In Flanders Fields, the Beer Flows

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Throughout most of the 20th century, a few regional styles of sour beers existed around the world, mostly as throwbacks to a different time. In the late 1990s, sour beers were almost unheard of in North America, except to a small number of brewers and craft beer aficionados. The Great American Beer Festival (GABF), renowned for having a category “for everything,” didn’t add a category for sour beers until 2002. Now, sour beers — both fresh takes on old styles and entirely new ones — are everywhere.

Sour beer production methods deviate from those of “regular” beers in a variety of ways. Obviously, specific steps are required to sour the beer. For a variety of reasons, however, the classic Belgian sour beers — the most famous being the lambics of the Senne River Valley and the sour beers of Flanders — have other production steps that aren’t found in other beers.

What Makes a Beer Sour?

Sour-tasting beers are made by allowing microorganisms to produce tart acids and sour the beer. The organisms most often involved with souring beer are bacteria in the genera Lactobacillus and Pediococcus. Bacteria in both of these genera can break down residual carbohydrates in beer that brewer’s yeast (Saccharomyces cerevisiae) can’t. These bacteria ferment the carbohydrates and release lactic acid, which has a distinctly tart flavor. These microorganisms can also operate in low-pH (less than 4.5), high-alcohol (over 0.5 percent ethanol) conditions. These conditions would inhibit most other microorganisms. Beer can be soured in the mash or the kettle, and some modern U.S. beers are made this way, but classic Belgian sour beers are soured after primary fermentation.

Basic Belgian Sour Beer Production

Brewers of Belgian sour beers produce their wort and ferment in a similar way to other beers. At one or more points in the process, however, the beer is inoculated with bacteria that’ll sour it. In most cases, the not-yet-soured beer is pumped to wooden barrels that’ve been previously used for conditioning sour beers. This inoculates the beer with the appropriate microorganisms. Brewers of sour beers are careful to assess the beer that comes from each barrel. Barrels that produced high-quality beer are used again. If the beer coming from a particular barrel is “off,” that barrel isn’t reused.

Photo by Bernt Rostad.
A guesze is a blended lambic that’s made by combining lambics aged in barrels for a number of years.

In lambic production, the wort is inoculated with a mix of wild, airborne microbes before it’s pumped to barrels. Some of these organisms, including enteric bacteria and the wild yeast Kloeckera apiculata, quickly begin feeding on glucose and briefly outnumber the brewer’s yeast. These organisms can leave behind a variety of flavors and aromas, not all of them pleasant. Two of the most off-putting aromas are described as smelling similar to fecal matter. However, these organisms can’t utilize maltose, nor stand the low-pH, high-alcohol environment that the brewer’s yeast will soon create, and so they quickly die. Shortly thereafter, the brewer’s yeast begins to multiply rapidly and dominates the mixed culture.

In other Flanders sour ales, fermentation begins when the worts are pumped to barrels, inoculating them with brewer’s yeast and souring microorganisms, but not the “wild” mix of airborne organisms found in lambics. The brewer’s yeast quickly begins to ferment, and the pH of the beer drops to around 4.5 in a day or so. After the brewer’s yeast has consumed all the carbohydrates it can take in, the bacteria begin transforming the beer into sour beer. The bacteria grow and ferment slowly, and the sour character from the lactic acid takes months to develop. The Pediococcus and Lactobacillus that work collectively in most sour fermentations are sometimes called “lactic acid bacteria” (LAB).

In most cases, the classic Belgian sour beers are aged for at least a year before packaging. After packaging, they may be aged for an additional year. Most finished sour beers have a pH in the 3.2 to 3.5 range, significantly lower than the pH values of 4.0 to 4.7 for most “regular” beers. In some cases, the bacteria used for souring are accompanied by other microorganisms. The most common of these are wild yeasts of the genus Brettanomyces. At low levels, the presence of Brettanomyces lends an earthy character to beer that’s sometimes described as “barnyard” or “horse blanket.” After the LAB have run their course, Brettanomyces continue to grow very slowly on the remaining carbohydrates in the beer.

Photo by Bernt Rostad

During the souring process, oxygen diffuses slowly into the porous wood barrels. In tiny amounts, oxygen can be beneficial to the souring, but in higher amounts, the barrel can become contaminated with Acetobacter or other aerobic yeasts. Acetobacter secretes acetic acid into beer and lends a vinegar-like quality, which is only tolerable in small quantities.

In general, sour beers that’ve been aged for more than one year are tarter and drier. Sour beers aged less than a year are less sour and may retain some residual sweetness. Blending the two can yield a beer that isn’t too sour, with a hint of sweetness for balance. With so many variables, consistency is hard to achieve in sour beers. As such, many sour beers are blended. In addition to achieving consistency, they’re blended to be as appealing as possible.

Sours from the Zenne

Lambics are pale sour beers made from malted barley and unmalted wheat. True lambics are spontaneously fermented after being inoculated by airborne microorganisms from the Zenne River Valley in Belgium, the area surrounding Brussels.

Lambic wort is made from a thick mash in which the liquid is drained from the mash tun and boiled, and then returned, several times. Lambic wort is boiled lightly for 5 to 6 hours with hops aged 2 to 3 years to achieve less than 10 IBUs (International Bitterness Units scale). The rationale behind using aged hops is that hops contain antimicrobial properties, and heavily hopped beers are difficult to sour. Lambic producers believe aged hops don’t inhibit the LAB, but may inhibit other less desirable types of bacteria. After the boil, the wort is pumped to a coolship — a wide, shallow vessel that allows the heat from the boiled wort to dissipate overnight.

As the beer cools, the microflora in the air settles over it, and microbial action commences. The beer is then pumped to barrels where it’s further inoculated by microorganisms. Some lambic brewers, responding to consumer preferences and inquiries by the EU into the wholesomeness of the product because of enteric bacterial action, have begun adding food-grade lactic acid to their worts during cooling. This drops the pH to 4.0 and suppresses the growth of enteric bacteria and Kloeckera, resulting in a “cleaner-tasting” lambic.

Gueuze, a blended lambic, is made by combining a mixture of different lambics, usually those aged one, two, and three years. Gueuzes are typically very sour, dry, and highly carbonated, although some lose carbonation with age. They have varying degrees of “funk,” with the best showing just a hint.

A young lambic may also serve as the base beer for a fruit lambic. Cherries may be added at a rate of 2 to 2½ pounds per gallon to a 1-year-old lambic to make a kriek. Raspberries may also be added, sometimes with a smaller amount of cherries and a hint of vanilla, to make a framboise. Other fruits, including peaches and black currants, may also be used to produce a pêche or cassis. After the fruit is added, the beer is left to condition for another year before packaging.

In the U.S., you can find lambics made by traditional lambic brewers, such as Drie Fonteinen, Boon, Hanssens, and — my favorite — Cantillon. I especially like its Lou Pepe Kriek (gueuze) and Rosé de Gambrinus (framboise). You can also find fruit lambics that are pasteurized and back-sweetened with sugar and fruit juices by Lindemans. Their Cuvée René is an authentic and tasty gueuze.

Flanders Sour Ales

The Flanders region of Belgium is home to both sour red and brown ales, the latter being called oud bruins. Flanders red ales are made from Vienna malts and some specialty malts, including Special B Malt for coloring, and some type of “Cara” malt (Caravienne, Caramunich, etc.) for the souring bacteria. They may contain up to 20 percent corn. They’re fermented in barrels and inoculated with a mix of brewer’s yeast and LAB. Brettanomyces is introduced from the microbiome of the barrel. The beer is fermented, allowed to sour, and then blended. The blend is usually a sour batch of aged beer and a younger, less sour batch of beer. In some cases, the beers may have been brewed differently with the intention of blending them. The bitterness is usually held to 10 IBUs or less.


Flanders brown ales are made from a mix of Pilsner malt and up to 20 percent Caravienne and Caramunich specialty malts, collectively. Up to 10 percent maize may also be used. The wort is inoculated with brewer’s yeast and all the souring bacteria it needs before primary fermentation. Brettanomyces isn’t part of the mix. Primary fermentation occurs in open fermenters until the brewer’s yeast is almost finished. Then, the beer is pumped to stainless steel fermenters. Oud bruins are slightly more bitter than the red ales, often reaching 25 IBUs. This is accomplished by blending a sour, lightly bitter batch with a less sour, bitterer batch.

In the U.S., you can find a few different types of Flanders red ales, such as Rodenbach Classic and Duchesse de Bourgogne by Brouwerij Verhaeghe. Rodenbach Classic is a 5.2 percent ABV beer that’s a blend of 25 percent aged beer and 75 percent young beer. Duchesse de Bourgogne is a blend of 8- and 18-month-old ales. (The proportions aren’t specified.) Petrus Oud Bruin and Goudenband, by Liefmans, are imported Flanders brown ales.

What’s Old Is New Again

Commercial yeast producers who cater to the homebrew market produce blends of microorganisms for lambic-style and Flanders-style ales. It takes patience, but you can make good Belgian-inspired sour beers with a little practice. To get started, see my recipe for a Flanders red ale on Page 31.

Although still overshadowed by IPAs, sour beers continue to gain popularity in the American craft beer market. The latest version of the GABF competition rules has several categories for sour beers. What’s old is new again, it seems.

Getty Images/Dr_Microbe
Brettanomyces is a genus of wild yeast used to sour beer.

Rhodan’s Back Flanders-Style Red Ale Recipe

Fermentation Time

Fermentation Type: Alcoholic
Primary Fermentation: 3 to 4 months
Secondary Fermentation: 6 to 9 months
Total Time: 13 to 16 months

Yield: 5 gallons (19 liters).


Malts (for an OG of 1.053 and 17 SRM)

  • 3 pounds (1.4 kilograms) Vienna malt
  • 8 ounces (0.23 kilogram) CaraMunich Type 2 malt (45 °L)
  • 5 ounces (0.14 kilogram) aromatic malt
  • 3 ounces (85 grams) Special B Malt
  • 5 pounds (2.3 kilograms) Pilsner liquid malt extract (fresh)
  • Hops (for 15 IBU total)
  • 4 AAU Mt. Hood hops (60 minutes)
  • (1 ounce, or 28 grams, of 5 percent alpha acids)

Yeast and Bacteria (for an FG of 1.006 or lower, and an ABV of around 5.4 percent)

  • 1 pack Danstar Nottingham dry yeast (11 grams)
  • Wyeast 3763 Roeselare Ale Blend or White Labs WLP655 Belgian Sour Mix 1 yeast (Don’t make a yeast starter.)


  • 1 ounce (28 grams) oak cubes (medium toast)


  1. Heat 5.5 quarts (5.2 liters) of water to 165 degrees Fahrenheit (74 degrees Celsius), and pour into a 2.0-gallon (7.6-liter) beverage cooler. Place crushed grains in a nylon steeping bag and slowly submerge grains in hot water. Stir the mixture thoroughly.
  2. Mash, starting at 154 degrees (68 C), for 45 minutes. Toward the end of the mash period, heat 1/2 gallon (1.9 liters) of water to a boil in your brew pot. In addition, heat 5.5 quarts (5.2 liters) of water to 180 degrees (82 C).
  3. When the mash is finished, recirculate 3.0 quarts (2.8 liters) of wort, and then run off all the wort. Add this wort to the boiling water in your brew pot.
  4. Add the 180-degree (82 C) water to the cooler until the liquid is at the previous level. Stir the grains, rest for 5 minutes, and then recirculate and run off the second wort.
  5. Bring the wort to a boil. Upon boiling, turn off the heat and stir in about a third of the malt extract. Resume heating and return the wort to a boil. Add Mt. Hood hops and boil for 60 minutes. With 15 minutes remaining in the boil, stir in the remaining malt extract.
  6. After the boil, cool the wort until the side of the brew pot is cool to the touch. Transfer wort to a food-grade plastic bucket fermenter, and add cool water to make 5.0 gallons (19 liters).
  7. Aerate the wort and pitch the dried yeast. Let sit at 70 degrees (21 C) overnight.
  8. The next morning, open the bucket and pitch the mixed liquid yeast culture. Let the beer condition, in the primary bucket, for 3 to 4 months at 70 to 80 degrees (21 to 27 C).
  9. After this period, rack the beer to a glass carboy. Add the oak cubes. Soak the oak cubes in beer for 2 weeks prior to adding them. Age them for another 6 to 9 months at 60 to 70 degrees (16 to 21 C), and then keg and carbonate the beer.
  10. Alternatively, add 6.0 ounces (170 grams) of corn sugar, and bottle it in heavy bottles.
  11. Let the bottles sit for at least 3 months, somewhere warm, for bottle conditioning.

Chris Colby is on the Fermentation advisory board, and is the author of two books, Home Brew Recipe Bible and Methods of Modern Homebrewing (Page Street Publishing), and over 100 articles on brewing beer, appearing in such publications as Brew Your Own and Zymurgy. He’s currently a contributing editor to Beer and Wine Journal. He lives with his wife and their cats in Bastrop, Texas. Find him on Twitter @ColbyBrew.

Inspiration for edible alchemy.