Oita Kabosu

Kabosu is a specialty fruit tree of Oita Prefecture, cultivated across the whole prefecture. The main growing areas cluster around Usuki, Taketa, Bungo-ono, Kunisaki, and Bungotakada, in commercial orchards and in scattered garden trees alike. Both the prefecture and the growers' association describe Oita as supplying almost all of Japan's kabosu — about 99% of national output in 2022.

Fruit sold under the name Oita kabosu is simply kabosu grown within the prefecture; the name fixes the growing ground to Oita rather than to any single town. Usuki and Taketa, by contrast, mark where cultivation was earliest and is densest, not the only places the trees stand.

A kabosu is about as big as a tennis ball, a size larger than the similar-looking sudachi. Cut it open and the flesh is juicy, the scent clean. Among fragrant acidic citrus its sweetness is comparatively strong and its sourness rounded, so a squeeze over grilled fish or grilled meat draws out the ingredient's own flavor while the acidity stays in the background.

It also carries a faint mineral saltiness, which is why using kabosu in place of some salt can help cut sodium, and its juice holds citric acid and vitamin C. Together these traits explain the habit of squeezing it only as the food reaches the table, so the aroma and acidity land fresh.

Kabosu was planted early in household gardens around Taketa and Usuki, kept as a medicinal plant. In Usuki's Otomi district a local account survives: that in the Edo period a doctor named Sogen brought a seedling back from Kyoto and planted it, taken as the start of cultivation. This is oral local tradition rather than a documented, dated origin.

Usuki does, though, keep living witnesses. One tree in the city is said to be around 300 years old, and several more of roughly 200 years are scattered about. The word kabosu first appears in writing only in the mid-Showa years after the war. Because trees this old are not found in other prefectures, Oita is widely taken to be where the fruit originated.

Kabosu is at its peak from August to October, when juice and aroma are fullest. To stretch the shipping window, growers link greenhouse, open-field, and stored fruit: greenhouse fruit runs from about mid-March to early August, open-field fruit follows from August, and from October cold-stored fruit takes over.

Storage is about holding the peel green and the aroma intact into winter. In November and December there is also yellow kabosu, left to ripen on the tree and harvested late. Through this arrangement Oita's kabosu reaches the table from late summer well into the following spring, beyond the two or three months of open-field harvest.

Much of a kabosu's green color and quality comes from handwork in the grove. Growers prune, clear the branches that block light, and expose each fruit to the sun so the peel colors a clear green; they also watch for the tree's tendency to bear in alternate years and even out the yield. Harvest in high summer is hard going, and many steps are still done by hand where mechanization remains limited.

The cultivated varieties differ too — among them Oita No. 1 and Toyo Midori. In Kunisaki, one grower works about 22 hectares and runs a juice-processing facility, shipping fresh fruit and making processed goods alike. These are the choices of individual farms, not a single rule across the whole prefecture.

In Oita the commonest use of kabosu is a squeeze just before eating. It cuts the richness of grilled fish, grilled meat, and fried food; a few drops into hot pot or miso soup freshen the bowl; and it turns up in cocktails, highballs, soft drinks, and sweets. Because its sweetness runs high and its sourness stays rounded, the juice carries the ingredient's aroma forward instead of masking it.

Many growers' own favorite way is simply to squeeze it fresh. The association, too, stresses how plain the method is: squeeze the juice into a dish or a drink, and that is all. That habit of a single squeeze is what lets the fruit slip into an everyday meal.

At the end of July 2020, the JA Oita kabosu section and the Usuki City kabosu promotion council ran an outreach class for about 30 fifth-graders at Shimominami Elementary School in Usuki. That year the pandemic pushed the start of summer break later, so the children happened to catch the season's first harvest.

The class ran in three parts: first a harvest in the grove of Kazuo Ninomiya, a member of the southern Oita kabosu section, where children picking with shears for the first time were nervous and delighted; then a lesson on the history and distribution of Oita kabosu; and finally making kabosu lassi, a yogurt drink. One child said they were surprised Oita grows more kabosu than anywhere in Japan; another saw how hard the harvest is for farmers. Oita kabosu reaches its busiest stretch just after Obon.