I have an old recipe for stuffed cabbage that calls for sour salt. I used to have some, but I've run out and none of the stores I've tried even knows what it is. Come to think of it, neither do I. What is it and where can I get some?

Sour salt is misnamed. It isn't a salt at all; it's an acid. They're two different classes of chemicals.

Every acid is a unique chemical having properties that distinguish it from all other acids. But it can have dozens of derivatives called salts. That is, every acid is the parent of a whole family of salts. However, so-called sour salt is not one of those offspring salts, but rather a parent acid: citric acid. It has an extremely sour flavor and is added for tartness to hundreds of prepared foods, from soft drinks to jams and frozen fruits.

In addition to its potent pucker power, citric acid retards the browning of fruits by enzymes and oxidation. It is obtained from citrus fruits or fermented molasses and is used in eastern European and Jewish dishes, most commonly in borscht. You can find it by its name "sour salt" in kosher markets or in the Jewish foods section of the larger supermarkets.

Citric acid isn't alone in its sourness. All acids are sour. In fact, only acids are sour, because of their unique property of producing so-called hydrogen ions (never mind!), which make our taste buds shriek "sour" to our brains. The strongest acids in your kitchen are vinegar and lemon juice. But sour salt, being 100 percent citric acid in crystalline form, is much more sour than vinegar, which is only a 5 percent solution of acetic acid in water, or than lemon juice, which contains only about 7 percent citric acid.

Citric acid is unique in that it contributes sourness virtually without any other flavor, whereas the assertive flavors of lemon juice and vinegar must be factored into the overall balance of any dish. American chefs could well benefit by experimenting with sour salt in dishes that need a touch of tartness without any accompanying lemon or vinegar flavors.

What is cream of tartar? Is it related to tartar sauce or steak tartare?

Not at all. The words "tartar" and "tartare" come to us from two different directions.

"Tatar" or "Tartar" was the Persian name for Genghis Khan's horde of Mongols who stormed through Asia and Eastern Europe in the Middle Ages. The Tartars were viewed by the Europeans as being culturally challenged and at the very least politically incorrect, inasmuch as they wore the skins of animals and often ate their meat raw. One of our contemporary, semi-barbaric delicacies was therefore given the Phony Phrench name of "steak tartare": ground or minced raw beefsteak mixed with chopped raw onion, raw egg yolk and salt and pepper (also raw), plus ad lib touches of Tabasco, Worcestershire, Dijon mustard, anchovies and capers. (James Beard ventured to civilize his with cognac.)

Tartar sauce is mayonnaise with chopped pickles, olives, chives, capers and such mixed in. It is usually served with fish (often, unfortunately, in tiny, plastic "individual servings") and, according to Larousse Gastronomique, it also goes great with calves' feet. Classic tartar sauce may contain vinegar, white wine, mustard and herbs, so it may have been dubbed "tartar" because of its potency and pungency. In fact, the French refer to a variety of highly seasoned dishes as a la tartare. The Tartars apparently take the rap for almost anything that's raw, pungent or crude.

The "tartar" in cream of tartar is quite another story. It comes to us via old Latin from the Arabic "durd," meaning the dregs or sediment that form in a cask of fermenting wine. Today's winemakers use the word "tartar" specifically for the brownish-red, crystalline deposits left in the bottoms of casks after the wine has been drawn off. Chemically, it is impure potassium hydrogen tartrate, a salt of tartaric acid. "Cream of tartar" is the fancy name given to the white, highly purified potassium hydrogen tartrate that's sold in stores for use in the kitchen--most often for stabilizing beaten egg whites.

The tartar that forms in wine casks comes from the tartaric acid present in grape juice. Tartaric acid is what gives wine about half of its total acidity. (Malic acid and citric acid contribute most of the rest.) The salt called tartar was known long before its parent acid was discovered, and when tartaric acid was ultimately nailed down by chemists they named it after the tartar in the wine casks. It's a case of the parent being named after the child.

Quite a few books mistakenly say that cream of tartar is tartaric acid instead of its salt, potassium hydrogen tartrate. That's an easy error, because cream of tartar, even though a salt, is slightly acid.

But that's in Chemistry 202.

Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh and the author of "What Einstein Didn't Know--Scientific Answers to Everyday Questions." Send your kitchen questions to wolke@pop.pitt.edu.