Saint Arnold Brewing Co. next week will release its first new year-round beer since February 2012.

Boiler Room Berliner Weisse is relatively low alcohol and also slightly tart due to the Lactobacillus bacteria introduced early in the production process.

Yes, bacteria. But not the kind that can harm you.

"We're just harnessing it to do good," says Saint Arnold founder Brock Wagner.

Wagner, often ebullient about his beers, seems particularly enamored of this one. It sounds like a classic labor of love.

"It's really, really hard to make and not very practical to make," Wagner says.

The beer started life as a mistake, though in hindsight it may sound more like a stroke of luck.

A misstep while adding grain for a batch of Fancy Lawnmower left Saint Arnold with 136 barrels of the pre-fermentation wort that was only half as strong as it was supposed to be. Before dumping the batch, brewer Stephen Rawlings set aside 10 gallons, mixed it with a Belgian beer known to have active Lactobacillus in it and left it in the boiler room to sour for a couple of days, with Rawlings checking the acidity level and then boiling it when it got to where he wanted it.

That led to the brewing of a test batch, which was found to be pretty good. Further test batches refined the process, Wagner says. Then came the scary part: brewing on a large scale and intentionally adding in bacteria.

"OK, we've done this in 10 gallons. Now we've got to sour 4,000 gallons," Wagner says. Adding to the anxiety, he recalls, "Trying to figure out how to spoil something on purpose is the opposite of everything we've trained for."

The beer is made with the brewery's regular Kölsch yeast, but it is held in the brew kettle much longer and at a lower temperature than usual to allow the bacteria to sour the wort. Once the beer reaches the desired pH level, the wort is boiled to halt the bacteria, a small amount of hops is added and then the yeast is pitched in to the fermenter.

The batch that will hit bars and restaurants starting Wednesday was brewed two weeks ago - on a Friday, since it would have to remain in the brew kettle for 48 hours to give the bacteria time to do its work. Brewers monitored the pH levels throughout the weekend.

The result, says Wagner, is a relatively light - 3.5 percent alcohol - and refreshing beer.

The bacteria is maintained the same way that bakers keep a starter supply of sourdough yeast, with a portion of the initial wort pulled aside from each batch for use in the next one.

The last Saint Arnold beer to go into year-round production was its Endeavour imperial India pale ale, although the brewery has released other specialty beers and seasonals in the meantime.