By Harold McGee
Green beans come from a climbing plant native to Central America and the Andes region of northern South America. Though the peoples who domesticated them have probably always eaten some immature pods, the breeding of specialized vegetable bean varieties is less than 200 years old. There are now chlorophyll-free, yellowish “wax” varieties, and purple, chlorophyll-masking anthocyanin varieties that turn green when cooked. The fibrous “strings” that normally join the two walls of the pod and are stripped away with the stem during preparation—hence the name “string beans”—were eliminated by a New York breeder in the late 19th century; these days only heirloom varieties tend to have strings. There are two general forms of green bean, one with round and thin pods, the other with flat and broad pods. Flat varieties have been found to have a more intense flavor. The flavor of cooked green beans is interestingly complex; it includes a number of sulfur and “green” compounds, but also the essence of fresh mushroom (octenol) and a flowery terpene (linalool).