Lorna Sass author of Pressure Perfect and Whole Grains Every Day, Every Way shares with us her definition of Pressure Cooking:

"In a pressure cooker, food becomes tender in one-third the standard cooking time. How does this happen?  Once you've locked the lid in place and set the cooker over high heat, the liquid inside comes to a boil and produces steam.  Because the lid has an air-tight seal, the steam gets trapped inside the pot and forces the internal pressure to build up to approximately 15 pounds per square inch.

Under 15 pounds of pressure, water boils at 250 degrees Fahrenheit rather than the standard 212 degrees.  Since the water is 38 degrees hotter than usual, the food fibers break down and the flavors mingle in one-third the standard cooking time.  That's why a pot roast becomes fork-tender in one hour instead of three, and split peas melt into a puree in 20 minutes rather than 60.

When purchasing a pressure cooker, examine the lid. First make sure that it's simple to lock the lid into place and to remove it after cooking. Then determine what kind of pressure regulator the cooker uses and how easy it is to determine when high pressure is reached.

There are three types of pressure regulators (so named because they regulate the amount of pressure in the cooker):

• The most sophisticated type is a spring-valve regulator, used in many cookers imported from Europe. Although designs vary, most spring-valve regulators have a small brightly colored rod or cylinder that pops up as the pressure builds.  When the rod comes up high enough to reveal a designated mark, the cook knows at a glance when high pressure is reached.  

• The second type is a removable, round metal weight that sits on top of the vent pipe.  This design is used in American pressure cookers like Presto and Mirro and in some imported brands. When high pressure is reached, the weight gently rocks back and forth, giving this cooker the familiar nickname of "jiggle-top".  

• The third type, called a developed weight regulator, is used in many imports.  Think of it as a jiggle-top in disguise since it sits on top of the vent pipe, but gets locked into position before cooking begins. This regulator lifts up slightly when high pressure is reached but, since it rarely jiggles, the cook must observe carefully to know when high pressure is reached.