Brewers Association

Style Spotlight: Bohemian Dunkel

Style Spotlight: Bohemian Dunkel

By Horst Dornbusch

When my friend Thomas Kraus-Weyermann and I both received invitations from Ivan Chramosil, brewmaster of the brewery and restaurant Pivovar a Restaurace U Fleků in the Czech capital of Prague, we did not hesitate to accept. After all, U Fleků—founded in 1499—is the world’s oldest brewpub. Who would not get excited at the prospect of brewing there?

On the appointed day, Tuesday, June 29, 2010, at four in the morning, Thomas and I found ourselves under the old clock in front of house number 11, Křemencova Street, waiting to be let in for the brewing experience of a lifetime. That day, we were going to help Chramosil, U Fleků’s brewmaster since 1971, make a Flekovský tmavý ležák, a dark lager, which is the only beer made at U Fleků nowadays. In North America, that beer style is generally known as Bohemian Dunkel. Here is how our day went.

Stepping Back into the 19th Century

Pub InteriorAt the start of the Flekovský tmavý ležák’s elaborate double-decoction regimen, we laid base in the mash-lauter tun with liquor at a temperature of roughly 70° C (158° F). U Fleků’s brew house is an iron-body, copper-domed, 70-HL (approx. 60-bbl), two-vessel system, made in the late 19th century. As was common then in central Europe, the kettle is used for boiling the mash and wort—but not for mashing-in, as is now standard practice in modern European two-vessel systems.

The U Fleků brew house was made by the Czech Škoda Works, a company founded as a heavy machinery manufacturer in 1859, which is now best known as an automobile manufacturer within the global Volkswagen Group. The Škoda system has remained virtually unchanged since it was first installed—except, as Ivan explained, for the modern, electric motor-driven agitators in the kettle and the lauter tun, and a gas burner under the direct-fired kettle. The burner was installed about three decades ago. While the lauter tun is uninsulated, the kettle is insulated by a mantle of concrete and a layer of tiles around its sides, though the tiles have largely come off above the brewer’s working platform. Both vessels are unjacketed, which means no steam-heating. This is why Ivan had pre-heated the old lauter tun with hot water overnight. This reduced the metal’s thermal absorption and ensured a sufficiently high and stable mash temperature during the first rest.

The Mash-In

Mash InBy 4:15 a.m., we mashed in at a water-to-grist ratio by weight of 3:1. To shorten the brew day for his guests, Ivan had pre-milled the grist the day before. Fifty percent of the grain bill consisted of a classic 19th-century, slightly under-modified, pale floor-malted barley that was grown and malted not far from the city of Prague. The barley strain was Bojos, one of only seven barley varieties allowed by the European Union to be used in authentic, appellation-of-origin Czech beers as a “Protected Geographical Indication.” The other varieties are Aksamit, Blanik, Calgary, Malz, Radegast, and Tolar.

Bojos pale floor malt has a color rating of roughly 1.8 °L (3.5 EBC) and contributes an earthy depth of malt flavors to the brew. The remaining grains were two specialty malts, Weyermann® Munich I (30 percent of the mash at approx. 6 °L; 14.5 EBC) and Weyermann® Caramunich® II (15 percent of the mash approx. 45.5 °L; II 120 EBC) for a reddish-coppery hue, extra mouthfeel, and a rich malt accent. For additional opacity and a mildly roasted aroma and flavor, Ivan used Weyermann® Carafa® II (5 percent of the mash at approx. 430 °L; 1,150 EBC). The Carafa®, however, was milled separately in the morning and kept aside during the mash-in. It was added to the mash after the second decoction. This is because Carafa® is in no need of conversion and thus would have added an extra and unnecessary load for the slurry pump.

The target color range of a good Bohemian Dunkel, according to Ivan, is about 38 to 45 °L (100 to 120 EBC). Ivan makes his beer slightly darker and more opaque in the summer, because his beer is unfiltered. This is a precaution, he said, against less experienced foreign beer tourists, who “usually do not drink unfiltered beers, and sometimes return my beer as defective, because they notice a little bit of unaccustomed turbidity.”

We gave the thick mash a thorough 30-minute hydration and acid rest at 38° C (100° F). At the end of that first rest, with the agitator running, we infused the mash with 11 HL (approx. 13 bbl) of water at 80° C (176° F) to thin it out and to raise its temperature to 52° C (126 °F) for a 30-minute protein rest.

The First Decoction

By 5:30 a.m., we started slurry-pumping the first decoction—about one-third of the main mash—from the lauter tun into the kettle. With the agitator running and the burner cranking, the decoction temperature was raised to 64° C (147° F) for a 10-minute beta-amylase rest. After a further temperature rise to the alpha-amylase range, the decoction was rested again for 10 minutes, at 72° C (162° F). This was followed by a 10-minute full decoction boil. The entire first decoction took about 40 minutes. Before pumping it back into the main mash, Ivan underlet the lauter tun, whose bottom is uninsulated, with hot water as a thermal cushion. As the decoction was mixed with the main mash, the latter’s temperature reached approx. 65° C (149° F). We kept it at that temperature for a 30-minute beta-amylase rest.

The Second Decoction

Now the decoction step was repeated, again with about a third of the mash, but starting from a higher temperature plateau. After the initial ramp-up to 72° C (162° F), there was a 10-minute alpha-amylase rest, followed by a second ramp-up for a 10-minute full decoction boil. The entire second decoction lasted barely 25 minutes. As the second decoction was being pumped back into the lauter tun, with the agitator running, the mash swirls of darker decoction were clearly visible. The decoction boil had resulted in a noticeable browning effect from the Maillard reaction. At this time, the Carafa® II was added as well, and the mash color darkened rapidly even further. Once thoroughly mixed, the main mash reached a temperature of 77° C (171° F). The result was a well converted, dark mash providing a strongly malty run-off.

Lautering, Boiling, Hopping

At the end of the first decoction, Ivan heated the hot liquor tank to 86° C (187° F) for lautering. After about 10 minutes of “Vorlauf” (literally: “pre-run” meaning recirculation), the rich dark wort was sent into the kettle. Once the grain bed was partially dry, it was sparged in the form of three successive, vigorous “Nachgüsse” (literally: “after-pours”), the traditional European way of flushing the sugars out in increments rather than in a continuous sparge, until the kettle was full. The boil lasted for two hours. Hop pellets—Saaz, of course—were added three times, in roughly equal amounts by weight: At 10 minutes into the boil, an amount calculated for an IBU value of 28; then at 20 minutes into the boil; and finally just before shut-down.

After a brief rest period of approximately 30 minutes for some initial trub sedimentation, the wort was cast with vigor into U Fleků’s old-fashioned, shallow copper coolship. In traditional brew houses, coolships used to perform several key functions: air-cooling the wort; evaporating any residual volatiles not already blown out of the brew stack during the boil; sedimenting the trub; and aerating the wort for a vigorous start of the yeast’s aerobic fermentation phase.

At U Fleků, the coolship is in a hall with louvers to the open air. The hall is adjacent to the brew house. As the wort spread out quickly over the entire surface of the shallow pan, the entire hall became engulfed in steam like an old laundry kitchen. There, the brew was allowed to cool off overnight. The following day, after Thomas and I were gone, Ivan drained the coolship into the primary fermenter with the pitched yeast.

The Cellar Process

Until about a year ago, U Fleků still used traditional, 19th-century open, wooden vats for primary fermentation. The brewery also has stainless-steel tanks, which it used only for lagering, however. Wooden fermenters, unfortunately, are notoriously difficult to maintain in prime microbiological condition. As a concession to modern standards of economic efficiency, therefore, U Fleků recently abandoned its wooden vats and now uses steel tanks throughout the entire cellaring process.

The yeast pitched at U Fleků is a typical, contemporary Czech lager strain, which Ivan purchases fresh from an adjacent large brewery on a regular basis. He does not cultivate his own strain because U Fleků does not have propagation equipment or a lab. He re-pitches his yeast no more than four times before starting with a fresh batch. In the 19th century, yeasts for Czech lagers used to be imported from Bavaria. Since then, however, these old strains are maintained locally. They have mutated by now and have taken on their own character. The optimum fermentation temperature for Czech-style yeasts is usually around 50 to 55° F (10 to 13° C). Fermentation should be completed in no more than 14 days. Lagering should take another 14 to 28 days, whereby a longer lagering period produces a mellower finished beer.

Incredibly, with the wort in the coolship, our brew day was over shortly after two o’clock in the afternoon. We had just enough time for a quick snack and a draft of Ivan’s Bohemian Dunkel tapped straight off the serving tank. Then Thomas and I headed back for the four-hour drive to Bamberg, the home of the Weyermann® Malting Company. The following morning, at 7:30, I boarded a train to the Frankfurt airport and from there I took a flight home to Boston.

Living History

Brewhouse TankI have been in many breweries in my life, but the brew house at U Fleků represented a special treat. It seemed as if time had stood still there for the past century. At the outbreak of World War I, the sturdy Škoda brew house was undoubtedly state of the art. It did its duty during the inter-war years, survived World War II, and then became trapped, for four long decades, behind the Iron Curtain when investments in new equipment, even repairs, were subject to the five-year plans of a top-down economy run by apparatchiks and bureaucrats. While many brew systems like U Fleků’s were being replaced in the West, they were being preserved in working order in the East. Inadvertently, therefore, the deep freeze under Communism salvaged a piece of living history from the 19th into the 21st century. Today, Prague is free again, and travel is easy, which makes a meeting such as ours between brewers from different traditions possible.

Bohemian Dunkel MealGlobally, the Bohemian Dunkel is perhaps a much underestimated beer style. It has great depth and complexity and has been cultivated at U Fleků in all its historic authenticity. U Fleků, no doubt, ranks among the world most important brew-historical sites along with, perhaps, the world’s oldest known modern brewery, a Roman excavation on the banks of the Danube, in Regensburg, dating from 178 AD; the world’s oldest continuously operating brewery at Weihenstephan, near Munich, which holds a brewing license from 1040; the world’s oldest monastery brewery at Weltenburg near Kelheim, also on the banks of the Danube, which was founded in 1050; the world’s quintessential rauchbier brewery, Schlenkerla in Bamberg, which was first mentioned in documents in 1405; the Hofbräuhaus in downtown Munich, founded in 1589, where the modern Bavarian dunkel, bock, and hefeweizen beer styles were developed; and the original Düsseldorf altbier brewpubs of Brauerei Schumacher and Zum Uerige, founded in 1835 and 1855, respectively.

To have been allowed to participate in the making of a batch of Bohemian Dunkel, under the expert guide of brewmaster Ivan Chramosil, inside the hallowed brew mecca that U Fleků represents, on a brew system almost twice as old as myself, was a true privilege and will remain one of the brewing highlights of my life! For both Thomas and me, U Fleků was definitely worth a pilgrimage!

Horst Dornbusch is a brewing industry consultant and a frequent contributor to The New Brewer and other American and European trade magazines. He can be reached at


Bohemian Dunkel in Brief

(Adapted for a modern brew system and modern ingredients.)

OG just from the mash: 1.048 (12 °P)
FG:                    approx. 1.012 (3 °P)
SRM:                 43.4 SRM (114 EBC)
IBU:                   28
ABV:                   4.8% (3.8% ABW)

For easy scaling, all quantities are calculated for 1 U.S. barrel (1.173 HL) net kettle volume produced by a brew system with a nominal extract efficiency rating of 75 percent. If your system is different, adjust all qualities up or down accordingly.

21.68 lbs. (9.83 kg) Weyermann® Floor-Malted Bohemian Pilsner Malt @ 1.8 °L (50%)
13.01 lbs. (5.96 kg) Weyermann® Munich I @ 6 °L (30%)
6.5 lbs. (2.95 kg) Weyermann® CaraMunich® II @ 45.5 °L (15%)
2.17 lbs. (0.98 kg) Weyermann® Carafa® II @ 45.5 °L (5%)
Total grain bill: 43.36 lbs (19.67 kg)

Hops (pellets)
3.9 oz (110 g) Saaz @ 4.5% AA (10 minutes into the boil)
3.9 oz (110 g) Saaz @ 4.5% AA (20 minutes into the boil)
4.3 oz (122 g) Saaz @ 4.5% AA (115 minutes into the boil)

American brewers have a wide range of Czech yeast choices for this brew. Suitable strains include Wyeast 2000 Budvar™; 2001 Urquell™; 2007 Pilsen™; 2124 Bohemian™; and 2278 Czech Pils™; as well as White Labs WLP800 Pilsner; and WLP802 Czech Budejovice.

Brew House Process
This brew ought to be multi-step-mashed, either by infusion in a jacketed mash-lauter tun or mash-kettle, or, more authentically, by double decoction. Suggested temperature and rest steps for the main mash and the decoctions (if used) are: 38 °C (100 °F), 30 minutes; 52 °C (126 °F), 30 minutes; 64 °C (147 °F), 20 minutes; 72 °C (162 °F), 20 minutes. Decoctions only: 10-minute boil. Mash-out at of 77 °C (171 °F). Lautering: approx. 90 minutes. Kettle boil: 120 minutes (not shorter than 90 minutes). Whirlpool: 30 minutes. Heat-exchange.

Primary fermentation at approx. 50 to 55° F (10 to 13° C) for 2 wks; lagering at approx. 32 °F (0 °C) for 2 to 4 wks. Filtration optional.

Photos courtesy of Horst Dornbusch

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Zach Yunker

Zach Yunker

General manager of Sockeye Grill and Brewery in Boise, Idaho. More