Introductory Physics 101 (Just Kidding)

To understand how a pressure cooker works, you need to know just a little about physics. Simply put, water boils at 212o Farenheit. At this point, no matter how long you continue to boil, it always stays the same temperature. As the water evaporates and becomes steam it is also the same temperature, 212o F.

Why isn't the food pulverized? It's all in the physics: As long as pressure is uniform on all surfaces of an object, it won't be distorted.

Pressure is effective throughout the food items, from the surface through to the center! No matter whether you're cooking a single chicken breast,or a 5-pound ham.

The only way to make the steam hotter (and/or to boil the water at a higher temperature) is to put the system under pressure. This is what a pressure cooker does. If we fit an absolutely tight cover to the pan so no steam can escape while we continue to add heat, both the pressure and temperature inside the vessel will rise. The steam and water will both increase in temperature and pressure, and each fluid will be at the same temperature and pressure as the other.

Steam has six times the heat potential when it condenses on a cool food product. This increased heat transfer potential is why steam is such an effective cooking medium.

If you put water into a pot and cover it with a tightly sealed lid, the steam will remain trapped and pressure will build and that rises the temperature at which the liquid boils. So at 15psi your food is cooking at 257oF instead of just 212oF. When you turn off the heat and begin to drop the pressure you will sometimes hear the food begin to boil inside the closed pot as the pressure drops and the contents come back down to the normal boiling temperature. See more about pressure settings.

Pressure cookers are especially designed cookware of aluminum or stainless steel. All models have a lock-on lid and a vent or pressure relief valve. Most pressure cookers are designed to be used on top of your stove, but there are some electrical models and also a one that is supposed to work in the microwave.

Food is cooked by the high temperatures inside the cooker. Click here to see how the internal pressure compares to temperatures. This high temperature is made possible by raising the pressure to a point greater than atmospheric pressure. Cooking times are based on pounds of pressure.

How Does it Do That Thing it Does?

Steam cooks many foods quicker than dry cooking methods. Steam is efficient in transferring cooking heat rapidly to foods upon contact without burning or damaging the final product, and for less energy than either electricity or gas. As a result, steam can be used to cook anything from delicate, tender-crisp veggies to large chunks of meat, either fresh or hard frozen.

Consider the difference in cooking between an oven and steamer for example. You can put your hand in a 400oF oven and not burn yourself, but put your hand over a boiling tea kettle and the 212oF steam will scald immediately. This is due to the different methods of heat transfer: Air is a poor conductor of heat; but water is a good conductor. Think of being outside when it's 70oF (quite pleasant) versus being in a pool of water at 70oF (feels very cold).

The increased pressure inside the pot delays the water and/or other liquids inside the pot from boiling under it reaches a much higher temperature. As a result, the cooking process is sped up considerably. A pressure cooker works by building up pressure from steam in the pot, which cooks food at a very high temperature. This method can cut cooking time by up to two-thirds, and will also retain the nutritional value and moisture of foods.

Pressure cookers create a closed environment that literally forces steam through foods to effect heat transfer. When the tightly sealed cooker is set over high heat, steam pressure builds and the internal temperature rises. This environment ranges from 5 to 10psi in low and medium pressure units and 15psi in standard high-pressure units which enables cooking at temperatures of 257o F. Under high pressure (15 pounds per square inch), the fiber in food is tenderized and flavors mingle in record time. What's more, fewer nutrients are lost because cooking is so speedy and nutrient-rich steam condenses in the pot instead of being lost in the air.

More cooks are becoming interested in pressure cooking as a way of creating healthful meals and to meet the demand of a busy schedule. Cooks are turning, or in many cases, returning to pressure cooking because of its versatility in preparing many different foods. Today's cook wants the convenience of faster cooking without the loss of nutrients normally lost to air, water, heat from cooking methods used in regular pots.