When hot molecules transmit some of their heat to cool ones in direct contact with them, the type of heat transference called conduction occurs. Pan-frying is a fish fillet exemplifies this principle. The heat of the flame is transferred - on a molecule to molecule basis - first through the pan, then through the thin oil layer, and finally through the fish. Another illustration is a metal spoon in a hot cup of coffee. At first the utensil's handle is cool. It then grows warm, and eventually hot.
The speed of conduction is relatively slow, and it varies by substance. Metal conducts heat more quickly than does wood, which helps explain why wood is a popular material for pot handles and cooking spoons.
Boiling or deep-frying is chiefly a process of convection, but since heat is simultaneously transferred directly from one water or fat molecule to another, conduction is part of the process too. Conduction heating takes place inside the food as well. When cooking a potato or other solid food in a pot of boiling water or hot fat, or in a hot oven, none of the circulating water, fat, or air molecules touch the subsurface molecules of the food. So if the potato's interior is to be cooked, conduction heating must take over where convection heating leaves off. In other words, the surface molecules of the potato pass along their acquired heat to the next layer of molecules in the potato, and so on.
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