The Science of Pressure Cookers

4.75 from 4 votes

Pressure cooker with steam coming off the top

This is an excerpt from Cooking For Geeks, Second Edition, reprinted with permission.
Get your copy today on Amazon.

Pressure cookers are like an old-fashioned version of the microwave: a convenient appliance that speeds up cooking. Our grandparents used a manual version, essentially fancy pots with locking lids that went on the stovetop. Manual versions are still available todayโ€”now enhanced with safety locks and over-pressure release valves to prevent accidentsโ€”and are a worthwhile investment for the serious pressure cooker enthusiast. Manufacturers also make electric units, which can safely be left unattended and are what I suggest for first-time buyers. If you have a tiny kitchen, get an electric unit that has modes for slow cooking and cooking rice.

Itโ€™s not the pressure itself that changes how foods cook, but the impact of pressure on physical and chemical processes. Increasing pressure always increases the boiling point of water. In wet cooking methods, the temperature differential between the food and the liquid heating it is what determines how quickly the food heats up. Increasing the pressure increases the boiling point of water, but itโ€™s not boiling water per se that does the cookingโ€”itโ€™s the larger temperature difference between the higher- temperature liquid and colder food that results in a faster rate of heat transfer.

How much faster heat will transfer into food depends on the maximum temperature at which the liquid can boil. Depending on the make and model, a pressure cooker can increase the pressure by 11โ€“15 psi (758โ€“1,034 hPa). Thereโ€™s no formal standard for how much pressure a cooker should operate at, but most recipes are written assuming a high pressure of 15 psi (1,034 hPa) and low pressure of 8 psi (550 hPa). (Underwriters Laboratories wonโ€™t certify units above 15 psi, which is why you donโ€™t see higher pressures than that.) You will need to adjust cooking times based on the pressure at which your model operates!

Pressure increases are relative to your current atmospheric pressure, so the maximum boiling point of water is based on your current air pressure plus the additional pressure your unit adds to it. If you live at sea level and have a higher-pressure unit, youโ€™ll be able to get water up to 29.7 psi (2,048 hPa) for a boiling point of 250ยฐF / 121ยฐC, but a mile up and with a unit that only adds 11 psi (758 hPa), youโ€™ll only raise the boiling point to 236ยฐF / 113ยฐC. Take a look at this zoomed-in part of the phase diagram of water from the previous section, cropped to the starting and maximum pressures possible in pressure cooking. Add the operating pressure of your pressure cooker to the air pressure at your elevation to look up how hot (and thus how quickly!) your foods will cook.

Graph of Pressure vs. Boiling Point of Water

Pros

โ€ข Speed! Raising the boiling point of water from 212ยฐF / 100ยฐC to 248ยฐF / 120ยฐF roughly quadruples how quickly culinary reactions can occur, cutting cooking times by ~60โ€“70% (it takes some time to heat up the food; otherwise, itโ€™d be closer to 75%). Pressure cookers are fantastic for cooking slow-cooking grains and legumes (rice and lentils in 5โ€“8 minutes instead of 30), beans (30 minutes for dry, unsoaked beans to be ready to serve), and higher-collagen meats (ribs, pot roast, and pulled pork can all be cooked in under an hour are deliciously easy).

โ€ข Electric units are extremely energy efficient, meaning theyโ€™re great for summer cooking where you want something thatโ€™s normally slow-braised for hoursโ€”say, pulled porkโ€”but donโ€™t want to adds lots of heat to your kitchen.

Cons

โ€ข You wonโ€™t be able to poke at food while it cooks to adjust seasoning or check if itโ€™s done. What goes in the pot at the start is what youโ€™re going to get at the end, as when baking a cake. If youโ€™re an intuitive cook, winging it as you go, treat pressure cooking as a way of cooking one ingredient that youโ€™ll then use as a component in your meal.

โ€ข With the faster rate of reactions, overcooking will happen much more quickly. Itโ€™s better to slightly undercook something and continue cooking it โ€œoff pressure.โ€ Take notes on cooking times on your recipes. (Use the low-pressure setting on vegetables to avoid overcooking.) Different pressure cookers will run at slightly different pressures, so treat times for recipes as starting points and take notes.

โ€ข Pressure cookers rely on boiling water or steam for transmitting heatโ€”itโ€™s a wet cooking methodโ€”and trap and condense most of the moisture, making it hard to reduce sauces. You may need to reduce the liquids after cooking. On the flip side, donโ€™t skimp on the liquid: make sure that you have at least a cup or two of water in the unit; otherwise, there wonโ€™t be anything to turn into steam and youโ€™ll end up burning the bottom of whatever youโ€™re cooking.

Tips and tricks

โ€ข If you want to adapt other recipes to a pressure cooker, think about things that normally cook via steaming, braising, or any wet cooking method, and try cooking for a third of the suggested time. Make sure not to fill the cooker more than two- thirds full; some ingredients will expand as they cook, and blocking the release valve is bad. If using ingredients that foam while cooking โ€” applesauce, barley, oatmeal, pasta โ€” donโ€™t fill the cooker more than one-third full. Be aware that dairy curdles under pressure, so add any dairy ingredients after pressure cooking.

โ€ข If you have a stovetop unit, you can rinse it under cool tap water to cool it down quickly after cooking; this is useful for quick-cooking foods like vegetables or polenta where the residual heat would continue cooking them.

โ€ข Many electric pressure cookers operate at 12 psi (830 hPa) instead of 15 psi (1,034 hPa), meaning cooking times for recipes based on the slightly higher pressure may need to be extended by 15โ€“20%. Check the manual for the operating pressure, not the rated air pressureโ€”manufacturers tend to list the maximum pressures their units reach and bury the true pressures at which they cook.

โ€ข Steam vegetables and artichokes using a metal steaming tray to raise them above the water level. You can also cook small quantities of food in a small glass or metal bowl this wayโ€”just remember to pour a cup or two of water into the pressure cooker! Donโ€™t use plastic containers in a pressure cooker; theyโ€™ll melt.

โ€ข Pressure cookers are great for making stock. Save bones from meals in a container in the freezer. Once the container is full, transfer the contents to the pressure cooker, cover them with water, and cook for 30 minutes. Cool and strain the liquid.

โ€ข Try using your pressure cooker to render tallow or lard: toss chopped-up fatty meats into a jar, cover them with water, add a cup of water to the pressure cooker, and render for about 2 hours. Let the fat cool to a safe-to-handle temperature and pour through a strainer.

mung beans


mung beans
4.75 from 4 votes

Indian Moong Dal Khichdi

By: Jeff Potter
There are a million ways to make khichdiโ€”an Indian dish that mixes lentils and rice with spices. Khichdi translates to โ€œmixture,โ€ and thatโ€™s exactly what this is. Hereโ€™s a recipe based on how I first had it, but experiment! Try adding other spices, such as fennel seeds, cumin seeds, garam masala, or curry powder.
Prep: 10 minutes
Cook: 5 minutes
Total: 15 minutes
Servings: 4 servings

Ingredients 

  • 2 tablespoons cooking oil, butter, olive oil, ghee, or coconut oil
  • 1 medium red onion, chopped
  • 1 tablespoon coriander seeds, whole, or ground
  • 1 tablespoon turmeric powder
  • 1/2 teaspoon cayenne pepper
  • 1/2 cup white basmati rice
  • 1 cup moong dal (split yellow mung beans), or red lentils
  • 6-12 cloves garlic, peeled
  • 1-2 tablespoons peeled, diced ginger root
  • 1 lemon, juiced
  • salt, to taste
  • cilantro or parsley, for garnish

Instructions 

  • Sautรฉ cooking oil, red onion, coriander seeds, turmeric powder, and cayenne pepper in a skillet, manual pressure cooker pan, or electric pressure cooker on sautรฉ mode.
  • Add rice, moong dal, garlic, and ginger root and stir to coat, and transfer to pressure cooker if necessary.
  • Add 3 cups of water and close the pressure cooker. Cook under high pressure for about 5 minutes.
  • Allow the mixture to cool, then open and stir it. Add juice from one lemon and salt to taste.
  • Serve with cilantro or parsley.

Notes

The first time I had this, it was served with lots of fresh raw arugula, which added a nice flavor and texture.

Nutrition

Calories: 344kcal, Carbohydrates: 54g, Protein: 14g, Fat: 8g, Sodium: 11mg, Potassium: 749mg, Fiber: 9g, Sugar: 3g, Vitamin A: 165IU, Vitamin C: 4.6mg, Calcium: 95mg, Iron: 4.7mg
Like this recipe? Rate and comment below!

Cooking for Geeks cover imageCooking For Geeks: Real Science, Great Cooks, and Good Food, Second Edition

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About the Author

Cooking for GeeksA science and food geek, Jeff Potter is the author ofย Cooking for Geeks: Real Science, Great Cooks, and Good Food, which the Washington Post called โ€œone of the most useful books on understanding cooking” (and is now in its second edition). He has appeared on NPR, The Today Show, CNN International, Food Network, and the Cooking Channel, and has spoken at science festivals both in the United States and overseas. Check out his website,ย Cooking For Geeks, or find him onย Twitter and Facebook.

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Laura
May 2, 2018 4:41 am

5 stars
If you soak brown Basmati rice overnight, you can cook it as if it were white rice? To do the soak: 1. Wash the rice til water runs clear. 2. 1 cup rice calls for 3 cups water, 1 Tbsp apple cider vinegar. 3. be sure to put in a quart sized sprout jar. Let sit on counter overnight, 8 hrs.
Next day, rinse rice in a strainer. Put in rice cooker with 3 cups water, 2 Tbsp butter, 1 tsp sea salt. When finished, fluff with a fork. I love all of your recipes!

October 21, 2017 4:02 pm

5 stars
This is such a cool article! I just got a pressure cooker 2 weeks ago and I’m loving it so far. Great info, thanks!!!

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Reply to  Carrie
October 21, 2017 7:46 pm

Carrie, you should totally get Jeff’s book! You will LOVE. IT. ๐Ÿ™‚