Dimocarpus longan, commonly known as the longan, is a tropical tree that produces edible fruit. It is one of the better-known tropical members of the soapberry family, to which the lychee also belongs. It is native to Southern Asia.
The longan is so named because it resembles an eyeball when its fruit is shelled. The seed is small, round and hard, and of an enamel-like, lacquered black. The fully ripened, freshly harvested shell is bark-like, thin, and firm, making the fruit easy to shell by squeezing the fruit out as if one is "cracking" a sunflower seed. When the shell has more moisture content and is more tender, the fruit becomes less convenient to shell. The tenderness of the shell varies due to either premature harvest, variety, weather conditions, or transport/storage conditions. In China, it is also called gui yuan, especially when dried.
The Dimocarpus longan tree is a medium-sized evergreen that can grow up to 6 to 7 metres (20 to 23 ft) in height. It is somewhat sensitive to frost. Longan trees prefer sandy soil. While the species prefers temperatures that do not typically fall below 4.5 °C (40 °F), it can withstand brief temperature drops to about −2 °C (28 °F). Longans usually bear fruit slightly later than lychees. The longan was listed as Near Threatened on the IUCN Red List.
The fruit is sweet, juicy and succulent in superior agricultural varieties and, apart from being eaten fresh, is also often used in Asian soups, snacks, desserts, and sweet-and-sour foods, either fresh or dried, sometimes canned with syrup. The taste is different from lychees; while longan have a drier sweetness, lychees are often messily juicy with a more tropical, sour sweetness. The seed and the shell are not consumed.
Dried longan are often used in Chinese cuisine and Chinese sweet dessert soups. In Chinese food therapy and herbal medicine, it is believed to have an effect on relaxation. In contrast with the fresh fruit, which is juicy and white, the flesh of dried longans is dark brown to almost black. In Chinese medicine, the longan, much like the lychee, is thought to give internal "heat".
Potassium chlorate has been found to cause the longan tree to blossom. However, this causes stress on the tree if it is used excessively, and eventually kills it.