Irtiza asked: “My oven has a convection setting that I’ve never used. What exactly is convection?”
In the kitchen, heat travels in three basic ways, all of them useful for different jobs.
- Radiation, or radiant heat, is what you get in a broiler: Heat travels (primarily in the form of infrared light) in a straight line, directly from the heating element to the food. It's very strong close to the heating element, but drops off fast as the distance increases. (The sun heats the earth via radiant heat too.)
- Conduction is the movement of heat through a (usually solid) material, like a metal pan. Heat up the bottom of a pan, and heat travels through the metal to make the cooking surface hot as well. That's conduction.
- Convection is when heat travels via moving air or liquid. In a pot of simmering water, for instance, the hottest water continually moves up from the bottom, circulating itself and its heat throughout the pot. The difference between convection and conduction is that in conduction, the material doesn't move, but the heat does. In convection, the material moves and the heat travels with it.
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What’s the Difference Between a Convection Oven and a Conventional Oven?
In a standard oven, the heating element heats up the air that is close to the element. That air moves as it heats up: Hot air rises, cold air sinks. As it moves, cooler air from elsewhere in the oven continually replaces the hot air next to the heating element, and the hottest air transfers heat to the food, circulating through the oven in a gentle, gradual, natural way. But wait, you say—moving air equals convection, right? Right: Even a conventional oven uses convection.
So a convection oven is more properly called a forced-convection oven. It doesn’t rely on that gentle, natural movement of the air inside the oven: It blows it around with fans. That accomplishes a couple of things (besides fluttering your parchment paper).
Circulating the air makes the temperature inside the oven very even, without the hotter and colder regions that are an inevitable part of natural convection. And the moving air removes the thin blanket of non-moving air and vapor—called the boundary layer—that otherwise clings to the surface of food in a conventional oven.
Non-moving air is a fantastic insulator. Down jackets, foam-walled coolers, and thermos bottles are all just structures to immobilize are. So in a conventional oven, the boundary layer insulates the food, slowing down cooking and evaporation. And by blowing it away, a forced-convection oven lets food cook, and its surface dry out, faster.
In theory, at least. As my estimable colleague Lan Lam learned in her testing of convection ovens, whether that difference really provides a noticeable effect depends greatly on the nature of the food, its shape and size, and the cooking time and temperature. Her guide to cooking with convection ovens is always helpful.
What About Convection In Other Situations?
The phenomenon of convection also comes into play in all sorts of ways outside of ovens. Try thawing frozen meat in a bowl of hot water and you’ll be waiting 30 minutes or more; but put the bowl and meat under a slowly dribbling tap and it defrosts in one third of the time. The active convection keeps warm water passing over the cold surface, transferring heat much more effectively.
A sous vide circulator is an even quicker way to defrost frozen food: It uses forced convection too, swirling water around with a motor to maintain a precisely even temperature throughout a water bath.