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Food Science Basics: Heat Transfer

Food Science Basics: Heat Transfer

The Culinary Institute of America

Convection

Convection is the transfer of heat through gases or liquids. When either of these substances is heated, the portions of the gas or liquid closest to the heat source warm first and become less dense, causing them to rise and be replaced by cooler, denser portions of the gas or liquid. Convection, therefore, is a combination of conduction and mixing.

Convection occurs both naturally and through mechanical means. Natural convection is at work in a pot of water placed on the stove to boil. Conduction transfers heat from the stove to the pot to the water molecules in contact with the interior of the pot. As these water molecules heat up, convection causes them to move away and be replaced by cooler molecules. This continual movement results in convection currents within the water. If a potato is added to the water, the convection currents transfer heat to the surface of the potato, at which point conduction takes over to transfer heat to the interior of the potato.

Mechanical convection occurs when stirring or a fan is used to speed and equalize heat distribution. When you stir a thick sauce to heat it faster and keep it from scorching on the bottom of the pan, you are creating mechanical convection. Convection ovens use fans to rapidly circulate hot air, allowing them to cook foods more quickly and evenly than conventional ovens. (Natural convection occurs in conventional ovens as air in contact with the heating element circulates, but the majority of heat transfer in a conventional oven is the result of infrared radiation.)

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