Extracted from the pulp inside the seedpods of the African tamarind tree, tamarind paste is sold in Indian groceries and sometimes in the Asian section of supermarkets. Tamarind pulp is quite sour, being about 20 percent acid (mostly tartaric), but it is also somewhat sweet and savory, with a complex roasted underpinning. In much of Asia, tamarind is used to acidify sauces, soups, preserves, and beverages. If you can't find it, you could substitute a combination of lime juice and Marmite and come close. Tamarind paste will keep almost indefinitely in the refrigerator in a tightly closed container.
To Mix with Seltzer:
1/3 cup tamarind tea syrup
1 cup seltzer
1 serving
Pour the syrup into a tall glass. Add the seltzer and stir just until blended. Add ice and serve.
To Carbonate with a Siphon:
1 batch tamarind tea syrup
3 cups water
3 servings
Pour the syrup into a 1-quart siphon. Add the water. Charge with CO2 according to the manufacturer's directions. Siphon-charged sodas can be stored in the siphon in a refrigerator for up to 5 days. Disperse as desired into tall glasses filled with ice, and serve.