I've always kept an open box of baking soda in my refrigerator to absorb odors. But I've noticed that there's now a new kind of baking soda box in the supermarket that supposedly works even better, even though the label says it contains nothing but pure baking soda. How does baking soda absorb odors, and how does this new contraption do it better?

Like every other householder in the U.S.A., I have kept an open box of Arm & Hammer (is there any other kind?) baking soda in my refrigerator, and I can testify that I have never smelled a bad odor. It also worked to repel tigers, because not once did I encounter a tiger in my house as long as that baking soda box was in the fridge.

Is it possible that I never saw a tiger because I live so far from India, or that I never smelled a foul odor in my refrigerator because I'm such a fastidious fridgemeister? Nah! Not according to the Arm & Hammer division of Church & Dwight Co. and every domestic maven in the United States, who would maintain that the baking soda absorbed all the odors. (They make no claims about tigers.)

What hard evidence do we have that baking soda really works, at least for odors? None that I know of. But here's the theory.

Baking soda is pure sodium bicarbonate, also known as bicarbonate of soda (chemical shorthand NaHCO3). It reacts with both acids and bases, that is, with both acidic and alkaline chemicals. (Techspeak: the bicarbonate ion is amphoteric.) But it is more than 20 times as effective in reacting with acids than with bases. And thereby hangs its odor-eating reputation, because should a wandering molecule of a smelly acid be unfortunate enough to alight upon a surface of baking soda, it will be neutralized, turned into salt (shades of Lot's wife!), and be trapped permanently. So there is no arguing with the fact that baking soda will gobble up acids -- if given the chance. And there's the rub, or rubs. How do we get the acid to come into contact with the baking soda, and why do we want to trap acids anyway?

First, why are acids the alleged stinkers? It goes back mostly to spoiled milk. In the old days of undependable refrigeration, and especially before pasteurization, milk quickly spoiled, not only by bacterial growth but by its butterfat breaking down into fatty acids, primarily butyric, caproic and caprylic acids. Butyric acid is largely responsible for the odor of rancid butter and neglected armpits, whereas caproic and caprylic acids are named after what they smell like: caper is Latin for goat. Get the drift?

So if you are in the habit of leaving month-old milk in the refrigerator while you visit your time-share, many of the sour fatty acid molecules may indeed find their way to an open box of baking soda, fall in, and be neutralized. But not all smelly molecules (smellicules?) that can pollute your refrigerator's air space are acids, or even bases for that matter; chemically speaking, they can be virtually anything. Claiming that baking soda absorbs "odors" generically is stretching the truth.

Let's put it this way: An odor is a puff of gaseous molecules floating through the air to our noses. Each type of molecule has its unique chemical identity and its own unique set of reactions with other chemicals. No single chemical, sodium bicarbonate included, can claim to react with and deactivate any and all gaseous chemicals that happen to smell bad.

Even for acidic odors that are bicarbonate's main quarry, note that the landing pad for a smellicule on a box of baking soda is a mere seven square inches (the box-top area), secreted somewhere within a 20 cubic foot (35,000 cubic inch) refrigerator air space. That's not a very efficient system for capturing smellicules. The open box does not attract odors, as many people believe. It has no come-hither power, in spite of its toplessness.

The new "contraption" you saw is Arm & Hammer's Fridge-n-Freezer Flo-Thru Freshener, a standard one-pound box of baking soda with removable sides to give gaseous molecules more access to the baking soda via porous paper inner seals. That sounds like a great idea, but this box, "specially designed to expose more baking soda than any other package," uncovers only another seven square inches of soda surface. And by the way, the air in the fridge doesn't "flo thru" the package anyway. There is no fan or other force blowing it into one side of the box and out the other. Nice advertising concept, though.

What about the odors that baking soda doesn't absorb? There is only one common substance that can gobble them all up: activated charcoal. It works not by trying to be a chemical for all seasons, but by using a physical stickiness that is essentially chemistry-blind. Gases find their way into its enormous interior network of microscopic pores, where they stick by a phenomenon called adsorption. That's not a misprint; it's adsorption, with a "d." It refers to the adhesion of molecules to a surface by means of what scientists call van der Waals forces. Okay, so that's more than you wanted to know.

You may be able to find activated charcoal in a drugstore, hardware store, appliance store or pet shop. Spread it out on a baking pan with sides and leave it in the offending fridge for a couple of days. Do not use charcoal briquettes; they may contain noxious, easy-ignition chemicals and wouldn't work very well anyway.

In the end, there is only one surefire route to a sweet, odor-free refrigerator. Three words: prevention, prevention and prevention. Seal all your refrigerated food, especially "strong" foods such as onions, in air-tight containers. Check frequently for signs of spoilage and throw away all suspects. Wipe up spills promptly. Clean the fridge thoroughly. Yeah, I know, but do it a lot more often.

Oh, you say there was a power outage while you were on vacation and all your refrigerated food spoiled and you could smell it all the way from the airport on your return? Poor soul. Neither baking soda nor charcoal nor cursing the power company will help. Make yourself a stiff drink, go to Louisiana State University's Web site for disaster information (www.lsuagcenter.com/Communications/pdfs_bak/pub2527Q.pdf) and follow the directions.

Labelingo: Perspicacious reader Diana Scheid of Chantilly informs me that the sauce for a Hunt's Manwich boasts of containing "crisp bell peppers." The ingredient label lists them as dehydrated peppers. "No wonder they're crisp," she writes.

(Have you noticed any silly things on food labels? Send your Labelingo contributions, along with your name and town, to Food 101, Food Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071 or to the e-mail address below.)

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.