Depicted in the 16th-century paintings of Pieter Breughel the Elder and criticized by the 19th-century poet Charles Baudelaire, lambic beer is perhaps the oldest of the modern beer styles. Its funky, sour and wild taste is a result of spontaneous fermentation, a brewing method where the beer is exposed to naturally occurring yeast and bacteria in the open air.

“It works just like any fermentation,” Greg Engert, beer director for the Neighborhood Restaurant Group in Washington, D.C., said in an interview about lambic beer. “It's just one that happens wildly.” We asked Greg to write a more technical explanation of how lambic beer is produced.

Lambic defined

Beer, the alcoholic beverage based on the fermentation of grain sugars, has been part of civilization since its very inception. For 10,000 years or so, brewing and fermentation methods have evolved and transformed largely in step with advances in science and technology, increasingly embracing the clean, replicable character born of highly controlled single-yeast fermentations in closed stainless steel tanks.

Yet perhaps the oldest of the modern beer styles, lambic, has managed to just barely survive -- and now thrive -- in almost diametric opposition to the global sea change in brewing practices. Over the past 600 years, brewers in Brussels and the Pajottenland, the rural farmland situated to the southwest of the capital city, have continued to craft beers of astonishing complexity from comparatively crude methods and techniques.

Lambic remains a seasonally brewed beer made with a hefty amount of unmalted wheat, boiled for hours on end with no more than an average addition of well-aged hops, cooled overnight in large shallow pans, then left to spontaneously ferment with a mixed culture of naturally occurring yeasts and bacteria while maturing for months or even years in oak barrels of varying sizes and ages.

What results is anything but simple: alluring, near vinous aromas of citrus, earth and a touch of barnyard funk lead to a balanced interplay of tantalizing tartness, tannin and rustic grain on the palate.

Lambic brewing ingredients and methods

The lambic brewing process shows a fascinating confluence of geographic influence and dynamic brewing tactics, all in the name of making the wort -- the hopped and/or spiced grain sugar-laden water -- hospitable to only certain naturally occurring microorganisms. Only these specific “bugs” will ferment the wort into lambic. The following ingredient choices and technical brewing decisions empirically evolved to influence -- if not control -- the types of spontaneous fermentations that could occur, and when.

Raw wheat:

The traditional lambic grain bill is uniquely comprised of 40 percent raw wheat. This is unique to modern brewing since wheat, particularly in its raw state, mostly offers complex starches that standard brewing yeasts -- the ones employed in the creation of all manner of ales (Saccharomyces cerevisiae) and lagers (Saccharomyces pastorianus) -- are largely unable to ferment. What’s worse, wheat has always tended to be more expensive to obtain.

The loamy topsoil of Brussels and the surrounding Pajottenland has long made wheat an available brewing alternative. And, it turns out, one particularly suited to encouraging the final flavors of lambic.

The turbid mash:

Nineteenth-century Belgian taxes on the size of the mash tun encouraged lambic brewers to downsize and to cram as much grain -- including plenty of unmalted wheat -- into these vessels. To heat the dense, wheaty wort more efficiently under these circumstances, liquid would be drawn out of the tun, boiled, and then returned, further increasing the kind of starchy richness (the preponderance of dextrins) modern brewing almost unanimously eschews.

The long boil:

Following the mash, the general brewer’s boil hovers around 60-75 minutes. The lambic boil can last 4 hours, with each additional minute further combining sugars and amino acids into complex melanoids, the kind that can -- like the starches contributed by raw wheat and a turbid mash -- resist the fermentative desires of certain microorganisms.

Old hops:

Lambic brewers actually add a very respectable amount of continental European hops to their boil, but in an effort to exclude the bittering effect, they utilize at least a portion of 2- or 3-year-old hops. The older varietals have lost their bittering quality, but maintain the antibacterial properties needed to keep a host of unwanted “bugs” at bay.

The koelschip and the seasons:

Beginning in the mid-19th century, countless Belgian brewers left behind their koelschip in favor of the newfangled Baudelot cooler, which would evolve into the modern day heat exchanger. Rather than leaving just boiled wort in rudimentary shallow cooling pans overnight, brewers began quickly cooling the wort in preparation for fermentation. In so doing, they dramatically decreased the possibility for unwanted spontaneous inoculations of the wort -- inoculations that could even threaten one’s health in the warmer months.

Through the usage of these heat exchangers, brewers could produce cleaner beers more quickly and -- especially with the introduction of refrigeration soon thereafter -- could do so year-round.

The lambic producers could not forsake the koelschip however, since this exposure to air introduces the initial arbiters of spontaneous fermentation. The process has had to remain seasonal though, and today’s lambic brewing season stretches from the first frost through early spring. In an effort to cool the 200-degree Fahrenheit wort below 70 degrees Fahrenheit, the brewing days should not exceed 50 degrees Fahrenheit, and nights cannot stay any warmer than 40 degrees Fahrenheit, nor cooler than 20-25 degrees Fahrenheit. Should the wort get too cold, the possibility for spontaneous fermentation would also be nullified.

Oak barrel aging:

Once the wort has cooled overnight, the liquid is transferred to oak barrels of various sizes. Many held red wine long ago, others have housed spirits, still others beer (as with 3 Fonteinen’s appropriation of old oak tuns once used to lager, or mature, Pilsner Urquell).

Some of the seeds of spontaneous fermentation are planted in the koelschip, while a majority of the microorganisms responsible for the character of the lambic wait in the wood. These bacteria and yeasts can be found as deep as 8 millimeters into the staves, happily feeding on cellubiose, a compound formed from the toasting of the oak in crafting the barrel. Once the wort is introduced, the attention of the “bugs” is shifted.

The lambic will spend months to years in barrel, slowly fermenting and gaining mild flavors from the wood.

Brewing Photo: Allagash Brewing / Flickr

The stages of spontaneous fermentation:

Despite the restrictive influence of brewing ingredients and techniques, as well as the seasonally based, airborne terroir of the region, lambic still contains nearly 100 different types of microorganisms, with 16 notable yeasts and 8 bacteria among them. What follows is a broad outline of the major influences on the progressive character of authentic lambic, each determined and affected by the compounds inherent within the beer, as well as the conditions within which it was brewed.

1. Bacteria Phase No. 1 (2 weeks-1 month): During the first few weeks, Enterobacter consumes the wort’s simpler sugars to create a touch of lactic and acetic acid as well as some mildly off-putting vegetal, smoky aromas. Foam begins to gather around the bungs at the top of the barrels. After this phase, the bungs of the barrels are sealed (preventing the lambic from tasting like sour celery).

2. Yeast Phase No. 1: Primary Fermentation (3-4 months): With the bacteria’s creation of acid, the wort’s PH lowers to a point where the first fermentation of yeast can begin. Ale yeast varietals (Saccharomyces cerevisiae) begin the fermentation, followed by lager yeasts (Saccharomyces pastorianus) after a few months. (This fermentation with standard yeasts would take 3-4 days in a standard brew.) The majority of the alcohol (ethanol) in the lambic is created during this fermentation, killing the Enterobacter, while a host of mildly fruity esters are also formed through this fermentation.

3. Bacteria Phase No. 2 (4-6 months): With most of the simpler sugars converted, the standard yeasts cease to ferment. The next bacteria arrives to consume the more complex sugars of the wort. Pediococcus is prevalent here, creating more lactic acid and a more acidic beer.

4. Yeast Phase No. 2, Secondary Fermentation (from months 6-8 on): Perhaps most pertinent to the character of lambic, the fourth phase is when Brettanomyces begins its work. This genus of yeast produces the wild, funky flavors we associate with lambic, the aromas and nuances that set the beverage apart as singular. Brett, as it has come to be colloquially known, can also slowly chew away at the complex dextrins and melanoids, drying out the brew and increasing its complexity of character. Though Brett can produce acidity, it tends to register below the threshold for taste recognition. As with Pediococcus, these Brett strains and the flavors they produce are specifically permitted by the ingredient and technical decisions that have long informed the style of traditional lambic.

Much work has been recently done on the genetics of Brettanomyces, and there are now five species agreed upon, two of which are pertinent to lambic. Of the others, Custersianus and Nanus offer restrained but pleasantly perfumed, peppery and fruity notes, while Naardenensis is just strange, generating sweaty, mousy, off-putting aromatics.

The two major species for lambic fermentation are:

  • Brettanomyces bruxellensis: Strains within this species can offer the classic horse blanket and barnyard funk associated with Brett, but also nuances of grass, leather, pie cherry and citrus. Once considered its own species, Brettanomyces lambicus is actually just a specific strain of Brett Brux, one that can tend to be supremely funky and earthy.

  • Brettanomyces anomalus: Strains within this species tend to be less funky than those of the Brux variety; bold pineapple and ripe fruit flavors dominate here. Also, once considered its own species, Brettanomyces claussenii is actually a strain within this species as well.

Speaking of Brettanomyces claussenii, the strain was named for Danish scientist N. Hjelte Claussen, the lab director for the New Carlsberg Brewery who in 1903 discovered the yeast while investigating the causes of spoilage in English beer. Pulled from oak barrels previously used to transport British Stock Ale, the yeast’s genus was immediately deemed British Fungus (Brettano-myces) by Claussen, but only later would the species be tied to the man himself.

Lambic styles

Old school:

Prior to the mid-19th century, all lambic was sold as meerts, faro or lambic. None would have been bottled (bottles only having arrived on the scene for the masses with the Industrial Revolution), nor from draft in the most modern American sense (that format did not arrive until the 1960s). Back then, these styles would have been largely served flat from the oak barrels in which they aged, or created in ceramic carafes and mugs.

Lambic: Served unblended from oak cask, this would be as funky and tart as the age of the brew allowed, but certainly flat. This is the beverage consumed by the peasants depicted in the 16th-century paintings of Pieter Breughel the Elder.

Today, straight lambic can be found on draft, on cask, and from bag-in-box formats in Belgium. They may range in age, in complexity and in vibrancy from jonge lambic (6-9 months old or less) to oude lambic (6-9 months or older, and can reach as old as 3 years).

Meerts/mars: A lower strength lambic produced from the second or third runnings of lambic grain. Each successive “running” or water addition and subsequent mash to already utilized grain would necessarily extract less sugar for spontaneous fermentations in the oak barrels, resulting in lower alcohol contents. These styles of lambics were cheaper and often spent a very short time in barrel. Referred to as table beers, meerts/mars were often consumed by children, women and the elderly, among others.

Faro: A sweetened lambic drink, faro was created tableside through the blending of meerts/mars with lambic, followed by an addition of brown sugar into the mug/glass (or by simply adding brown sugar to a glass/mug of meerts/mars).  Either way, the lumps of sugar would be ground into the drinking vessel using a stoemper to ensure that the sweetness dissolved into the beer faster. Consistently consumed in Brussels and the Pajottenland, faro became a dubious style, one increasingly bereft of actual spontaneously fermented lambic. Baudelaire was famously quite critical of the offering.

New school:

Gueuze/gueze: The mid-19th century brought a bevy of imported beer to Brussels, including styles that showed a very modern carbonation when served from bottle (pilsner) and cask (British bitter). These new brews made lambic seem antiquated, and the response was the lambic en bouteille, a blend of lambics of different ages, typically 1- (60 percent), 2- (30 percent) and 3-year-old (10 percent) lambic mixed and stored in, then served from, used wine bottles -- the presence of the still fermenting jonge lambic guaranteeing a bottle refermentation over a 6-month period.

This evolved into the wildly popular gueuze/geuze at the end of the 19th century, a beverage interestingly prepared by café owners, who would each also be known as a guezestekerij, or gueuze blender. Unfortunately, this noble beverage was largely bastardized into the capsulekensgeuze of the post-WWII sweet-tooth era (a blend of some lambic with non-spontaneous ale that was then filtered, pasteurized, sweetened and artificially carbonated). Today, the label presence of oude and/or vieille is helpful in determining if the offering is the former, an authentic, traditional and dry example, or whether it is sadly the latter (typically labeled as mere gueuze/geuze in this case).

Kriek: Initially a homespun concoction most likely crafted by the cherry farmers of the region, kriek lambic, or cherry lambic in Flemish, gained popularity and was soon being prepared by the same aforementioned gueuze-blending café owners. Two-year-old lambic was steeped in cherries for a few months then bottled exceedingly dry.

Sales were strong prior to WWI, but then a similar fate befell kriek as had gueuze after the second world war. The traditional and locally grown Schaarbeek cherries were disappearing as farms turned to suburbs, and mechanized harvesting proved incapable of cultivating the small heirloom fruit that remained. By the late 20th century, sweetened kriek was dominant and followed by a mélange of sweet fruit lambic styles (framboise (raspberry), but also apricot, apple, banana, strawberry, etc.).

As with gueuze, the terms oude and/or vieille -- found on bottle labels -- can be used to help distinguish between the real stuff and the rest. However, since even the most authentic of producers are increasingly experimenting with new fruits, new styles and new naming approaches, and since some of the more commercial producers have been known to employ the terms of authenticity without merit, simply selecting based on label terminology may prove ineffective; more effective may be to seek out only the lambics produced by a handful of the most highly, and rightfully, regarded lambic brewers and blenders.

Lambic producers

In the early 20th century, there were nearly 3,400 breweries in Belgium, a country the size of Maryland. (As a comparison, we have roughly the same number of breweries in the entire U.S. today, up from 70 or so in the late 1970s, and the previous high of 2,800 or so in the late 19th century.)

Of these, likely 500 or so were involved with the brewing and/or blending of lambic in and around Brussels. By 1993, the long impact of changing tastes as well as succession issues and the ever-increasing drive to consolidate had left 12.

But traditional lambic has experienced a renaissance in production and appreciation, one that sees the most authentic brewers and blenders thrive more and more each day, while the larger, commercial, and sometimes even multinational concerns dabble with the traditional lambics of old. The following is a list of the lambic producers -- the brewers and blenders -- of Belgium, with an asterisk denoting the finest, independent and most classically attuned.

Lambic brewers:

  • Belle-Vue (AB In-Bev)

  • Boon (Palm)

  • 3 Fonteinen*

  • Cantillon*

  • De Troch

  • Girardin*

  • Lindemans

  • Mort Subite (Alken-Maes/Heineken)

  • Timmermans (John Martin)

Lambic blenders:

  • De Cam*

  • Hanssens*

  • Oud Beersel

  • Tilquin*

Coolship brewers in the U.S.:

My continued obsession with all things spontaneous made Bluejacket’s coolship and adjoining sour barrel room a requisite when we designed the brewery for Navy Yard in Washington, D.C. The lambic producers of Belgium were not alone in inspiring our plans, but were joined by the bourgeoning cadre of Amercian coolship brewers equally consumed with the craft. Here is a list of our friends in spontaneous brewing, a list likely growing by the day.

  • Allagash (Portland, Maine)

  • Bluejacket (Washington, D.C.)

  • Crooked Stave (Denver)

  • De Garde (Tillamook, Oregon)

  • Hill Farmstead (Greensboro Bend, Vermont)

  • Jester King  (Austin, Texas)

  • Logsdon Farmhouse Ales (Hood River, Oregon)

  • New Glarus (New Glarus, Wisconsin)

  • Russian River (Santa Rosa, California)

  • Side Project (St. Louis)