Koshu is said to be Japan’s only indigenous grape. It is named after its supposed prefecture of origin: Yamanashi. Koshu is a traditional name for the prefecture, so the grape is literally ‘the grape of Yamanashi’. It has two origin stories. One suggests Koshu was found growing wild in Yamanashi in 1186, allegedly grown from grapes introduced from China by Buddhist monks. The second suggests its seeds were planted by a famous monk Gyoki in 718AD. Reality is more prosaic, if not exactly clear. Wine Grapes tells us that while it belongs to the Vitis vinifera L family, that family was never grown naturally in Japan, so it must have been either introduced from Eurasia or bred in Japan from two imported vinifera parents. Joint Japanese Canadian research into the grape’s DNA reveals it contains something like 30% wild Vitis material, but its journey to Japan remains obscure.
Today Yamanashi prefecture remains the heartland of Koshu in Japan. The region is responsible for 8,600 tonnes or about 30% of total Japanese wine production (2015). Of this half is Koshu. Production is not significant in the second and third most important prefectures (Nagano and Hokkaido) and it represents just over 1% of production in Yamagata, the fourth most important wine producing region.
Koshu is a late-ripening grape, with large, pinkish-purple, thick skinned berries and some susceptibility to mildew but, thanks to those thick skins, it is resistant to bunch rot (botrytis). Typically the wines are delicate, light and fresh. Until recently Koshu was mostly grown as a table grape on pergolas. As a wine grape it was low in sugars and lacked both real flavour and acidity. Selection of better clones was a first step in the 1970s, when serious attempts were made to improve wine quality. The shift in emphasis towards wine production has seen some experimentation with different trellising systems and innovative canopy management techniques have enabled significant quality improvements.
The 1970s also saw a move away from sweeter styles towards dry, fresh styles, with a new emphasis on hygiene in the cellar. Critics of this style might see the wines as simple and boring, whereas alternatively they might be said to be subtle and elegant. Koshu’s thick skins can also provide an interesting textural dimension, not dissimilar to that of some Italian varieties, such as Verdicchio. Professor Tominaga of Bordeaux University, working with producer Chateau Mercian, has identified precursors of 3-mercaptohexan-1-o1 (3MH), which provides citrus aromas. Mercian has subsequently exploited this discovery with its grapefruit-scented Koshu Kiiroka. Along with other larger producers like Manns, Mercian has constantly shared its research with the wider wine industry in Japan, helping to raise collective standards. New winemaking techniques designed to give more flavour and texture include extended lees contact; cryoextraction and skin contact. Other innovative wine styles include barrel-fermented versions and sparkling wines.
For over 30 years as a Master of Wine, consultant, speaker, educator, trainer, writer and judge, Peter McCombie MW have enjoyed promoting the world of wine. Peter is a Co-Chairman of the International Wine Challenge and judge wine competitions all around the world.