There is a joke among cider makers when they open a bottle and its contents taste disappointingly sour or flawed:

"We say, 'Oh yeah, this cider went bad, so we just put it into green bottles and called it Spanish,' " says cider maker Nathaniel West, owner of Reverend Nat's Hard Cider in Portland, Ore.

The joke refers to the fact that northern Spanish cider, which has been made for centuries along Spain's Atlantic coast, is characteristically tart, almost entirely flat, cloudy with sediment and a bit funky tasting — as though the fermented juice might have been filtered through a hay bale. It can taste a bit off to first-timers more accustomed to the sweeter, and sometimes one-dimensional, ciders of America's rather limited, if growing, craft cider market.

However, Spanish cider is a beloved tradition in its homeland, mainly in the lush coastal state of Asturias. Here, apple orchards blanket the emerald slopes that plunge toward the Bay of Biscay, and sidra natural — so called because it is fermented with wild yeasts found naturally on apple skins — is made in scores of commercial cider houses, homes and roadside bars. Sidra natural of Asturias is, in fact, a product protected by European Union regulations, with growers restricted to using 22 specific varieties of apples in order to label their beverage "sidra de Asturias."

America's craft cider culture has been growing rapidly for about a decade, with cider makers both experimenting with flavored styles as well as following old traditions from France and England. But Spanish-style cider has only recently begun to gain traction among American craft cider fans.

Imports of Asturian brands seem to be on the rise, according to Jim Asbel, a Portland-based importer who deals exclusively in Spanish ciders and has closely studied the American cider market. Within the United States, a growing pool of cider makers is making their own impressions of the style. Some American producers even serve their sidra in the traditional Spanish way — from the spigot of an elevated barrel or poured from a bottle held overhead. The idea is to create some effervescence, as the otherwise almost bubble-less cider splashes into the glass.

West makes a sidra natural — a sharp and musky-smelling rendition called Sidra Bravo, which he first introduced in 2015. Tilted Shed Ciderworks, in Sonoma County, Calif., first introduced theirs, a lively interpretation called Inclinado, in 2014. Other riffs of the tart Asturian style are made by Millstone Cellars in Maryland, Finnriver Farm & Cidery, near Seattle, and Angry Orchard — a side project of the Boston Beer Co.

"A lot of this has to do with sour beers becoming so popular," says Tim Prendergast, cider maker and co-owner of ANXO Cidery & Pintxos Bar, a cider-centric tap house and restaurant in Washington, D.C. "Even people who don't necessarily like sour beers now know that sourness is something that can be intentional and doesn't necessarily mean, 'Oh, this cider has gone bad.' "

ANXO just released its first sidra natural. In early November, Prendergast put almost 600 gallons of apple juice into a huge oak barrel. Here, the juice spontaneously fermented for about six months, much the way sidra natural is made in Spain. Yeasts and bacteria worked slowly away at the sugars, and finally, in late April, Prendergast transferred the sour cider into kegs and small bottles. It was served at a small onsite festival on April 30.

In Portland, Ore., Asbel founded his import company, Ciders of Spain, in 2013 — a career move that was a culmination of decades of traveling in Spanish cider country. Asbel remembers his first taste of Asturian sidra natural. It was 1974, and he was touring by bicycle. While passing over the Picos de Europa, the spectacular mountain range that crowns the state of Asturias, he stopped in at a roadside bar, starved and ready to eat anything — but he wasn't expecting sour cider.

"I probably wrinkled my nose at it," he says.

But it only took a few sips for Asbel to fall in love with the sidra's prominent tartness — partly the effects of acetic acid (what makes vinegar sour) — and complex flavors. Today, Asbel calls himself "the torchbearer" for sidra natural.

He says many cider aficionados have reacted the way he first did to sidra natural. He recalls a few years ago when he periodically presented Asturian ciders at tastings and competitions.

"They were horrified by this sour, horse-blanket-smelling stuff," he says.

Now, people want funk and sourness, especially in the craft beer world, where sour ales have become wildly popular.

But Ellen Cavalli, co-owner of Tilted Shed Ciderworks, is reluctant to call her sidra rendition — and the style as a whole — "sour."

"I think it's a bit reductive," she says. "These ciders are so much more complex than that. They're lively and fruity, and they've been making them in Spain for hundreds of years. But if the word sour is going to push the category forward in the U.S., then I'll go with it."

In the United States, few of the designated apple varieties used in Spanish sidra natural are commercially available — but it's debatable whether this makes much of a difference in taste or quality. ANXO's sidra was made in large part from bitter crabapples harvested along D.C.'s sidewalks. Angry Orchard has used a blend of sour and bitter varieties. Tilted Shed's sidra natural is made with Gravenstein apples, the storied heirloom variety of Sonoma County.

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