Sunday, April 02, 2006

Cooking from the inside out? hating free radicals?

On April 2, NJN broadcast episode 801 of christinacooks (I hate free radicals), which included a statement about how pressure cookers cook food from the inside out.

A pressure cooker works by allowing heating by water, which normally is limited to 212 deg F (100 deg C) at 1 atmosphere pressure, to occur at higher temperatures (at pressures in excess of 1 atmosphere). As the missvickie website notes:

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.

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 [ie 15 psi above 1 atmosphere]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.

Of microwaves cooking from the inside out, from

While microwave radiation does penetrate the surface of food and start to heat the inside at roughly the same time as the surface, it's not necessarily accurate to say the food is cooked from the inside out. Microwaves heat food by being absorbed primarily by liquid water molecules, and to a lesser extent fats and some sugars, imparting energy to them in the form of heat. (Contrary to what many think, the frequency at which microwave ovens operate, 2.45 GHz, is not tuned to the maximum absorption frequency of water. That frequency is actually closer to 10 GHz, and if ovens operated there, food would be heated even less inside, since the bulk of the radiation would be absorbed at or near the surface due to the short wavelength.) If a food is of uniform consistency and high in water content, most of the microwave energy will be absorbed by the water near the surface before it gets into the center of the food, and the food will heat from the outside in, as with traditional ovens. On the other hand, if the surface of the food is drier than the center, as with bread or a baked potato, the center will heat up faster. You can see this clearly if you microwave foods with a dry outer crust and a moist filling, like a McDonald's apple pie. If you cook it for about 20 seconds in a typical oven, you'll find that the crust is fairly cool to warm, while the filling can be quite hot.


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