Hassaku

Hassaku is probably a natural pomelo × mandarin hybrid, with pomelo-like characteristics. An allele sharing test gives its progenitors as pomelo (Citrus maxima) and the Kunembo mandarin (Citrus nobilis). The pomelo is the largest citrus fruit. It is native to Southeast Asia and is one of the wild ancestors of cultivated citrus species such as the bitter orange and the grapefruit. The bitter orange is itself a natural hybrid between the pomelo and the mandarin.

Origin

Hassaku was discovered during the Edo period at the Mitsugon Jōdo temple in Innoshima, now part of Onomichi, Hiroshima Prefecture, Japan. It was not propagated and planted commercially until about 1925.

The Tree and the Fruit

The hassaku tree is vigorous, upright, virtually thornless. The leaves are broad and pummelo-like, but the petiole wings are narrower, approaching sweet orange. The fruit is medium to large, 9-10 cm in diameter, and slightly oblate. It is seedy and monoembryonic, with numerous segments and a broad axis that is semi-hollow at maturity. The skin is orange-yellow, moderately thick and slightly grainy. The flesh is light yellow. It is not very juicy, but the flavor is good. The fruit matures early in midseason and stores only moderately well. The fruits are harvested in winter, but eaten several weeks or months later, when the bitterness has become milder.

Cultivation

In 1964, hassaku cultivation in Japan was reported to exceed 2,500 acres, mostly in Hiroshima Prefecture, where the fruit originated. During the 1960s the tree was increasingly planted elsewhere. Today, most hassaku is produced in Wakayama Prefecture, which represents about 60% of the Japanese output. Several populations of the trees were infested with Citrus tristeza virus, which caused stunted growth and destroyed the harvest after 15-20 years.

Eating

Hassaku is one of the citrus fruits that Japanese people often eat as they are. Japan is home to more than 25 citrus varieties.

Medical Use

An extract from the young fruits has been used medically in skin care treatments for patients with atopic dermatitis.

Constituents of the Peel

The peels of Citrus hassaku have been studied for their chemistry. A methanol extract of the peels yielded two new limonoids, 1-acetyl-sphaerocarpainic acid I and 1-acetyl-sphaerocarpain I, and a new eremophilane-type nor-sesquiterpenoid, 12-nor-11S-hydroxy-11-hydronootkatone. Eleven known compounds were isolated with them, among them nomilin, limonin, obacunone, nootkatone, umbelliferone and auraptene. The new nor-sesquiterpenoid showed weak antiproliferative effects against human glioblastoma U-251 MG cells.