The label on my bottle of barbecue sauce says it contains "liquid smoke." Isn't that an oxymoron?

Pardon me while I drink a glass of liquid ice, with cubes of solid water floating in it to keep it cold.

Ahh! That was refreshing. Now to your question.

While its terminology may be a bit suspect, liquid smoke is a legitimate and useful product. It adds a smoky flavor to foods without our having to go out and chop wood, build a fire and all the rest.

Smoking is one of several ancient methods of curing or preserving foods, primarily meats and fish. Other long-used methods are drying (think jerky), salting (think bacalao, or salt cod), and pickling (think . . . well, pickles). Drying works because bacteria can't live without moisture; salting works because the salt draws water out of the bacteria's cells; and pickling works because bacteria can't live in acid environments such as vinegar. Of course, our ancestors discovered these methods empirically, long before there was any knowledge of pathogenic microorganisms.

Smoking undoubtedly originated as an unintended consequence of drying meats over open fires. Wood smoke can be lethal not only to bacteria but, as all firefighters know, to humans as well -- if there is enough of it. On the other hand, when exposed to only a little bit of smoke we love its aroma (think fireplace on a winter's night) and its flavor (think smoked trout). But smoke is a mixture of hot gases and microscopic suspended particles, which are more difficult to capture and put into a bottle than a genie. So manufacturers of processed foods invented liquid smoke.

Gaseous Smoke

Today, we still smoke foods, but usually for flavor rather than for preservation. Smoking is commonly done in two ways: cold smoking (in which the food isn't allowed to exceed about 90 degrees Fahrenheit) and hot smoking (in which the food can reach 190 degrees and become partially cooked, that is, its proteins can become partially denatured by the heat). Some processed meats are hot-smoked and are therefore considered to be cooked (bologna), while others are cold-smoked and sold raw (bacon). Smoked hams may or may not require further cooking; the labels will tell you.

Sausages are a challenge to classify. The ground meat may be fresh or cured (with nitrites, for example); the filled casings may then be cooked or not, smoked or not, dried or not and/or fermented or not. Frankfurters are usually cured, cooked and smoked; Italian salami are usually cured, fermented and then dried; fresh (uncooked) pork sausage is neither cured nor smoked.

In a traditional smokehouse, meat is hung from the ceiling and smoke from an outside fire of moist sawdust is blown in through ducts. In modern commercial smoking plants, the density of smoke, the temperature and the humidity are all carefully controlled to produce specific effects on the product.

Vegetables can also be smoked, of course. In the village of La Vera in the Extremadura region of western Spain I saw bright red Capsicum annum (chili) peppers being simultaneously dried and smoked in long, low, two-story bungalows. The peppers were piled on wooden-slat platforms above smoldering hollow oak logs on the concrete floors below. The dried peppers were then ground to a velvety, brick-red paprika-like powder called pimenton. (Historical note: The Hungarians, renowned for their paprika, obtained it originally when King Carlos V of Spain sent some pimenton to his sister, Queen Mary of Hungary. But Hungarian paprika doesn't have the rich, musky flavor of pimenton because the peppers are dried without smoke.)

Liquid Smoke

Unfortunately, much of a smokehouse's smoke eventually finds its way up the stack to pollute the atmosphere. And in today's environmentally conscious society, where there's smoke there's ire. Liquid smoke comes to the rescue.

To make liquid smoke, real smoke is first generated by burning moist hardwood chips or sawdust. The moisture partially deprives the fire of oxygen to ensure maximum smokiness (Techspeak: incomplete combustion). Hardwood is used because soft woods contain resins that burn to produce noxious fumes. The smoke is then led to chilled condensers, where many of its chemical components (hundreds of different chemicals have been identified in wood smoke) condense into a brown liquid, which is then purified by fractional distillation to remove undesirable -- and toxic -- components. What remains is usually mixed into acetic acid (vinegar) and can be added as such to your barbecue sauce. The FDA doesn't permit a food to be labeled "smoked" unless it has been exposed directly to real smoke from burning wood. Read the label on the package of your favorite hot dogs; some are only "smoke flavored" by having been sprayed with or dipped in liquid smoke.

Is It Safe?

You have undoubtedly been unable to forget what I said earlier about smoke being lethal, and you are now wondering whether liquid smoke is safe. What should I say?

Nightmare scenario: I say it's safe. You eat some and get a headache. An opportunistic lawyer tells you, "You have a case." He sues the food company and me, stacks a jury box with migraine sufferers and wins a $2 million settlement from the company plus $500 from my threadbare pockets. He takes a million for himself, runs off after another ambulance, and you're left with an aspirin for your headache.

So should I say it's safe? Okay, I'll take the plunge. Yes, it's safe in the small amounts you'll encounter in smoke-flavored foods. So sayeth the FDA.

Real, gaseous smoke, however, is quite another story. The decomposition of wood (and tobacco and grilled steaks and hamburgers) by heat (Techspeak: pyrolysis) produces highly carcinogenic 3,4-benzopyrene and other so-called polycyclic aromatics. But none of these chemicals has been found in commercial liquid smoke products. On the other hand, liquid smoke, like its gaseous parent, contains bactericidal and antioxidant chemicals such as formic acid and phenolics that may even make a positive contribution to your health.

You may be able to find liquid smoke in four-ounce bottles under the brand name Colgin, in pecan, mesquite, hickory and applewood flavors.

Robert L. Wolke (www.professorscience.com) is professor emeritus of chemistry at the University of Pittsburgh and the author, most recently, of "What Einstein Told His Cook: Kitchen Science Explained" (W.W. Norton, $25.95). He can be reached at wolke@pitt.edu.