once it reaches pressure. The valve already holds the pressure steady. A fierce flame only blows off more steam, boils the water away and wastes fuel.
6. Watch out for steam. Never put your face or hand over the valve while steam is escaping. Steam burns worse than water at the same temperature, because when it touches the skin it condenses and releases there the large amount of energy it took up when it evaporated.
7. Follow the manual for your own cooker. Children should not use a pressure cooker on their own.

Three common misconceptions

“Adding salt makes water boil hotter.” Only very slightly. A tablespoon of salt in 2 litres of water raises the boiling point by only about 0.15 °C. The sudden fizzing when you add salt has nothing to do with heat: bubbles simply form easily on the rough surfaces of the salt crystals. Salt is added for flavour.

“If I turn the heat up full, boiling water gets even hotter.” In an open pan, the temperature of boiling water is set by the pressure. Extra heat only turns more of the water into steam. Turning the heat down once the water is boiling, as long as it keeps boiling, makes almost no difference to how fast the food cooks.

“The more the whistle blows, the better the food cooks.” The whistle tells you that the pressure inside has already reached its limit. A whistle that keeps on blowing only shows that surplus heat is being thrown out as steam.

Summary

• Water boils when its vapour pressure equals the pressure outside; at sea level that happens at 100 °C. Higher up, both the pressure and the boiling temperature fall: to about 97 °C in Ankara, 93–94 °C in Erzurum and about 70 °C on Everest.
• Cooking speed is set by temperature, not by bubbles. With 0.5–1 bar of extra pressure, a pressure cooker boils water at about 110–120 °C and greatly shortens the cooking time of long-cooking foods.
• Safety comes from physics: do not open the lid before the pressure has dropped, keep the valve and sealing ring clean, respect the fill limit and turn the heat down once it reaches pressure.