Spontaneity: Prospecting for Bugs

With fairly simple tools and the right attitude, you could create a local blend of microflora that delivers something valuable.

Dave Carpenter Feb 21, 2017 - 12 min read

Spontaneity: Prospecting for Bugs Primary Image

Within the gables of a crumbling gray row house facing a cobblestone lane in the shadow of Brussels’s Tour du Midi, a bit of magic is unfolding. As a venerable brewer shuts the front doors to his family-owned brasserie three floors below, a gentle breeze wafts through open windows and louvred vents. Swirling currents of air percolate through the brewery’s attic, dispersing a unique blend of microbes carried aloft from the banks of the Senne. The microorganisms settle upon a shallow open vat of tepid, turbid wort and begin the years-long process of transforming malt sugars into lambic, pride of the Pajottenland.

Such sepia-toned scenes are as romantic and alluring as the myth of the Wild West, but successful spontaneous fermentation leaves less to chance than legend suggests. Airborne bugs do make their way into Belgian wild ales, to be sure, but it is largely the resident microbes—caked on walls, joists, and barrels—that hold the signature terroir found in every bottle of lambic, gueuze, kriek, and framboise.

Indeed, the virtuosos at Brasserie Cantillon recently gained a bit of press for having sprayed down the brick walls and wooden rafters of their new building with a healthy dose of the brewery’s elusive lambic. The goal? Ensuring that the new location maintains the same blend of critters as the old one. It takes years—more than a century, in fact—to develop just the right mix for a classic example such as Cantillon Gueuze.

But, for New World craft brewers and homebrewers just staking their claims, the draw of spontaneous fermentation is just too powerful to ignore. With the right tools—and the right attitude—they’re out to create their own terroir.


Own the Risk

Let’s get something out of the way: You probably shouldn’t bother with spontaneous fermentation if you’re impatient or if you fear uncertainty. It’s unlikely (though not impossible) that you’ll capture a beautiful blend of microscopic creatures on your first try, so patience and persistence are key. You must let go of the need to be in total control, be willing to accept unknowable outcomes, and take things as they come.

Understand that embracing the indeterminate does not, however, mean shirking all responsibility. Even when you place your trust in the proclivities of anonymous microbes, you can still tilt the odds of success in your favor. Try to adopt a laissez-faire approach, yes, but don’t forget to measure, sanitize, and take good notes. You might never perfectly replicate the best wild ale you’ve ever made, but you don’t stand a chance if there’s no record of how you got there.


Most of the equipment you use to brew standard ales and lagers can be repurposed and reused for spontaneously fermented beer, but it’s advisable to maintain a separate set of plastic pieces for wild fermentations. Stainless steel and glass are easy enough to sanitize that they can safely perform double duty (if your sanitation practices are good), but plastic is prone to scratching: Donate old buckets and bottling wands to the wild side and buy some brand-new equipment for your clean fermentations.

A couple of special pieces you might want to consider include the coolship and the horny tank. A coolship (from the Dutch koelschip) is nothing more than a large metal pan. The pan’s wide, shallow shape promotes rapid cooling of hot wort by exposing a relatively large surface area to the air. It won’t, however, cool off nearly as quickly as is possible with modern heat exchangers and wort chillers, thus leaving open the critical window of 80–120°F (27–50°C) within which most wild bugs thrive.


Coolships have largely gone the way of gas lighting and other nineteenth-century technologies, but they remain an important part of the traditional lambic brewery, and it’s easy enough to fashion one at home. All you need is a large shallow pan. Commercial roasting pans offer the ideal overall shape and are available from restaurant supply stores and online. Finding one of sufficient capacity can be a challenge, but if you have the cash (or know someone in the restaurant business), roasting pans as large as 42 quarts (10.5 gallons/40 liters) are available.

I’ve found that a full-sized stainless steel steam-table pan does the trick for less than $20. These measure roughly 20-by-12-by-6 inches (51-by-30-by-15 cm) and hold 5.25 gallons (20 liters). Reducing your batch size from 5 gallons to 4.5 ensures that there’s enough room for Kräusen, should you wish to conduct an open fermentation directly in the coolship.

In theory, spontaneous fermentation is as simple as brewing up some wort, transferring it to your open coolship, and waiting. In practice, this method may or may not yield satisfactory results because the bugs you capture will change with the season and the weather. Professional lambic brewers have the advantage of decades-old crusts of yeast and bacteria lining their attic walls. Chances are you don’t. That’s where the horny tank comes in.

Apart from its evocative name, there’s nothing terribly special about a horny tank: It’s little more than a vessel that harbors a colony of microorganisms. In large craft breweries it might be a foeder. Smaller brewers might use an oak barrel. In either case, it’s usually a wooden container that hosts yeasts and bacteria. Wort is transferred to the horny tank after it’s spent a night in the coolship coming down to room temperature.


A homebrew-scale horny tank might simply take the form of a carboy to which you rack fresh wort after having transferred the previous batch. Purists might feel that this method isn’t very “spontaneous,” and that it’s nothing more than repitching, but I claim that it maintains the spirit of spontaneity for those of us who do not enjoy the luxury or HEPA requirements of filthy wooden rafters. Including a few chips or cubes of oak in your horny tank can enhance the wood character of your wild ales and help maintain your bug colony, to boot.


Whether you let nature take its course in an open coolship-cum-fermentor or transfer cool wort to a horny tank, the microorganisms at work in spontaneously fermented beer are numerous and diverse. In Wild Brews, author Jeff Sparrow observes that more than 200 unique microbial species have been identified in lambic. Despite such biological diversity, most of the bugs that do the heavy lifting fall into just a handful of broad categories of similar species that dominate the brew at different times. Sparrow offers the following rough timeline for lambic fermentation:

Enterobacter and Kloeckera are the first microorganisms to begin working on the wort, reaching peak activity within the first week. You might not notice much activity during this period.

Saccharomyces species of both the ale and lager variety establish themselves next, efficiently converting wort sugars into ethanol and carbon dioxide for a period of two weeks or more. As with normal ale fermentations, an observable Kräusen develops. If you conduct open fermentation in a coolship, falling Kräusen is a good indicator that it’s time to transfer the beer to a carboy for aging.


As Saccharomyces fermentation activity subsides, the lactic-acid bacteria Lactobacillus and Pediococcus get to work, converting sugars into ethanol, carbon dioxide, and lactic acid. The timeline for lactic-acid bacteria is on the order of three to four months.

Brettanomyces is the last major species to establish itself, taking eight months or more to contribute its character, which can range from fruity to horsey.

Keep in mind that these are not discrete steps but a blend whose dominant species varies with time and conditions.

Once the Lacto, Pedio, and Brett get going, your spontaneously fermented beer may or may not develop a pellicle—a rather sickly-looking floating layer of scum. A pellicle is no cause for alarm; in fact, it’s a sign that souring bugs are at work and protecting themselves from oxygen infiltration. Fuzzy black or green rafts, however, indicate mold, and should be skimmed off immediately to avoid ruining the batch beyond repair. Mold is typically only a potential issue in the very early stages of an open fermentation.


Other Considerations

Just about any wort you conjure up can become fodder for your foeder, barrel, or carboy. However, when you’re just starting out, it’s probably best to keep things simple. Traditional lambic is made from about 60 to 70 percent pale or Pilsner malt, with the remainder unmalted wheat. Unmalted wheat contributes plenty of complex starches in the wort that provide long-term sustenance for a wide variety of hungry microbes.

Lambic brewers historically have relied on large additions of aged hops to prevent spoilage with little to no bitterness (aging hops reduces their bittering potential). Suppliers such as Puterbaugh Farms (Hops Direct, LLC) will sell aged, debittered hops by the pound to homebrewers at very reasonable prices, or you can do it yourself if you have time. Just put some hops in a paper bag and wait two or three years.

I prefer to let someone else do the aging and purchase mine already aged. If you don’t have aged hops at the ready, then at least keep the IBUs low: In wild ales, acidity and funk provide the counterpoints to malt, not hops. A few scattered pellets of a low alpha acid hops such as Hallertau or Strisselspalt ought to do the trick. Remember, you’re after antiseptic compounds, not bitterness.

Uniquely Yours

Spontaneous fermentation gives you the opportunity to create something that is truly, uniquely yours, but as Peter Bouckaert of New Belgium Brewing Company famously observed, “If you want to be good, you’re going to have to dump some beer.” If making sour beer were easy, everyone would be doing it.

Wild ale is aptly named, not just for the untamed nature of its microflora, but because its a bit like the Wild West: The rules are few, and there’s a very good chance you’ll go bust when you stake your first claim. But if you keep at it and don’t get discouraged, it’s entirely possible that you’ll discover riches beyond your wildest dreams.

From Berliner Weisse to Gose and points in between, quick souring is rapidly becoming the time-constrained brewer’s choice for building pleasant tartness on a schedule. In CBB’s online course, Quick Souring Methods, Funkwerks Cofounder Gordon Schuck explains how to use _Lactobacillus _bacteria, experiment with sour mashing, test acidity levels, and more. Sign up today!