The trifoliate orange is the hardiest of all the citrus tribe — a deciduous, ferociously thorny shrub of China and Korea whose small, downy, golden fruit is far too sour, bitter and resinous to eat raw. Grown worldwide as the cold-tough rootstock that lets sweet oranges survive frost, it has a real culinary and medicinal life of its own in Korea and China: the pectin-rich fruit is boiled into fragrant jam and cordials, and the dried immature fruit is a classic ingredient of Korean and Chinese herbal medicine.