Visit Sante Adairius Rustic Ales, and it’s clear that “rustic” describes more than just a beer style. At the end of a warehouse park in a residential neighborhood in Capitola (Santa Cruz County), the brew house looks like an antique relic, with crusty wooden foudres (oak vats), old dairy equipment and discarded Lipton tea barrels. Bottling is done by hand, nearly every day. There are no buttons to push.

“The word ‘rustic’ has become synonymous with saison,” says Tim Clifford, the brewery’s co-owner. “But what we meant by rustic is just the way we make our beer.”

“We started out scrappy,” laughs co-owner Adair Paterno. Altogether, the brew house cost less than $10,000 to set up. “We’ve since kind of upgraded,” she adds. “But not really.”

That scrappiness has no doubt intensified the impassioned following of SARA, as the business is known, which in its short life span has skyrocketed to a cult of obsessive proportions. Demand for SARA’s beers far outruns supply: When I meet them, on a Tuesday, a 10-barrel batch they brewed the previous Friday has already sold out. Limited-edition beers sold to their club are getting flipped for many multiples of their price on the secondary market. Since they don’t distribute — heck, they don’t even deliver — the beers are hard to find, even in the Bay Area’s top beer bars.

Now, at 5 years old, SARA is at a crossroads. So far, rusticity has defined the brewery: its tiny production levels (just 1,350 barrels last year; maybe 1,500 this year), its off-the-path location in Capitola, its scrappy-warehouse vibe.

But SARA is growing up. To increase production capacity, and to reach more customers, Clifford and Paterno are about to open a second, 2,000-square-foot taproom and barrel-storage facility in downtown Santa Cruz. And this time around, it’s going to take more than $10,000 and some Lipton tea barrels.

If the expansion detracts somehow from the SARA cult fever, the co-owners are OK with that. “Sure, being limited creates some buzz,” Paterno says, “but saying no all the time is hard.”

“We’re transitioning from this fun thing we started to, five years later, having to run a real business, with real bills,” Clifford says. “We have responsibilities to our employees. We have to be more practical.”

It’s not only their business that’s coming to an end of innocence — it’s Paterno and Clifford themselves, too. When SARA began, the two were partners in life as well as in business. But they’ve since ended their romantic relationship.

They had been a couple for 20 years when they opened the brewery together. At the time, Paterno was a litigator for the city of Santa Cruz, and Clifford worked at Seven Bridges Organic home-brew supply shop, moonlighting as a home brewer. His best beer, which kept winning home-brew competitions, was a brett-infected saison: Adairius, named for his girlfriend.

In 2011, ready to go pro, Clifford decided to open a combination brewery-winery with local winemaker Jon Benedetti, largely because he wanted access to Benedetti’s discarded wine barrels. They called it Sante (after Benedetti’s great-grandfather) Adairius (after Clifford’s signature beer).

But they couldn’t get the hybrid facility approved by the state’s Department of Alcoholic Beverage Control. To salvage her boyfriend’s dream, Paterno bought Benedetti out of the business. (His winery, Sante Arcangeli, found a home in Pescadero.)

It was a slow start. “It’s been glacial,” Clifford says of their growth rate. Describing themselves as “extremely risk-averse,” the two took on no investors and little bank debt. Establishing infrastructure took a while; hence the old dairy equipment. In exchange for used barrels, they’d trade local wineries kegs of beer.

As for a recruitment strategy, all six of SARA’s current employees began as customers. One night, for example, the tasting room’s credit card machine broke, so Paterno gave away beer for free. A customer, Jon Drinnan, returned first thing the next morning to settle up his tab. He became SARA’s head brewer.

The business was just starting to find its footing when, in 2014, Clifford and Paterno ended their romantic relationship. It was not an amicable parting. And it didn’t stay secret for long: They were outed publicly by another brewer at the state’s annual craft beer conference. Trying not to burden their staff with the drama, Clifford and Paterno took to renting a nearby co-working space whenever they needed to have a meeting together, because they knew it would inevitably escalate to a screaming match.

The question became: Who would get the brewery? Both considered leaving. Clifford took a sabbatical. “For the sake of the business, it was the best call,” he says.

What kept them together as business partners was that “we agree a lot on beer,” Paterno says. “On what’s good and not good — and on what our brewery should be.”

The choice to stay, in the end, didn’t even feel like a choice. “SARA had become our identity, for both of us,” Clifford says.

Today, they discuss the breakup with what feels like remarkable candor. When Paterno says, “It was a rough year and a half,” she wears that memory’s still-fresh pain.

But like many modern families, the two still exude an undeniable intimacy, of the sort that after 20 years cannot be erased. They acknowledge that their relationship can be difficult for new romantic partners to process. But “in some ways, I like my relationship with you better now,” Clifford says to Paterno.

Silence, for a beat.

Then she turns to me: “Business relationships are hard no matter what.”

The rustic brew house, so far, has been more help than hindrance. Physical spaces matter a lot when you’re making the kind of microbially driven beers that have become SARA’s calling card. They like to refer to that style — which incorporates a mix of yeast and bacteria to carry out alcoholic and lactic fermentations — as “mixed fermentation,” which they prefer to “farmhouse ale.” (“We’re not in a farmhouse,” Paterno says.) The moniker “saison” also works.

If the flavors of an IPA are determined largely by its hops, and a porter by its malt, then saison is defined by its yeast. As a home brewer, Clifford attended talks by Vinnie Cilurzo, brewer-owner of Russian River Brewing Co., a mixed-fermentation pioneer. “Vinnie would end the talk by handing out oak chips soaked in his culture,” Clifford recalls. The charge: Here’s my bacteria, now go make your own beer with it.

Clifford spent years developing his proprietary yeast strain — tirelessly experimenting with combinations of commercial strains and still-alive bottle dregs — before arriving at the “house culture” used on SARA beers today.

Arguably, that house culture is SARA’s most valuable asset. “Over time, the culture changes, and takes on a life of its own,” says Clifford. “The beers we make today are direct descendants of the beers we’ve made before.” They consider their flagship, Saison Bernice, a granddaughter of sorts to the original Adairius. (Bernice is Clifford’s mother’s middle name — a more suitable namesake, he felt, after the breakup.)

The proprietary nature of a brewery’s culture is lately a bone of some contention, and raises issues analogous to the open-source software movement. You can’t copyright a yeast strain. But you can protect it. To throw off would-be plagiarists, for example, some brewers will add a Champagne yeast just before bottling, so that anyone who buys a bottle can’t isolate the fermenting strain.

Inspired by pro-open source Russian River, SARA began issuing a friendly message on bottles of Saison Bernice: “The beer in this bottle is alive. So is the yeast. Grow it, keep it, use it. We encourage you to brew your own.”

But Clifford and Paterno’s commitment to open source was tested when they got a call from Jim Withee of Giga Yeast, the laboratory where they bank their culture. A “very large” brewery had called Giga Yeast asking for some of the SARA strain. Withee did not release it, but Clifford and Paterno’s ire had been provoked.

“As a brewer, I’d want to have my own identity,” Clifford says. Incorporating other house cultures into your own, in his view, “is like being a scholar — you should build on other people’s ideas, not steal them entirely.”

Withee has gotten several more requests for SARA’s magic potion. As time goes by, Clifford’s views on open source have changed. “My idols in this business are people like Vinnie, who is so open,” he says. “But I see the merit in being private, too.”

Imitation is flattery, of course, but Clifford and Paterno fear the proliferation of bad imitations — a legitimate concern when you’re dealing with funky, sour, bacterially precarious beers. “The danger is that, when the quality isn’t there, you’re dumbing down a product that turns people off from the whole category,” Clifford says.

SARA isn’t just a saison house, despite the presence of that saison-connoting word (rustic) in their name. They make IPAs, lagers, porters and more. “We had one of their stouts a couple months ago,” says Christian Albertson, owner of Mission beer bar Monk’s Kettle. “And my staff was like, ‘They should just do stouts.’ Except that they do hoppy really well, and tart saisons really well.”

Most brewers would go to extreme lengths to separate the equipment used on funky beers, which incorporate spoilage agents like brettanomyces, lactobacillus and pediococcus, from “clean” beers like IPAs. SARA does not. They have, admittedly, had bacterial issues, but that’s the price of biodiversity. Once, a huge foudre became infected with something nasty, and they had to dump all its beer, plus the foudre. But the clean beers come out clean.

“I’m realizing they have a freakish gift for new styles,” says Ben Henning, in charge of beer at State Bird Provisions and the Progress. “The sours are the ones that, when the bottles are released, get the lines out the door. But the lagers are pretty stunning. The hoppy beers are consistently incredible. Somehow, the character, whether it’s the brewing style or the yeast, is always harmonious.”

Those who want SARA beers have to put in some legwork — or at least gas mileage — to get them. Everyone has to come to Capitola, including bar and restaurant buyers like Henning and Albertson. Ninety-two percent of their sales come from direct-to-consumer purchases, making their business model more like a boutique winery than a typical brewery. Membership in their beer club, the SARA Cellar, is coveted, and the limited-edition bottles acquired through it are getting resold online; much to Clifford and Paterno’s dismay, three $20 bottles were recently flipped for $550.

Paterno and Clifford are uncomfortable with the hype, in part because they still feel like they’re figuring out this whole brewery thing — not to mention their own relationship. They’re nervous about the Santa Cruz taproom, which they hope to open by August, and the challenges of staffing, permitting and increased public visibility that will come with it. For the moment, the new space will just be barrel storage and a tasting room. But Clifford is already envisioning adding brewing equipment there and making a bona fide second brew house.

“There’s a high demand for our beer, and even five years in, we can’t meet it,” Clifford says.

Paterno quickly adds: “But we want to.”

Tasting Notes: Sante Adairius Rustic Ales

Esther Mobley is The San Francisco Chronicle’s wine, beer and spirits writer. Email: emobley@sfchronicle.com Twitter: @Esther_mobley Instagram: @esthermob