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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.
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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.
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