A pressure cooker is a kind of pot with a lid equipped with a rubber gasket. The lid creates a seal that prevents liquid, steam and air from escaping, except at a certain pressure under controlled conditions, and only by means of a regulator on the lid. The result of pressure cooking is the food is cooked faster than with a stove or oven. Food also retains moisture, vitamins and minerals. Less tender cuts are made tender by this cooking method.
Since the regulator is the path that steam uses to escape from the pressure cooker, it must not be blocked. Before cooking, you have to look through it to make sure you see light, poke a toothpick through it or take other measures advised by your particular pressure cooker's instruction manual to make sure the regulator stays open. As some foods break down under heat and pressure, bits of it can be carried into the regulator or various safety valves, clogging it. These foods should not be used in a pressure cooker.
Liquids: Don’t Use Milk
To prevent a pressure cooker pot from warping and also to aid in steam build-up--needed for increasing pressure--liquid must be added. The amount will vary with the recipe, but no matter the recipe, the pot does need a minimum amount. Just how much depends on the size of your cooker. Most often, the amount is about 2 cups. Usually the liquid called for will be water, but you can experiment with other liquids. The liquid you cannot use is milk or milk products. They boil over easily and scorch.
No Frothy or Foamy Foods
Frothing, foaming foods can gunk up the pathway that lets steam escape, causing pressure to build up too much and putting a quick end to your intended meal. Check your pressure cooker manual for a list of foods likely to do this. Mirro brand's manual, for instance, lists • Applesauce • Oatmeal and other cereals (you can cook rice, though) • Pasta • Split peas • Pearl barley • Cranberries or rhubarb
Beans and Oil
You can cook dried beans, but you have to make sure they, too, don't froth. To do this, add a tablespoon of oil to the liquid before you close it up. Make sure to check your instruction manual for advice about cooking beans. Speaking of oil, you can't deep fry food in a pressure cooker. In fact, you're likely to ruin your pot if you try that. That's not to say you can't brown or saute food in advance of subjecting it to pressure cooking--doing so can increase the flavor of the finished dish--but don't deep fry. Especially don't close the lid and heat oil to a high temperature and pressure.
Any Food: Overfilling Hazard
Food and liquid of any kind can pose a problem in the pressure cooker if you overfill the pot. Hot items expand, so you want to make sure you don't fill a cooker more than two-thirds full with liquid or half-full with food. Some pressure cookers have fill lines on the inside to help you know when the pot is at capacity. Overfilling a pressure cooker can make food clog the pathways for steam release, causing too much pressure to build in the pot.
Modern pressure cookers don't explode. They have a number of safety features, including safety valves and plugs that lock or give way if pressure builds up too much. Some models will make a mess doing that, and make no mistake, what is released is hot and dangerous. But the lid won't fly off explosively, injuring someone. Besides the safety plug, other features to prevent disaster include a lid lock, which won't let you open the lid until the pressure has reduced. Clogging the regulator, steam pathways or valves is unlikely to hurt you, but you'll probably have a problem with your pot. Until it is fixed or replaced, you won't be able to use it.
How Pressure Cookers Work
The pressure cooker is a one-pot meal maker. You add the food, add the liquid, then close and lock the lid. After the cooker is sealed, pressure is built up by applying heat. A properly functioning regulator lets enough steam out to keep pressure from building beyond a certain level. Pressure and temperature is maintained at a constant rate until the food is cooked. That's because the pressure in the pot causes water to boil at a higher temperature so foods cooked in the pressure cooker cook at higher than normal temperatures in a moist environment.