Making chili from dried beans, hummus from dried chickpeas, or a delicious meat stew. If you have less than an hour, they’re a no go. You need hours. Unless…
You happen to own a pressure cooker!
A pressure cooker speeds things up considerably. Allowing you to make them within an hour. But how come pressure makes food cook faster?
Pressure cooking = High-temperature cooking
Pressure cooking is called pressure cooking, because you’re literally cooking food under pressure. However, it might just as well be called “high-temperature cooking”, because, effectively, that what’s going on.
Food doesn’t cook faster because of the high pressure per se, it cooks faster because of the effect that pressure has on the temperature in the cooker!
Water boils at 100°C (212°F)
At sea level, water boils at 100°C (212°F). At its boiling point water evaporates and turns from a liquid into a gas. This takes some time, but if you leave a pot of boiling water on a stove for some time, it won’t take that long before all the water has left the pot.
Because water transitions from liquid into gas there’s no way to heat up your soup, stew, or rice dish higher than this temperature. First, the water need to evaporate, only then can you increase the temperature further.*
Cook faster at higher temperatures
If we would be able to cook at higher temperatures, cooking would go faster. All sorts of processes occur faster because of an increase in temperature. This is because the molecules that make up your food move faster at higher temperatures. They can move to other locations faster, interact with each other faster, and more.
So how do we increase the cooking temperature?
Cooking in the mountains & a pressure cooker
If you go up in the mountains you’ll find that water boils at a lower temperature. That is because the surrounding pressure of air is lower up in the mountains than it is at sea level. As a result, the pressure on the water is lower and it becomes easier for water molecules to escape the water and form a gas. That is, the boiling point, the temperature at which water transitions from a liquid into a gas, goes down.
The opposite though is true in a pressure cooker. In a pressure cooker, you build up a high pressure that’s higher than in a regular pot or pan. The high pressure in the pressure cooker presses down on all the liquid water molecules. This makes it harder for water molecules to escape the liquid and form a gas.
As a result, the boiling point of water in a pressure cooker is considerably higher.
Depending on your model of pressure cooker, the boiling point of water may increase up to about 120°C (250°F). That’s considerably higher than it would normally be. And it’s why pressure cooking cooks your food so much faster than a regular pot on your stovetop would!
10°C hotter is ~2 times faster
So how much faster can you cook? That’s hard to say precisely, it depends on the actual food you’re cooking. But, as a rough guideline, keep in mind that for every 10°C increase, cooking happens roughly twice as fast.
Why? Because chemical reactions occur approximately twice as fast for every 10°C increase. The Arrhenius equation, which describes the rate of chemical reactions, states this. Of course, cooking isn’t just chemical reactions, so this formula doesn’t hold up perfectly, but it’s the best science-based estimate there is.
How pressure builds in a pressure cooker
Regardless of the type of pressure cooker your have, stovetop or electric, it’s core mechanism is the same.
Once you fill and close your cooker, you’ll start heating the contents, whether that’s beans + water, the ingredients for a soup, or maybe a stock. Once the water in your pot is boiling, water starts to evaporate. However, that water vapor can’t leave the pot. It’s locked inside.
This causes the pressure in the pot to increase. The water vapor presses on the walls of your cooker. The hotter it gets, the higher the pressure gets. As long as the vapor can’t escape and as long as you continue to heat the pressure will keep on increasing and as a result, the temperature in the pot rises!
There’s a limit to the maximum pressure
However, we don’t want it the pressure to get too high either. Even though a pressure cooker is made up of a strong container, with a well secured lid, it can only handle so much pressure. If the pressure gets too high, it will explode, not a desirable scenario for sure!
It’s why pressure cookers contain relief valves. When the pressure inside a pressure cooker exceeds a certain pressure, the cooker will automatically let out some steam. By letting some of the water vapor escape, the pressure goes down a little, protecting the pot from exploding.
Maximum pressure in pressure cookers
So how hot can it get in a pressure cooker? Most importantly, that depends on the model of pressure cooker you own. Theoretically, we could build pressure cookers that can handle extremely high pressures, however, these would no longer fit comfortably in your kitchen and would be way to heavy to handle. The walls simply need to be too thick to hold all that pressure inside.
Instead, most pressure cookers are rated at a pressure that allows for them to still be carried around easily.
15 psi gives +20°C
The strongest pressure cookers, this includes many stovetop models and higher rated electric cookers, can handle pressures up to 1 bar (15 psi). That is, the pressure will be 1 bar higher than the surrounding air pressure.
At sea level, that would result in a boiling point of water of 120°C (250°F).
11 psi gives +16°C
Slightly lower rated models, including many electric pressure cookers, have a lower maximum pressure of approximately 0.75 bar (11 psi). That gives a water boiling point of 116°C (241°F).
That difference may sound minor compared to the higher rated cookers. But, the relationship between temperature and rates is exponential, so even a few degrees has a considerable impact on cooking rate.
Low, high Pressures for InstantPot
The InstantPot is one of the most well known electric pressure cookers. It definitely was one of the first affordable models and became highly popular. InstantPots often have at least 2 different pressure settings: low and high. I’m not too sure of the use cases of the low setting (I personally never use it), because why would you use a pressure cooker to speed things up only to then slow things down again?
That said, the pressure settings for InstantPot models are as follow:
- High Pressure: 10.2 to 11.6 psi (0.7 – 0.8 bar) – boiling point water ~115-117°C (239-242°F)
- Low Pressure: 5.8 to 7.2 psi (0.4 – 0.5 bar) – boiling point water ~109-111°C (229-233°F)
- There’s also a model that can reach 15 psi, but most can’t get that high.
Electric vs stovetop pressure cookers
Both electric and stovetop pressure cookers work pretty much the same way. The only difference is that electric pressure cookers regulate their pressure and cooking program automatically. It’s pretty much ‘set it and forget it’. You can set the duration of the cooking program, and it will stop automatically once it’s done.
Stovetop pressure cookers however need a little more experience and ‘handholding’. Whereas electric pressure cookers have sensors that determine whether more heat it needed, you need to do this yourself with a stovetop cooker. By regulating the amount of heat you put in your pot through your stovetop, you control how fast pressure builds up.
What to make in a pressure cooker
Notice how we’ve been discussing water all the time in this article? That’s because pressure cookers are made to cook food that contains a lot of water. You need the water to evaporate and build up pressure. It’s why soups, stews, rice, beans, all work great. Pressure cookers are not meant for frying or very dry dishes.
Yes, you can make cakes and puddings in a pressure cooker. To make them, you’ll need to add a layer of water at the bottom of your pot. You simply place the cake or pudding on top. Do make sure the cake is properly covered so water and steam doesn’t get in. Also, keep in mind that you’re cooking the cake in a very humid environment. So, don’t expect crunchy dry crusts. It’s why cheesecakes work particularly well for instance.
Dulce de leche / Caramel sauces
It’s surprisingly easy to make dulce de leche, or caramel sauces in general for that matter, in a pressure cooker! Whereas making them in a pot often requires a lot of stirring and waiting, making them in a pressure cooker is way less effort.
Slow cooked meats
Stews, ribs, and other meat cuts that would normally require hours to cook can be made in a pressure cooker well within the hour. In these meat cuts collagen needs to break down in order to make them tender. In a pressure cooker this simply happens much faster.
*When grilling or frying you can create temperatures higher than 100°C (212°F). However, your food only becomes this hot once enough water has evaporated. Most of the time, it’s just the outside of the food that gets that hot.
Engineering Toolbox, Water boiling point at higher pressures, link, visited Oct-2023; a tool to calculate the water boiling point at higher pressures
InstantPot, Cooking times, link
Jopson, Marty. The Science of Food: An Exploration of What We Eat and How We Cook. United Kingdom: Michael O’Mara, 2017, Section: Under pressure. link
B. Srilakshmi, Food Science, New Age International Publishers, 2003, p.18-19, link