One rare citrus stands out: the exceedingly sour and bitter, yet exquisitely delicious, sanbokan.

When you think of Japanese cooking, chances are you may not think of citrus. But Japan produces some of the most diverse varieties of citrus in the world, and, after salt, the acidic juice from sour citrus is the single most important seasoning in the Japanese pantry. It finds its way into condiments like ponzu marinades and fermented chilli yuzu kosho; it is used to brighten the flavour of almost every dish – including sushi and sashimi – and it is made into an infinite variety of preserves, desserts and drinks.

Today, thanks to its ideal growing conditions, Wakayama remains Japan’s top citrus-producing prefecture. 

Straddling the Earth’s temperate and sub-tropical zones, the region benefits from long, hot summers and winters just cool enough to ripen citrus. And because Wakayama is a peninsula that juts out into the Pacific Ocean, it catches the heavy rainfall caused by the warm Kuroshio Current that flows up from the South Pacific along Japan’s eastern coastline, making the region the wettest semi-tropical area in the world.

Wakayama is Japan’s top fruit-producing prefecture, producing many varieties that form an essential part of Japanese cooking (Credit: Credit: kookookoo/Getty Images)

Wakayama is Japan’s top fruit-producing prefecture, producing hundreds of varieties that form an essential part of Japanese cooking 

As a result, Wakayama is commonly referred to within Japan as “The Fruit Kingdom”. And among the prefecture’s myriad varieties of juicy sweet mandarins, oranges and tangelos – and distinct lemon-like yuzu, lime-looking sudachi, mandarin-esque jabara, grapefruit-sized hassaku and bitter-orange daidai – one rare citrus stands out: the exceedingly sour and bitter, yet exquisitely delicious, sanbokan.

According to legend, it comes from a single tree that grew inside the castle

With its thick peel and unmistakably pronounced nipple, the sanbokan is instantly recognisable. It has a taste that is somewhere between a blood orange and a bitter grapefruit. Scientists have no idea how this unique citrus formed. Most say it’s a variety of orange. Others that it’s related to Japan’s ethereally fragrant yuzu. Some even say it’s a type of lemon. But according to legend, it comes from a single tree that grew inside the castle of the former feudal lords who ruled Wakayama prefecture until 1867.

I first heard about sanbokan from Shigeru Muroi, chef-owner of the Michelin-starred kaiseki restaurant Muroi in Kyoto, who told me that it’s his favourite citrus because, though very tart, it also has a sweet richness. 

He uses it when it’s in season each winter to finish his multi-course meals with a sophisticated, palate-cleansing flourish. Muroi also told me about the sanbokan legend: a samurai allegedly found the fruit growing wild sometime between 1818 and 1829 and brought the tree to the castle. Wakayama’s ruler restricted its cultivation and distribution so only those in the castle could taste it, and this “secret fruit” became part of the bracing diet of the feudal lord of Wakayama and his samurai followers.