Which acid gives lemons and other citrus fruits their sour taste?

Citric acid gives lemons and other citrus fruits their characteristic sour taste.

It is a weak organic acid that occurs naturally in lemons, limes, oranges, grapefruits, and many other plants. Lemons and limes contain especially high concentrations, sometimes reaching about 8% of their dry weight. The acid releases hydrogen ions, which activate sour-taste cells in the mouth.

The compound has an interesting history: Swedish chemist Carl Wilhelm Scheele first isolated citric acid from lemon juice in 1784. Modern industrial production usually relies on fermentation, commonly using the mold Aspergillus niger, rather than extracting it from citrus fruit.

A common mix-up is confusing citric acid with ascorbic acid. Ascorbic acid is vitamin C, which is also present in citrus fruits but is not the main cause of their sour flavor. Citric acid is also widely used as a food preservative, flavoring, and descaling agent.