For years I was familiar with citric acid only as a mysterious additive listed on the labels of various packaged foods. Then I visited the western state of Gujarat, in India, where I saw a home cook mix granules of it into a batter to provide the requisite astringency in dhokla, a spongy sweet-and-sour cake made with chickpea flour. More recently, I came across it as an ingredient in the 19th-century recipe for Fruit Punch, where it adds a tart kick. Curious, I did a little investigating and learned that citric acid, first isolated in 1784 by the Swedish chemist Carl Wilhelm Scheele, used to be derived from lemons and limes but is now produced primarily through a fermentation process involving beet- or cane-sugar molasses. In this country, where it's commonly employed in the canning of low-acid items like tomatoes, citric acid is sometimes labeled lemon salt or sour salt and can be found in certain supermarkets and specialty stores. The discovery I liked the most, though, was this quick and refreshing "lemonade": stir together water, sugar, and citric acid to taste; pour over ice and enjoy.