There are several different methods of cooking, each utilizing a different method of heat transfer. Understanding how heat transfer works under different conditions will help you to understand why certain foods are better cooked one way than another.
Heat is energy, and cooking is simply the transfer of energy from a heat source to food. Heat always moves from hot temperatures to cooler temperatures, and three basic methods cause this to happen.
Conduction is the transfer of heat via contact. For instance, when a pan touches a heating element, such as a stove burner, heat is transferred to pan. When food touches the pan, heat is transferred from the heating element to the food via the pan.
Some materials conduct heat better than others. Aluminum, copper, and iron all conduct heat very well, but stainless steel does not. This is why many pots and pans have aluminum cores in their stainless steel walls.
Radiated heat is transferred by energy waves emitted by the heated object. An oven, for instance, has a heating element. When food is placed into the oven, it absorbs the heat waves emitted by the heating element.
Radiant heat also reflects off of the oven wall and into the food. Radiant energy reflects off of smooth, shiny, bright surfaces. Dull, dark, rough surfaces absorb radiant energy. This is why it is important to have a clean oven.
Convection is the transfer of heat via the movement of material. For instance, when boiling water, the water at the bottom is heated first. As heat is transferred to the water via conduction, the water begins to move around more and more as its temperature rises. The movement—convection—causes the water at the bottom and top to perpetually trade places, thus causing the entire body of water to heat to a uniform temperature.
While some cooking methods are more direct, others use combinations of heat transfer methods.
For instance, broiling involves placing the food directly underneath a flame or heating rod in your broiler oven. Radiation is the only method being used. The same is in the case of a microwave oven, where magnetic energy is sent directly into the food, causing asymmetrical molecules—such as water—to vibrate, thus heating the food.
However, placing food in an oven to roast or bake involves not only radiation, but also convection. The heat from the heating element inside the oven creates air currents, which offers convection. The amount of convection is raised in a convection oven, whereby heated air is circulated about the oven cavity by a fan.
Boiling, too, is a hybrid method. Though heat is transferred via convection, the food placed in the boiling water is also in contact with the hot water. Thus, conduction is also used.
- I’m Just Here For the Food: Food + Heat = Cooking; Alton Brown; 2002
- Cooking for Engineers, “Heat Transfer and Cooking”
- University of Arizona, “Heat Transfer”