How pressure cooking works – Using an InstantPot

If you’ve ever cooked dried chickpeas or dried beans you know it’s something that has to be planned well ahead if you intend on having dinner on the table before midnight. Same goes for stewed meats, delicious, but they take a long time to make.

However, these modern days there are solutions to speed this up. By using a pressure cooker you can cut down this time drastically, to easy within an hour. That’s why a pressure cooker had been on my wish list for a long time. It sounded great to be able to experiment with pressure cooking and see how that would affect (and speed up) cooking processes.

Most pressure cookers though are expensive or quite complicated/tricky to use. So when I realized that the InstantPot is actually an affordable type of pressure cooker, we decided we had to buy it and try it! Of course, the first three dishes were those chickpeas, dried beans and stew meats, all of them turning out great! A chili from dried beans within the hour and stewed beef in a little over half an hour cooking. Enough reason to dive deeper into the InstantPot itself and pressure cooking.

What’s an InstantPot?

The InstantPot seemed to be everywhere on the internet a while back. At the time, I wasn’t really that sure what it was all about. Everyone was raving about it, but what makes it so special? Actually, the InstantPot is several things combined into one:

  • A pressure cooker ; and a well priced one for that matter, I’d been looking for pressure cookers before, but they can easily cost hundreds of dollars/Euros. You will see that there are a lot of specific programs in the InstantPot for rice, beans, soup, etc. However, they’re all just different programs of the pressure cooker.
  • A slow cooker ; I haven’t tried this yet, and have never used a slow cooker before, but it works just like a slow cooker
  • An electric cooker (sauté function) ; you can saute food in the pot. You could theoretically use it just as you would use a regular pan to make sauces, etc. although it’s a bit more clumsy.  I wouldn’t buy an InstantPot just for this function, but it definitely is very beneficial to have this in combination with the pressure cooking functions.
  • A yogurt maker (not all InstantPots have this function) ; I certainly haven’t bought the InstantPot for this function specifically, but am planning to test it in the future. What the option does is to hold the milk for making yogurt at a constant temperature (which is sometimes a bit of hassle to do at home).

Really, the main reason to buy the InstantPot was that pressure cooking ability. The InstantPot is quite an advanced electric pressure cooker which has a lot of advantages. First of all, the InstantPot controls the heat and pressure by itself. No need for you to heat it on the stovetop and release the pressure yourself. Also, you can time the cooker, meaning it will give you a signal once it’s done. All in all, very convenient and we’ll focus on pressure cooking from here on.

There’s pressure all around us

Before we dive into pressure cooking, a note on pressure and air in general. All around us is air which constantly presses downwards on us resulting in a pressure in the air (at sea level) of approximately 1 bar. This is a normal pressure and we wouldn’t be able to live under far higher or lower pressures. Up in the mountains, pressure is lower, resulting in lower concentrations of oxygen. Down in the sea, the pressure is a lot higher, due to the water and our bodies would collapse under that pressure.

The pressure in the air determines the boiling temperature of water as well. If the pressure is low, water boils at a lower temperature. If the pressure is high, water boils at a higher temperature. If you’ve ever tried to boil an egg high up in the mountains you may have realized that you need to cook it for a lot longer to get the same consistency as would when cooking at sea level!

What is pressure cooking?

This is where pressure cooking comes in, as the name says, it truly is cooking under (increased) pressure. What a pressure cooker does is that it increased the pressure in the vat that you’re cooking your food. As a result, the boiling temperature of water goes up. In a decent pressure cooker this can go up to 120°C, which is significantly higher than the 100°C it would normally be!

This significantly higher temperature greatly increases the speed of all sorts of chemical reactions as well as the movement of molecules. Since cooking is chemistry and movement of molecules, it drastically increases the speed of cooking!

instantpot chili
A chili made from dried beans well within the hour.

Pressure cooking with the InstantPot

The InstantPot has a lot of pre-set programs you can use to cook beans, rice, meat, soups, etc. However, being the scientist I am, I don’t tend to like these programs. Whereas many will greatly appreciate them, I prefer understanding what’s going on myself. Therefore, let’s take a better look at the settings of an InstantPot (please note that I refer to the Duo Plus model since that’s the one I got, but I found that most have very similar settings and ranges).

Pressure: high & low

The InstantPot has to two settings for pressure:

  • High Pressure: 10.2 to 11.6 psi (0.7 – 0.8 bar)
  • Low Pressure: 5.8 to 7.2 psi (0.4 – 0.5 bar)

This means that while cooking in your pressure cooker the pressure in the cooker is: the pressure around you (ambient pressure, about 1 bar at sea level) + the pressure mentioned above. So at high pressure, you’re cooking at 1.7-1.8 bar, which will drastically increase the boiling point of your water. At 1.7-1.8 bar the boiling temperature of water is 115.2-116.9°C. At the lower pressure setting (1.4-1.5 bar) this temperature is 109.3-111.4°C.

If you’re wondering when you’d use the ‘low pressure’ setting, you’re not the only one. If you’re using a pressure cooker to speed up cooking you’d want the highest possible pressure and thus temperature. There only seem to be a few exceptions I found online, cooking eggs and rice are sometimes done at low pressure. Let me know if you have any others.

How is the pressure controlled?

Pressure inside a pressure cooker is controlled in two main ways. The first one is the amount of heat you put in. Remember, the pressure cooker is a closed off vessel. Water cannot escape. If you keep on heating the vessel the water will become warmer and warmer. Once it’s boiling and if water vapour cannot escape, it will start building up pressure. Without heating the pressure cooker, no pressure will be build up.

On the other side, the pressure may not be too high either. You can do this by stopping the heating process, but you can also do this by release some of the water vapour. Pressure cooker will always have a valve that lets out some vapour from inside the cooker if the pressure becomes too high. Different designs have different mechanisms, but the overall concept is very similar!

Comparing to stovetop pressure cookers

Instead of an electric pressure cooker you could also buy one that you use one your stovetop. You simply heat the pressurized pan on the stove until you’ve reached the pressure you want. These tend to require more skill to use, but work just fine. It’s good to note that these can often go up to 15 psi (1.0 bar) which results in an even higher boiling temperature and thus even faster cooking time!

Does a pressure cooker need water?

By now you should be able to answer this question to which the answer clearly is: yes. Since the pressure cooker works because of an increase in boiling temperature of your water, it does need enough water. How much greatly depends on your pressure cooker and the recipe you use though, but without water, it definitely won’t work!

Pressure cooking – A beef stew recipe

My experience with pressure cooking in an InstantPot so far has been great. That said, I would only buy a pressure cooker if you really plan on making dishes that normally take hours. Manufacturers advertise with reduced cooking times for just about anything, but those 3 minutes gained on cooking your broccoli don’t seem worthwhile to me. Dishes that normally take hours and can now be done in way less than an hour, that’s where the real benefit kicks in.

So, to close this lesson on pressure cooking, an example of a dish that worked wonderfully: fast pressure cooked beef stew! The recipe is inspired by one from Pinch of Yum.

InstantPot pressure cooked beef stew
 
Prep time
Cook time
Total time
 
Author:
Serves: 2 portions
Ingredients
  • little oil
  • 250g chuck steak (or other stew meat)
  • 1 large carrot
  • 6 celery stalks
  • 1 decently sized onion
  • 1 generous tbsp tomato paste
  • 125ml water
  • 1 tsp corn starch
Instructions
  1. Cut the meat into smaller chunks and cut the carrot, celery and onion to a similar size.
  2. Turn on the InstantPot on 'normal' saute and brown the meat with a little oil. Add the vegetables and brown a little further. This gives a great flavour!
  3. Add the tomato paste and water (note: it depends on your pressure cooker model whether this quantity of water will be enough to create pressure)
  4. Switch the InstantPot to pressure cooking mode, set on 'high pressure' and cook for 35 minutes once it's reached pressure.
  5. Once it's finished, release the steam through the release valve. It'll be pretty hot, a lot warmer than you might be used to due to the high boiling temperature so take care when you start eating!
  6. Eats well by itself or with some bread or mashed potato.
Tip 1
  1. If the stew is very liquid, take the corn starch and mix it with a little bit of cold water (to get rid of clumps) and add it to the stew Set the InstantPot on saute 'high' and cook for a few minutes until it's thickened just slightly. Add more corn starch if you prefer even thicker.
Tip 2
  1. Always follow the instructions of your own pressure cooker, you might have to adjust the recipe to make it work!

 

Sources

The InstantPot website

Calculate boiling temperature of water based on the pressure, Engineering toolbox

Initially I thought that pressure cooking sped up cooking not only because of the increased temperature, but also because of the pressure itself breaking down the food. However, I have not found any evidence for this, most literature only refers to temperature as well as increased energy transport of steam vs. air. Here’s a few of the sources I looked into: Food Science, The Science of Cooking, An Introduction to High Pressure Science and Technology p. 361

The Kitchn, When to use low or high pressure on the Instantpot

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