Laboratory tests have revealed Foster’s super premium $90-a-bottle beer contains brettanomyces, an unwanted yeast characteristic which an expert has described tastes like a “dry stable”.

John Cozens, head brewer at Carlton and United Breweries, confirmed to Crikey that Crown Ambassador Reserve had been found to contain ‘Brett’ — a wild yeast character that produces a sour taste and smell.

“It wasn’t our intention for Brett to feature in the 2010 vintage but we’re still incredibly pleased with the end result,” Cozens told Crikey. “We can’t 100% pinpoint the source of the Brett given some time has passed since we brewed the beer.”

The revelation comes after Crikey reported last month the brewery had been forced to send the top-shelf beer to be tested by microbiologists after an esteemed beer writer wrote that he thought the beer contained the feared yeast strain.

Willie Simpson, a noted beer reviewer, triggered the investigation when he wrote in Epicure he thought the beer contained an “unwanted presence” and he would “send it straight back” had he ordered it at restaurant.

“I reckon it’s chockful with feral yeast flavours and the tell-tale presence of wet horse blanket,” he wrote. “Brettanomyces to be precise, favoured by Belgian lambic beer producers and some English farmhouse cidermakers, but feared by most winemakers and brewers alike.”

Crown Ambassador Reserve, which costs $90 a bottle and is designed to be cellared for up to 10 years, is part of a marketing push by Foster’s to drive interest in its premium Crown Lager brand. Just 7000 bottles have earmarked for release and Foster’s is hoping the beer’s burgeoning reputation amongst fine diners will see it included on wine lists at some of Australia’s big restaurants.

A feature of the 2010 Crown Ambassador vintage is that it was blended with last year’s popular release. Cozens told Crikey it was possible the oak barrels that housed the 2009 reserve brew could have been the brettanomyces culprit. As a result, next year’s edition will not be blended with the 2010 vintage.

Nic Baxter, a lecturer in wine production at the Northern Melbourne Institute of TAFE, told Crikey that while brettanomyces was common in wine, it was a real problem for brewers.

“The reason being is that it has a strange taste characteristic,” he said. “In low levels, Americans call it ‘barnyard’, but here we would liken it to the inside of a dry stable. There’s that little edge of horse sh-t, dry straw and maybe a sweaty smell.”

Baxter also told Crikey that brettanomyces was usually only found in Belgian lambic beers and that in the Australian brewing fraternity it would be typically seen as a serious fault.

“Their argument may be that they are trying to create a complex beer, but if that’s the case it’s the first time that Foster’s has done that,” said Baxter. “Typically their style of beer is ultra clean and, I guess I would say, non-descript.”

But John Cozens is adamant the brew will sit well with beer aficionados: “The beer has had so many great reviews and we’ve had terrific feedback from sommeliers, beer writers and the public.”