7.1 How heat moves: conduction, convection, radiation

When food warms up, its molecules take in energy. They vibrate faster and bump into each other. That energy spreads through the food and cooks it.

Heat travels in three ways: conduction, convection and radiation. In real cooking they often work together.

Conduction: heat by touch

Conduction moves heat through direct contact. It is the most common kind. A burner heats the bottom of a pan, and the pan then heats the food sitting in it.

A deep fryer also counts as conduction, because the hot oil touches the food.

It is the slowest of the three. But because the food sits right against the hot surface, it cooks from the outside in. A steak in a cast iron pan, for example, gets an even, browned outside and stays moist inside.

Everyday examples of conduction:

• touching a hot piece of metal and burning your hand
• grilling chicken breasts, pork chops or steak
• dropping steamed vegetables into ice water so they keep their color
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Convection: heat that circulates

In convection, the molecules closest to the heat warm up and rise. Cooler ones move in to take their place, get heated in turn, and rise too. This movement carries heat around.

Natural convection happens on its own. Put a pot of water on the stove. The burner heats the pot by conduction, and the pot heats the water. Warm water moves up and away, cooler, heavier water sinks, and a steady current forms that spreads the heat evenly through the pot.

Mechanical convection uses an outside force to move the heat. Stirring a pot is one example. A convection oven is another: a fan blows hot air over and around the food. Moving the heat this way cooks food faster and more evenly.

Convection also works for thawing. Running cold water over frozen food carries heat into it and thaws it faster. Air at room temperature moving around frozen food does the same job.

Radiation: heat with no contact

In radiant cooking, waves of heat and light hit the food and enter it. Nothing touches the food. There are two main kinds: infrared and microwave.

Infrared heat comes from an electric or ceramic element. Its waves travel at the speed of light and heat food quickly, but they are taken in mainly at the surface. Glowing coals, broilers and toaster ovens all cook this way.

Microwaves are short, high-frequency waves. They go into the food and shake its water molecules, and the friction makes heat. After that, the heat moves through a solid food by conduction and through a liquid by convection.

Because microwaves reach several inches into food, they usually cook faster than infrared heat. They work best on small batches.

Warming your hands at a fire, sitting in the sun and reheating a meal in a microwave are all examples of radiant heat.