Food 101

Radiant vs. Convection, Flames vs. Coils and Other Hot Questions

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By Robert L. Wolke
Wednesday, March 28, 2007

While scoping out new ranges (to replace an old clunker), we've encountered three types of ovens: radiant, convection and just plain ordinary. Can you bring us up to speed on the technological differences among them?

Unfortunately, erratic terminology pervades the entire, pardon the pun, range of oven technology. There may be more hot air in sales pitches than in the ovens being sold.

A case in point: The word "radiant" is thrown around quite loosely because it sounds scientific. In truth, all ovens -- including the "just plain ordinary" -- use radiant heating, in that they heat the food primarily by radiation.

The word "range," which has replaced "stove" except among some of our more mature citizens, usually refers to a free-standing appliance consisting of two parts: a cooktop that provides a cooking surface and an oven that provides a heated enclosure. These days, you can buy those parts separately and install them wherever your designer genes dictate. The cooktop can be set into the counter or on top of an island; the oven (or two) can be set into the wall.

Wherever it sits, the oven is about getting heat into your food.

Heat energy can move from one place to another via any of three paths: conduction, radiation or convection. Conduction is the direct transfer of heat from a hot object into a cooler one in contact with it. In an oven, your food isn't actually in contact with the heating element, so we can eliminate conduction, except perhaps for a small amount of conduction through the air, which is a terribly poor conductor of heat.

Almost all of the heat in any oven is transferred to your food by what sales engineers -- and scientists -- call "radiant" heat: infrared radiation emitted by the heating element itself and the hot oven walls. When this radiation strikes a substance, its energy agitates the substance's molecules into a frenzy, making it hotter.

Convection is the physical movement of hot air from the heat source to the food. There is a small amount of moving-air convection taking place in any oven, but in a "convection oven" a built-in fan intensifies the circulation, bathing all parts of the food equally. That extra heat shortens the cooking time. When you are using the convection feature, most recipes will tell you to curtail the cooking time and/or decrease the oven temperature by 25 to 50 degrees.

Now to cooktops, which are sometimes confusingly called ranges in a misguided attempt to distinguish them from ovens.

Today's cooktop heating units may be flaming gas, electrically heated coils or hot plates, halogen lamps, or induction coils or electric elements encased in a smooth, ceramic surface. And your salesperson may feel free to call any one of them "radiant."

I don't have to tell you how gas flames work. They're great for making quick, accurate adjustments in heat, but most of their heat may go into the kitchen, not your pot.

The familiar electric coils and their flat, round hot plate cousins work like most electric heaters: That is, when electricity passes through a conductor it generates an amount of heat proportional to the so-called resistance of the conductor. Electric cooktop elements are highresistance conductors that get very hot. They're fairly energy-efficient but are slow to heat and cool.

Ever notice how hot a halogen lamp gets? (Never touch one until it has cooled completely.) Unlike with incandescent bulbs, the halogens' high temperature is essential for producing their bright light and long life. So why not use that heat for cooking? Halogen cooktops are instant-heating, but not very energy-efficient.

Smooth cooktops may contain any of several types of heat sources, but they most often involve electrically heated coils or ribbons embedded within a flat glassceramic surface. They are a delight to keep clean.

The newest kid on the block is the induction-heating cooktop, the most energy-efficient of all the technologies. It doesn't get hot itself, but by generating a high-frequency magnetic field, it heats any iron or stainless steel cooking pan that is magnetic. (Test your pans with one of your refrigerator magnets.) Magic? No. Expensive? Yes. But it can boil a pot of water in no time flat, even one that's watched, and that's anything but ordinary.

Robert L. Wolke (www.robertwolke.com) is professor emeritus of chemistry at the University of Pittsburgh and author of "What Einstein Told His Cook: Kitchen Science Explained" (W.W. Norton, 2002). He can be reached atwolke@pitt.edu.


© 2007 The Washington Post Company

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