You can't see it but you can certainly taste it. It worries winemakers and delights a growing breed of brewers. What is it? It's a funky little wild yeast called Brettanomyces (Brett for short), increasingly prized by the most adventurous craft brewers because - when used skilfully - it adds complexity and depth to their beers.
Not that winemakers are convinced. "I can see why wineries are paranoid, as their wines can age on oak for years, giving Brett plenty of time to alter the flavours," says James Brown of La Sirene, an Alphington brewery that uses the yeast in a beer called Wild Saison. "I've just been to [Northern Californian brewery] Russian River in the Sonoma Valley, where some winemakers won't even come in for a beer, lest they leave with some Brett on their shoes!"
The reason for this is to do with Brett's tenacity (it can live for many years in oak) and its opinion-polarising flavour. "I would describe Brett as 'horsey/goaty' or 'Band-aid'," says Brown, 38. "I know this doesn't sound very appealing but … there's a sense of 'Old World'. These days, everything is manufactured so cleanly, so purely, that these more rustic, challenging flavours are rarely experienced."
The most famous breweries to use wild yeast are the Lambic producers of Brussels and the Senne Valley in Belgium. The likes of Cantillon, which has been brewing in the Belgian capital for more than a century, believe that the use of wild yeast ensures that their beer perfectly expresses the local terroir. Winemakers fear Brett can do the opposite: it destroys fruit character and therefore masks their product's native flavours, they argue.
Cantillon's devotion to the traditional method used to produce Lambic has inspired brewers around the world, including Josh Uljans at Moon Dog Craft Brewery in Abbotsford.
"A few years back, [my business partner] Karl van Buuren and I made the pilgrimage to the Cantillon Brewery," says Uljans. "It was so inspiring to taste beers that were so perfectly crafted, but made in a way that is so completely different to modern brewing methods. We knew that we wanted to make wild beers that paid homage to the traditional Lambic breweries, but at the same time we wanted to put our own spin on the beers."
Brett's rise in popularity has come courtesy of the growing interest in experimental brewing, and the increasing demand for sour beers - Brett does not produce sourness but is often used alongside bacteria that does.
Moon Dog uses Brett in conjunction with oak barrels and fruit in order to produce a complex, unusual beer. "We've got a pretty decent collection of oak barrels that we use: ex-bourbon barrels, cognac barrels, pinot and shiraz barrels, French, American and Hungarian oak," says Uljans. "They all provide their own unique characters that we pick to work with the flavours of the Brett. Right now we've got heaps of shiraz barrels from Mitchelton Winery filled up with our Perverse Sexual Amalgam [a dark wild ale] with cherry plums."
The rising popularity of Brett is shining new light on the critical role yeast plays in brewing (and, indeed, winemaking). Much of the focus of the recent craft-beer movement has been on hops, but cannier brewers have always known that yeast is the real star: you only have to taste beers made by British family brewers like Fuller's, in London, and Adnams, in Suffolk, to realise what a big difference yeasts make to beers produced using fairly similar ingredients.
"Yeast's unpredictability appeals to me," says Brown. "You get the same, repeatable flavours from malt and hops, but yeasts are so sensitive to environmental factors (such as temperature, nutrients, etc), that you can make completely different beers with the same strain. We have only scratched the surface of Brett's potential in brewing."
Leading the way in investigating this potential are American brewers like the aforementioned Russian River, which has built a worldwide reputation despite distributing only in a handful of American states. It's a sign that craft-beer drinkers are now ready for more challenging flavours, Brown believes. "The reaction to the Wild Saison has been very positive," he says. "I think that beer tends to attract the more adventurous drinker. I don't think Brett beers will ever be top-sellers, but I think there is still a lot of room for growth."
Uljans agrees. "Some people that haven't tried them before are a bit shocked by the flavours, but once you explain where the flavours come from and what you're trying to achieve - the balance of flavours, structure and mouth feel - most people want to try more."