After vinegar and lemon or lime juice, many American cooks would be hard put to name the world's next most popular sour flavoring. Yet tamarind, from the pods of a large tree grown throughout the tropics, is hardly a secret.

It adds its prune-like sourness to curry-style dishes and chutneys in India, pickles in Iran, sour and hot soups in Thailand, refreshing beverages in Latin America and to worcestershire sauce.

The edible pulp of the pod is savored like a fruit in West Africa even though the sugar it contains does little to mitigate its mouth-puckering sourness. In the Middle East and India a shabat (syrup) is made from cooking tamarind paste with sugar; it's kept on hand to be diluted and poured over ice whenever a thirst-quenching beverage is called for, or for making medicinal teas.

As a tonic, tamarind, rich in vitamins, is said to be good for the liver and kidneys; it's used to bring down a fever in Southern Asia and is a mild laxative. Tamarind also finds its way into jams, candies, cookies, cakes and relishes everywhere it grows.

The name comes from the Arabic tamr hindi, meaning "date of India." It is not a relative of the date, however, and while it has grown in India for centuries, the tree is thought to be native to tropical Africa.

Like lemon and vinegar, tamarind is usually added in a liquid form that is obtained by soaking and straining the pulp of the pod. The pods are 3 to 4 inches long and resemble fava beans. Tamarind in various forms is available in specialty and Asian markets specializing in Thai and Vietnamese staples.

When ripe, the way they are almost always sold, the brownish gray shells of the pods are often cracked, revealing the reddish brown pulp. Occasionally the immature pods, with a greenish cast, are sold in Southeast Asian markets to be cooked whole in soups or stews.

Ripe or unripe, the pods sell for about $2 a pound. For half as much one can buy just the pulp, which is perfectly acceptable for cooking. It comes in 8-ounce or 1-pound blocks, wrapped in clear plastic. Indian stores carry a liquid concentrate that eliminates soaking and straining but lacks the full flavor of the freshly soaked pulp.

Recipes call for the amount of tamarind to be soaked in ball-shaped sizes, e.g., "the size of a walnut." To soak, simply take the amount of pulp called for from the brick -- if using a fresh pod, it first must be shelled -- put it in a bowl and soak it in hot water for 15 minutes. A ball 1-inch in diameter requires about 1/4 cup of water. After soaking, the fibrous material should be worked with the fingers while still in the liquid to remove every bit of pulp. This should be poured and worked through a fine-mesh strainer to extract all that is flavorful. The resulting liquid is added when called for. It should be pointed out that although the fresh pods must be peeled, the pulp will dissolve faster and, as a rule, will be less fibrous than that from the brick. TAMARIND GINGER RELISH (India) (Makes about 1 1/2 cups)

This smooth textured relish goes well with fried foods. It should keep in the refrigerator for two weeks or more.

1/4 pound tamarind pulp (2-inch ball)

1 tablespoon or more chopped fresh ginger

2 red chilies, cut into 1/4-inch cubes

1/2 cup brown sugar

2 teaspoons cumin seeds, pan-toasted and ground

1/2 teaspoon white pepper

2 teaspoons salt or to taste

Soak the tamarind for 30 minutes in 1 cup of water that has been brought to a boil. When soaked, work the pulp with the fingers to loosen it, then pour the liquid through a fine mesh strainer, again working the pulp to push as much through the strainer as possible.

Stir the rest of the ingredients into the tamarind liquid until the sugar dissolves. Allow the relish to rest for 4 hours or overnight before serving.