Pressure Cooking Explained

Several of you have asked (mostly off-blog) for more information on pressure cooking—and here at Doris and Jilly Cook, we aim to please!

The Science

First, take a listen to the “Chemistry In Your Cupboard” segment on the “Time” episode of Distillations, a podcast one of us is involved with. You’ll find out a little bit more about my obsession with pressure-cooked chickpeas, and learn how they operate on the principles of the ideal gas law. Remember PV=nRT? This equation basically means that, if the volume and the quantity of a gas are held constant, the pressure and temperature will be directly proportional. So, if the pressure doubles, the temperature doubles. And there’s one other chemical principle you need to know: a substance will maintain the same temperature throughout a phase change. So, ordinarily, when you boil water at or near sea level, it will never get hotter than 212°F until all the water has converted to steam. In a pressure cooker, the steam produced by a heating substance can’t escape, so the pressure increases. And this in turn makes the temperature increase, possibly up to about 250°F, depending on how much pressure you’re using.

Equipment

In my last post, on chicken broth, I referred both to a pressure cooker and a pressure canner. What’s the difference?

Mainly size. The one on the left is a massive, old-fashioned 22 qt. pressure canner. It can hold up to 7 quart jars, or, if you use the rack, 14 pint or jelly jars. The one on the right is a modern 6 qt. pressure cooker. You can use either one to cook with, but since the big one is so bulky I only use it to can or when I want to make something very large, like, say, stock from an entire turkey carcass. It’s also nice for steaming grains or making custards.

The above pictures show the top and bottom of the little one’s lid, with the key parts labeled. The steam escapes through the vent—you should make sure that the vent is clear every time you use the pressure cooker. Just hold the lid up to the light and look through it. The rubber ring, or gasket, ensures a tight fit. The overpressure plug is a safety feature: theoretically, if the pressure got too high or if the vent pipe became clogged, this piece would be forced out. Both the ring and the plug should be soft and flexible; if not, you need to order new ones. Presto is the largest manufacturer of pressure cookers and offers replacement parts for both old and new cookers. The pressure indicator pops up when the cooker is at pressure. NEVER attempt to open it if the indicator is up.

I also referred to something called “the pressure regulator.” Both of my pressure cookers are of the jiggle-top variety. That means that the pressure is created by placing a small weight on top of the vent.

Most modern pressure cookers give you one option: 15 pounds pressure. If your weight isn’t labeled, or if you have an electric cooker that doesn’t tell you anything about the weight, you can assume that it’s 15 pounds. At 15 pounds of pressure, your cooker will reach approximately 250°F.

Another style of jiggle-top regulator gives you the option of 5, 10, or 15 pounds of pressure. Just remove or add weights until you have the weight you want.

5 pounds = 220°F

10 pounds = 235°F

15 pounds = 250°F

Presto steam valves are a standard size, so if you have a new cooker and see the variable size at a yard sale, chances are it will fit.

How to Use It

With the lid off, a pressure cooker is basically a very large, heavy-bottomed pot, so you can sauté your onions or whatever right in the pot. Always add some sort of liquid, but never fill it more than 2/3 of the way full. If you’re cooking beans or anything else that expands, only fill it to the halfway mark. Put in your food, close and lock the lid, and put on the pressure regulator (unless you’re canning…then you need to evacuate some steam first). Turn the heat up to high. Start timing when the regulator starts jiggling, and turn the temperature down to medium or so, so that it jiggles at a reasonable rate. Don’t be alarmed: some hissing and sighing is normal; however, if it starts to sound like a runaway freight train, you should probably turn down the heat. Most recipes will tell you “Let the pressure drop of its own accord.” That simply means turn off the heat (or move to another burner) when the timer is done and wait for the indicator to fall. Depending on what you have in the pot, this may take as long as a half an hour, so plan accordingly. If the recipe says “Release pressure immediately,” VERY CAREFULLY move the pot to the sink and run some cold water over it. As soon as the indicator drops, it’s safe to open.

Safety

Please don’t worry. Modern pressure cooker have a number of features that make them safe for home use. Really and truly, if you follow the directions, they won’t explode. The key points to remember are:

1) Never, EVER attempt to remove the lid if the pressure indicator valve is up.

2) When the cooker is at pressure, you want the regulator to jiggle several times a minute. If it sounds angry, turn it down.

3) Always lift the lid away from you. Remember, it gets up to 250°F in there.

4) Double-check your recipes if you’re canning. To stay on the safe side, check out the helpful documents on pressure canning prepared by the folks at the National Center for Home Food Preservation.

And finally, if you’re really, really scared, considering buying a European brand. I’ve never used one, so I can’t recommend a particular brand, but apparently they are foolproof.