Humans and yeasts have evolved side by side for millennia, so it's no surprise that we love fermented foods: yogurt, soy sauce, kimchi, sourdough, wine, beer. Plenty of foods will ferment on their own given time and the right conditions. Fruit, for example. Milk. Beans. Anything that contains sugar for the yeast to eat.
When it comes to brewing, letting nature do its thang is nothing new. Our single-celled helpers were busy doing us a solid on the regular long before we doltish humans even knew that yeast existed. Which is just another way of pointing out that not so long ago all beer would have been what we now call spontaneously fermented (“spon” for short) or wild beer. It's only relatively recently that “clean” brews using cultured yeasts have become the norm.
Most brewers making wild beers will switch up their technique a little to suit the process. Instead of making wort in the usual way they will use a turbid mash. This involves taking portions of liquid out of the mash tun at different stages. The aim is to end up with a thick, soupy wort that contains a mix of short- and long-chain sugars. You'll understand why in a moment.
When it's ready, the brewer pumps the wort into a coolship. Picture an open-topped stainless-steel paddling pool wider than it is deep. These are usually housed up near the brewery's roof, where louvred windows can be left open to the cool night breeze—and anything that blows in on it. Yeast, you see, is everywhere.
Yeasts riding on the wind will settle overnight onto the surface of the cooling wort. Steam rising from the wort will condense on the rafters above, picking up yet more yeasts that live in the wood as it does so. When the droplets finally grow too heavy to cling on, and fall into the wort below, they ferry these yeasts down with them. This is why some brewers will suspend old barrel staves above their coolships like a rustic baby's mobile for their young beer.
Brewers usually prefer to brew coolship beers when the nighttime temperature outside will fall to 46 degrees Fahrenheit (8 degrees Celsius) so that by the next morning the wort will have cooled from near-boiling to about 68 degrees Fahrenheit (20 degrees Celsius), an ideal temperature for fermentation. Sometimes the cooled wort goes directly into wooden barrels. Sometimes it first spends a couple of days in a stainless steel “horny tank,” where the overnight incomers have a chance to get their breed on before going into wood to meet the rest of the party guests.
The bugs on the invite list are wild strains of Saccharomyces, Brettanomyces, Lactobacillus, and Pediococcus. Occasionally the food poisoning duo Enterobacter and E. coli will crash the party and bring along their pal Acetobacter (a.k.a. Mr. Vinegar). Luckily for us, wild Sacch has got our back. It will quickly turn the wort into a hostile environment for this unpleasant gang of microbes by lowering the pH, multiplying quicker than they can, eating all the food and getting them shitfaced.
When fermentation takes hold, wild Sacch does its usual thing, eating up the short-chain sugars and pumping out carbon dioxide and alcohol. You can think of it like a picky preschooler; it only eats the simple stuff. It leaves behind the complex, long-chain sugars upon which Brett, Lacto and Pedio will chow down once the Sacch has filled its boots. This takes a long time, and results in all sorts of funky flavors. Spon beers are sour but they're also complex, not like the quicker kettle sours which can seem thin and one-dimensional in comparison.
The final ingredient for a great wild beer is time. These beers sit in barrels for one, two, three years, sometimes even longer. As the beer slumbers and the bugs do their slow work, the wood lends its own character to the beer. Most wild beer brewers prefer barrels which held white or sometimes red wine in their former lives. Over time and with repeated use these will change; the oak and wine characters will soften and a “house style” flavor will develop as the brewery's unique blend of microflora colonise the porous wood.
Some wild beers are bottled right out of the barrel in which they fermented. This is known as a “monoblend” in the trade. Others are blended from a variety of barrels, often using beers of different ages, to balance out the various characteristics of sourness, woody notes, yeasty funk, and overall awesomeness. Some also undergo a secondary fermentation in their bottle; this is what makes a traditional gueuze so effervescent.
While the classic examples of spon beers are Belgian, American brewers have been making great wild sours for some time as well. Look at beers from Jester King, Crooked Stave, and Jolly Pumpkin to name just a few. You can also find some top notch wild brews from the likes of Burning Sky and Mills Brewing, both in the UK, and La Sirène in Australia.
Top photo via Jester King on Facebook