How Pressure Cookers Work

MC / February 5, 2013

Pressure cookers are fantastic tools. They develop the characteristic flavors and textures of foods so quickly that what is conventionally a long, labor-intensive process becomes one hardly more time-consuming than a casual sauté. Risotto takes six minutes instead of 25. An intense chicken stock takes only 90 minutes. You can even pressure-cook food in canning jars or in oven bags or FoodSaver bags rated for high temperatures–which means grits and polenta, for example, no longer require constant stirring to avoid sticking. The high temperatures inside the cooker also promote browning and caramelization, reactions that create flavors you can’t get otherwise in a moist cooking environment. If you aren’t a believer, try our Caramelized Carrot Soup recipe.

A pressure cooker is essentially just a pot with a semi-sealed lockable lid and a valve that controls the pressure inside. It works by capturing the steam that, as it builds up, increases the pressure in the vessel. The pressure increase in turn raises the boiling point of water, which normally limits the cooking temperature of wet foods to 100 °C / 212 °F (at sea level; the boiling point is slightly lower at higher elevations). Because the effective cooking temperature is higher in the pressure cooker — as high as 120 °C / 250 °F — the cooking time can drop substantially.

Take a look below at our cutaway photo from Modernist Cuisine at Home. The letters correspond to an explanation of each part of the pressure cooker.

    1. High-pressure steam rapidly transfers heat to the surface of any food not submerged in liquid.
    2. A spring-loaded valve is normally open so that air can escape. As heating begins, expanding vapor pushes this valve up, closing off the vent. (At very high pressures, it rises farther and reopens the vent to release excess steam.) The valve regulates the pressure inside the cooker to a preset level: typically 0.7 or 1 bar / 10 or 15 psi above atmospheric pressure; this value is called the gauge pressure. At these elevated pressures, water boils at 114 °C or 121 °C / 237 °F or 250 °F, respectively. As soon as the cooker reaches the correct cooking pressure, reduce the heat to avoid over-pressurizing it.
    3. The sealing ring, typically a rubber gasket, prevents steam and air from escaping as they expand. This causes the pressure in the vessel to build as the temperature rises. Any food particles stuck in the seal can cause it to leak steam, so check and clean the gasket regularly.
    4. The lid locks with a bayonet-style mechanism that pushes against the sides of the cooker. Frequent over-pressurization can damage this mechanism and render the cooker useless. Other designs use bolts that clamp around the outside.
    5. The handle locks as well, to prevent the lid from opening while the contents are under pressure.
    6. There is too much liquid in this cooker. Generally, you should fill the pot no more than two-thirds full.
    7. Water vaporizes into steam, increasing the pressure inside the cooker as it heats. Because the boiling point of water depends on pressure, it rises too, just enough to keep the water and steam temperature hovering around the boiling point for the higher pressure. The pressure continues to rise until it is stabilized by the valve.
    8. Add enough water to the pot, either around the food or under a container of food elevated above the bottom of the pot, to enable plenty of steam to form.

Ready to start cooking? Check out our library for our Carnitas, Caramelized Carrot Soup, Risotto, and Garlic Confit recipes.

–adapted from Modernist Cuisine at Home


36 Responses to “How Pressure Cookers Work”

  1. How do you prevent burning?? I tried to make the refried bean foam in my Fagor 8-qt and followed the directions to a T and after an hour I opened the lid and the beans were burnt to the bottom. I doubled the recipe as well cause I felt like my pressure cooker was to big for just the single amount.

    • Hi Happy Valley Chow,

      One trick the chefs often recommend is to give the pressure cooker a shake every now and then, especially as it’s coming up to pressure. This is also good to do for recipes such as the vegetable soups, if you’ve had similar problems.

      Hope that helps!

  2. I made the garlic confit as per the instructions. When the process was complete, some of the olive oil in the jar had escaped onto the water in the pressure cooker. Did I loosen the screw band of the jar too much before putting it into the PC or is there another reason for the oil escaping? Thank you.

    • Elsie,

      When the food heats while processing, steam escapes pushing out the air. If you screwed your lids tighter, this would affect your ability to properly seal your jar in an airless condition.

      (Is this garlic confit recipe from Modernist Cuisine or another source?)

      Regards,
      Cathy

      • Thank you, Cathy, for your reply. In my case, not only steam escaped, but the olive oil as well. The recipe I followed is from Modernist Cuisine and I followed the instructions. So my question remains – did I loosed the screw bands too much after loosening them? I gave it what I thought was a quarter turn but maybe it was too mich?

        • Allan Risk

          Not entirely true. A pressure cooker that can reach and maintain 10 psi can – in theory – be used to can. The problem with using a pressure cooker stems from the fact that the volume inside a cooker is relatively small compared to a canner. When you fill the space up with filled mason jars, it’s almost impossible to maintain constant the 10 psi required for most canning.

          However, if you put in, say, only one or two jars, and add water to come ⅓ of the way up, there is less chance of the pressure fluctuations that result in insufficient processing to occur.

    • Allan Risk

      This can happen if the pressure fluctuates during processing OR if the pressure is released too rapidly. In either case, the too-rapid drop of pressure in the cooker causes the contents of the jar – which is temporarily hotter than the surrounding environment in the cooker – to boil violently, and force out some of the liquid contents.

  3. i was wondering what is the best size pressure cooker to get seeing as you don’t use much liquid do you buy a bigger one so as you always have enough space and the smaller amounts of food still cook well I’m tossing up between the duromatic 8ltr and 12ltr the 12ltr looks big but would be great for stocks but i thought for 4 portions of anything else it would be overkill.. any advice would be welcome

  4. Modern pressure cookers offer many safety features that pressure cookers of the past simply did not. If overpressure occurs modern pressure cookers vent excess steam from a valve stem with an audible “hiss”. If pressure were to continue to rise the sealing gasket on a modern pressure cooker would be pushed out through a designed safety aperture in the lid safely venting the pressure.

  5. I have a 25 pound rocker for my Presto but unable to find what the temperature is at this level. Use it mostly for cooking Bones for our dogs. 10 minutes under pressure and turn the heat off and leave it to cool on its own. Later we add potato and rice etc and fat hamburger to the now very soft bones. Some will be concerned about feeding chicken bones to dogs but when cooked at pressure as above they turn to mush.

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