The true mints are mainly small natives of damp habitats in Europe and Asia. There are about 25 species in the genus Mentha and some 600 varieties, though the family tendency to hybridizing and chemical variation confuses the picture. The mints of most interest to the cook are spearmint (Mentha spicata) and peppermint (M. piperata), which is an ancient hybrid between spearmint and watermint (M. aquatica).
Both of the major culinary mints have a refreshing quality, but they are quite different. Spearmint has a distinctive aroma thanks to a particular terpene, L-carvone, and a richness and complexity thanks to pyridines, nitrogen-containing compounds more typical of roasted foods than raw ones. Spearmint is widely used in the Eastern Mediterranean as well as in India and Southeast Asia, in large quantities, both fresh and cooked, and in both sweet and savory contexts. Simpler, clearer-tasting peppermint contains little or no carvone or pyridines; instead it makes a terpene called menthol, which gives it a uniquely cooling quality. In addition to having its own aroma, menthol actually binds to receptors on temperature-sensing nerve cells in the mouth, and causes those cells to signal the brain that they are cooler than they really are by 7–13°F/4–7°C. Menthol is a reactive chemical that rapidly degenerates when heated, so peppermint is usually not cooked. Its concentration increases with the age of the leaf, so older leaves taste more cooling; hot and dry growing conditions cause menthol to be transformed into a noncooling, somewhat harsh by-product (pulegone, the characteristic volatile in pennyroyal).