By Elin McCoy, Bloomberg

INSIDE a 19th-century silk merchant’s house in Katsunuma, Japan, about 70 miles west of central Tokyo, the three Aruga brothers are pouring several white wines in their timbered tasting room. All are made at their Katsunuma Jyozo Winery under the Aruga Branca label from the country’s unique grape variety koshu, and all are delicious: One is elegant and sparkling; another fresh, bright, and lemony; a third succulent and tangy; still another savory and smoky; and a fifth barrel-fermented version is round, rich, and smooth.

About 15 years ago, when an Aruga Branca bottle won medals in a French wine competition, Bernard Magrez of famous Bordeaux château Pape Clement was so intrigued he proposed a joint wine project that introduced koshu to France. And now third-generation winemaker Hiro Aruga, who studied and worked in Burgundy, has joined his father, Yuji, and is experimenting to create wines with even higher quality.

Aruga Branca is part of the vinous revolution that’s making Japan the world’s newest serious wine frontier. Since 2010, koshu has been on the International Organization of Vine and Wine (OIV) list of varieties, so it can be displayed on labels in Europe. And last year, to insure quality, government regulations were enacted to restrict labeling of Japanese wine to vintages made wholly in the country, from vine to barrel. The volume of exports went from 45,000 liters to 58,000 liters from 2015 to 2017, up almost 30%, according to the National Tax Agency. Ambitious vintners anticipate more demand during next summer’s Olympics.

Although many grapes are grown in Japan, wineries in Yamanashi, the most important of Japan’s four major wine regions and where Katsunuma is located, are betting on koshu. “The grape is ideal for Japan’s humid, rainy climate. It’s thick-skinned and resistant to rot,” says Mr. Aruga.

The vineyards, too, seem unique. At nearby Lumière, which claims to be the oldest family owned winery in Japan (established in 1885), koshu vines look like small trees, with branches spread-eagled on wires six feet off the ground to create a pergola. Folded paper hats are tied over hanging bunches like miniature umbrellas to shelter grapes from rain.

Japan’s enticing whites fit neatly with the latest global wine trends. Koshu wines have floral aromas, delicate, distinctive flavors (of yuzu, savory minerals), and naturally low alcohol levels (11% to 12%), and they’re exotic but not odd, like, say, Georgia’s rkatsiteli which hasn’t truly taken off. And, hey, the name koshu is easy to remember and pronounce. Plus, the wines are perfect matches with popular Japanese cuisine mainstays sushi and sashimi.

All this is why I recently spent a few days in Yamanashi, a 90-minute train ride from Tokyo. It’s home to 81 of the country’s 300 wineries and has a 1,000-year history of grape growing and nearly all Japan’s plantings of koshu.

My first stop, in pouring rain, was the local Daizen-ji Temple touted as the grape’s legendary birthplace 1,300 years ago, when a monk named Gyoki saw a vision of the Buddha of medicine holding a bunch of grapes, which led him to discover a grapevine.

The real story turns out to be only slightly less fanciful. DNA analysis at the University of California at Davis showed koshu is a hybrid of mostly vitis vinifera (the species of European grapes like chardonnay) and Asian grapes. Scientific consensus is that it came to Japan from the Caucasus via the Silk Road.

But for most of its history, pretty pink-skinned koshu was a table grape for eating. Only in the last 130 years has it been turned into wine.

“For most of that time the wines were sweet and reviled,” explains Ernie Singer, a Tokyo wine merchant, who produces Shizen sparkling koshu on Mount Fuji and has been a key player in reorienting Japan’s wine industry. Along with other koshu boosters such as Château Mercian and Shigekazu Misawa, the intense owner of Grace Wine, Singer enlisted the help of Bordeaux white wine wizard Denis Dubourdieu as a consultant 15 years ago. Now almost all koshu is dry.

As Misawa drove me around his vineyards in the northwest part of Yamanashi, he pointed out ways he’s been experimenting to improve quality. He’s planted vineyards at higher elevations and in neat rows like those in Europe instead of the traditional pergola. He says that helps boost ripeness during wet summers and results in more body and richness in the wines. He founded organization Koshu of Japan, which began holding annual tastings in London in 2010.

His daughter Ayana, who studied in Bordeaux and is now the winemaker, hand picks and sorts grapes meticulously. She’s pioneering making koshu from single vineyards with different terroirs, from volcanic soil to slate. She ages most of them in oak, but she says “you have to be careful, especially with oak, because koshu aromas are very delicate.”

You could think of koshu like Cincinnati’s Five Way Chili. Most wineries make five or six styles — sparkling like Champagne; crisp like a cross between chablis and sauvignon blanc; aged on the lees with a tangy taste like Muscadet; round, rich, and aged in oak barrels; and even “orange” versions, fermented on grape skins the way red wines are, and which is the best match with sea urchin, if you’re wondering.

The big Japanese drinks companies — Kirin, Suntory, Sapporo — all have wineries in Yamanashi. Château Mercian, now a member of the Kirin group, was the first to make a dry koshu on the lees and turned to both Dubourdieu and the late Paul Pontallier of Château Margaux for help in making more elegant wines. Suntory, famous for whisky, actually began as a wine company more than 100 years ago. From its vineyards you have a panoramic view of the valley surrounded by densely forested mountains including a glimpse of snow-capped Mount Fuji. Both make excellent wines from familiar Western grapes like sauvignon blanc and merlot, as well as koshu.

New boutique wineries reflect today’s boom. Hiroshi Matsuzaka founded modern, Western-style MGVs winery three years ago in a former semiconductor plant where he used to make precision parts for smartphones. Except for the signs in Japanese, his hipster-style tasting room with shiny green metal chairs and a bar could be in Napa.

We donned disposable white hairnets and booties to tour the industrial winery. Where he once used liquefied nitrogen gas to protect silicon wafers from oxygen and moisture, he now uses it to prevent grape juice from deteriorating through oxidation. Despite his scientific bent, every tank is named for a tarot card. His wines will be in the US next year.

Yamanashi is an easy day or weekend trip from Tokyo, and most wineries have tasting rooms and cafés open daily. One of the most popular lunch stops is small family run Haramo Wine housed in a building once used for silkworms, where you can enjoy simple Japanese vegetable plates with maitake mushrooms and sausages as well as meat curries on rice, then superb coffee.

Like many wineries it also makes a red wine from native grape Muscat Bailey A, a hybrid developed in 1927. How can I put it? I’m not a fan of its candylike scents and intense, cherry-candy flavors. Stick with koshu.

“The next generation will make great red wines as well as white,” predicts Haramo owner Shintaro Furuya, “but we’re on our way.”

Some of these wines are available in the UK and US, while others will arrive next year. You can order now from Dekanta, a Japanese online retailer. If they don’t list it, they may be able to source it for you.

• 2015 Shizen Sparkling Koshu ($50) — Crisp and lemony and made by the traditional method used for Champagne, this sparkler has an almondy richness and salty character. It’s poured at top Tokyo restaurants such as RyuGin and Esperance.

• 2017 MGVs K131 Shimokawakubo (¥5,400, or about $50) — Very dry, savory and smoky, this intense wine offers aromas of ripe grapefruit, herbs, and honeysuckle with round, citrusy, and mineral flavors. Next year it will be available through Joto Sake in the US.

• 2017 Grace Koshu Private Reserve Koshu ($25) — The elegant koshu has the tang of a golden delicious apple overlaid with notes of lemon and chalk. All the winery’s koshus are brilliant. Especially Cuvee Misawa Areno, with its herb and grapefruit flavors and aromas of jasmine.

• 2017 Château Mercian Yamanashi Koshu ($27) — Light and citrusy, with hints of green apple, this has the rich, savory quality of koshu aged on the lees. Also look for Koshu Cuvee Ueno and Koshu Gris de Gris.

• 2017 Haramo Koshu Lees Contact ($18) — Fresh, very subtle and delicate, it’s a bit like richer Muscadet with spice and tang. Also look for their barrel-aged version.

• 2017 Aruga Branca Issehara ($99) — This single-vineyard white mostly made in stainless-steel tanks has floral, peach-and-lime aromas and deep, complex mineral flavors. Every koshu from this producer is stunning.

• 2016 Lumière Prestige Class Orange ($34) — Koshu grapes make terrific orange wines. This one has a light apricot color, umami flavors, and a rich texture. The winery’s sparkling koshus are also worth seeking out.

• 2017 Suntory Tomi no Oka Koshu ($60) — With fresh acidity, this has bright aromas and a lot of snappy richness for a white wine. The grapes come from 40-year-old vines grown at a high altitude.