Conduction. Convection. Radiant heat. And the science of heat transfer in cooking.
Heat transfer is the key to cooking. Whether we are boiling, grilling, frying, microwaving or steaming, our food is cooked by a combination of various forms of heat transfer.
There are three forms of heat transfer:
1) Conduction (which happens inside the food)
2) Convection (which happens in the air or liquid the food is cooked in)
3) Radiant heat (which comes from an external heat source)
Conduction occurs when the surface of our food, e.g. a piece of meat or a pot of soup, is heated and becomes hotter than the rest of the food. The heat energy causes the molecules at the surface to move more rapidly than the molecules in the colder regions.
When the former molecules bump into the latter, the latter begin to vibrate quickly too, and the heat energy is transferred (hence the phrase “heat transfer”). The process continues until the center is heated and all parts of the food are cooked.
Or in the case of the molten lava chocolate cake, the baking is stopped before the heat energy is fully conducted to the center. This way, the center remains liquid (read the details in the 1-Minute Marvel “Half-baked but still lava-able“)
Convection occurs in the liquid or gas that the food is cooked in. For example, when we’re boiling food, the water at the bottom of the pot is heated by the stove. Hot water is less dense, so it moves upward.
As it does so, it heats the surface of any food that comes into contact with it. It is also replaced by the colder and denser water from the colder regions. This in turn is heated and rises, to be replaced by more cold water, and the process is repeated.
Radiant heat occurs because of electromagnetic radiation. The energy from a heat source, such as hot coals or within the microwave oven, is transferred to the food via electromagnetic waves that cause the molecules in food to vibrate. The food then heats up either via conduction or convection or both.
By now it should be clear that the three forms of heat transfer can occur in concert to cook food. For example, when grilling meat over hot coals, the coals’ emit radiant heat. This heats up both the surface of the meat via electromagnetic waves, as well as the air around the coals. The convection currents in turn also heat up the surface of the meat (read about how to do a perfect grill in the 1-Minute Marvels “Wind beneath my wings“).
Or take the earlier example of boiling food. The water first heats up because of the conduction from the fire to the pot to the water at the bottom of the pot. This is followed by convection within the water, which heats up the surface of the food in the water. The insides of the food are then cooked by conduction.
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photos: in order – depositphotos/rocharibeiro; uroszunic; Ha4ipiri; tepic; vvoennyy