SOMETHING smells about Not Your Father’s Root Beer, and I’m not talking about the heavy dose of vanilla extract that flavors the suddenly popular brew.

Described as “ale with the taste of spices,” it tastes exactly like soda and contains 5.9 percent alcohol. Two other versions sold in large bottles contain 10.7 percent and an astonishing 19.5 percent alcohol.

The brew has social media and beer aficionado websites buzzing with excitement and high ratings, and buyers in Pennsylvania have been scooping up $50 cases since they arrived earlier this year. Its manufacturer went from an unknown startup to national distribution in under five years, with one sales survey calling it the fastest-selling new craft-beer product of 2015.

What is this stuff, and who’s behind its incredible success?

Tim Kovac, founder and brewmaster of Small Town Brewery, in tiny Wauconda, Ill. (population 13,823), where Not Your Fathers Root Beer was born, says he’s astonished at his success.

“It has been very much an amazing ride,” Kovac told me. “Going from a few dozen Chicago bars to one of the most sought-after beers in America – it’s a phenomenal beer, it really it is. ”

At first, Small Town Brewery sounds like the prototypical independent craft brewery with a quaint back story: The owner is a graphic artist who stumbles upon his best-selling recipe during a carefree day of stovetop home-brewing with his son. It takes him two years to perfect it, finally producing an authentic, old-fashioned hard root beer.

One day, he serves it to a woman and watches a tear roll down her cheek as she declares, “You just brought back memories of me being a little girl. ”

There are other gems, including the discovery of a 17th-century “leather-bound scroll” filled with brewing recipes from a seafaring ancestor who, legend has it, won a brewery in a card game.

Kovac shared the homespun tale with me last week during a phone call arranged and monitored by his public-relations agency, Sard Verbinnen & Co., a high-priced New York City firm known mainly for representing Wall Street scoundrels, including the Madoff family and Lehman Brothers’ Dick Fuld.

When I asked for details on how the root beer is brewed, the PR rep interrupted and said, “Parts of the recipe are proprietary. ”

Kovac said it’s “brewed and fermented just like any other beer. ”

Perhaps, but this is what else we know:

The brewhouse at Small Town Brewery, tucked into a small industrial center that also houses a body-jewelry outlet and a smoke shop, is capable of making fewer than 15 kegs a day.

That’s the equivalent of about 2,500 bottles – or would be if the brewery owned any bottling equipment.

Most of the root beer is brewed and packaged 238 miles away, at the former G. Heileman Brewing plant now owned by City Brewing in La Crosse, Wis.

Kovac said that City Brewing uses his original recipe.

The La Crosse plant, however, is known primarily for the production of Mike’s Hard Lemonade, Smirnoff Ice and other so-called malternatives.

These drinks, which are not generally regarded as real beer, are fermented from grains and sugar, then stripped down to their essential taste-free alcohol and reflavored artificially.

It’s a fairly advanced technique, one that no small, largely inexperienced craft brewer would likely tackle on his own.

Kovac and the names of two other area men are listed on Small Town’s state liquor license. Nonetheless, there is ample evidence that the brewery is either controlled by or in a partnership with a much larger company called Phusion Projects LLC.

For example:

The label for Not Your Father’s Root Beer was registered by Phusion. Small Town’s Illinois state business registration lists Phusion’s Chicago offices as its main address. Small Town and Phusion shared the same director of strategic marketing. And, tellingly, Small Town Brewery’s own website includes a contact address that is the same as Phusion’s. Or, at least it did, until the address was erased from the website sometime this spring.

Why the subterfuge?

Possibly because Phusion is responsible for the most notorious alcoholic beverage to hit the shelves in the past decade: Four Loko.

Made with caffeine and marketed as an alcoholic “energy beer,” Four Loko was linked to dozens of hospitalizations and at least one death from excessive consumption before the U.S. Food and Drug Administration forced it off shelves in 2010.

The drink has since returned but without added caffeine.

Phusion did not reply to a request for comment about its relationship, and Small Town declined to answer further questions about its ownership.

However, a source familiar with the companies told me that the brand (but not the brewery) was recently acquired by Eugene Kashper, the new CEO and chairman of Pabst Brewing.

Pabst will distribute the root beer in all 50 states

Meanwhile, Small Town Brewery is developing other brands, including Not Your Father’s Ginger Beer and a barrel-aged root beer with 24 percent alcohol.

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