Cold as Ice(bock)

Eisbock, (Eis meaning “ice” in German), is a higher-alcohol version of the classic barley-based Bavarian strong lager, the bockbier (or bock for short), or alternately, a wheat-based ale. Eisbock has a unique place in the world of fermented beverages, because its foundation liquid is a “normal” beer that is made much stronger through a post-fermentation process called freeze-distillation. For comparison, the high alcohol content in Scotch, Bourbon, or Cognac, for instance, comes from heat-induced distillation. In such beverages as Port, Madeira, Marsala, or Sherry, it comes from fortifying the fully fermented wine with pure ethanol (also known as neutral or rectified spirit, with an ABV of up to 96.5%). In eisbock, by contrast, the high alcohol content comes from a reduction of the water content of the fermented beer. This subtraction of water involves dropping the beer’s temperature below the freezing point until a portion of the water crystallizes into icy slush. Once the unfrozen liquid is drained from the ice, it is eisbock, a beer with a higher alcohol concentration than the starting bock.

The Magic of Freeze-Distillation

Freeze-distillation works because water freezes at 32 °F (0 °C) while ethanol does so only at a low temperature of -173.2 °F (-114.1 °C). Thus, the amount of the ABV increase in eisbock depends on the extent to which the beer’s temperature drops below freezing. In commercial settings, the foundation bock is usually chilled to about 25 to 28 °F (-4 to -2 °C). At this temperature, about one-quarter of the beer’s water content freezes. Eisbocks made in Bavaria tend to have an ABV of between 9 and 12%, rarely more. When making an eisbock at home, you can calculate the approximate ABV of the finished eisbock by using the following equation:

V₁ x ABV₁ / V₂ = ABV₂ 

whereby:

V₁ = The volume of beer prior to freezing

ABV₁ = The alcohol by volume (in percent) of beer prior to freezing

V₂ =  The volume of drained beer after freezing

ABV₂ = The alcohol by volume of the finished beer after freezing.

To help illustrate this equation, let’s use an example: 

If a 5-gallon (19-L) batch of bock with 7% ABV yields 3.75 gallons (14.25 L) of finished beer after freezing, the ABV of this eisbock is: 

5 x 7% / 3.75 = 9.3% ABV

Vintners, by the way, use an analogous process to produce icewine. They freeze the watery component of grape juice prior to fermentation. In fact, the water is frozen inside the grapes by allowing the clusters to remain on the vine until after the first frost. They then harvest and crush the frozen grapes in the cold, usually at night, to produce a must with an exceptionally high content of fermentable sugars for a high-alcohol dessert wine. This is similar to how ice cider is made, where the apples or juice are partially frozen prior to fermentation to concentrate the sugars.

The Classic Bavarian Eisbock is a Maibock at Heart

In theory, you can subject any beer — ale or lager — of any beer culture, from a helles, the classic Bavarian summer quaffing lager (“hell” is German for light in color), to an English brown ale, to a pitch-black American imperial stout to the freezing process. In practice, however, in Bavaria, home of the eisbock, the base beer is traditionally a barley-based strong lager, often a Maibock (May bock), as it is in the recipe I’ve included at the end of this article.

The only notable exception to a barley-based bock as a starting beer for eisbock in Bavaria is the wheat-based ale weizenbock. The most famous example of this is probably the Aventinus Eisbock, made by the Schneider Weisse brewery in Kelheim, on the banks of the Danube, some 67 miles (108 km) northeast of Munich. Schneider makes its wheat eisbock only once a year, shortly before Christmas, as a freeze-distilled version of its hallowed Aventinus Weizenbock. This weizenbock already has a substantial ABV of 8.2%, which turns the eisbock made from it into a 12% ABV brew. 

Making a Maibock to Create an Eisbock

In Bavaria, the month of May is often still too cold to sit outside in the beer gardens, while it is already too bright and spring-like to hide away in the dark beer halls for a liter or two of strong liquid libation. When spring is in the air but old man winter is still fighting his rearguard battles, this is the right time for a transition beer, and out comes the Maibock. 

This beer is a real bock, that is, it is a typical Bavarian strong beer, but its appearance is golden-yellow to almost as blond as a Bavarian helles, which is why a Maibock is also called heller bock. The key difference between a bock and a Maibock is mostly in the composition of the grain bill, which, in the latter, is without the preponderance of dark caramel and Munich malts of the other bocks.

Traditionally, all Bavarian bocks were decocted, but nowadays many breweries make perfectly tasty bocks using a multi-step infusion mash. Dough in for a very thick mash at roughly 85 to 105 °F (about 30 to 40 °C) for a 30-minute hydration rest. Infuse the grain bed with hot brewing liquor to lower its viscosity and to raise its temperature over 2–3 hours to the mash-out temperature of 170 °F (77 °C), while allowing for two 30-minute rests during the ramp-up, one at 149 °F (65 °C), and another at 162 °F (72 °C). Recirculate thoroughly. Lauter slowly until the kettle content reaches the target original gravity, while taking evaporation losses into account.

For Extract Brewers

Because an eisbock can be made from any foundation bock, there seems to be no need to specify an extract-plus-steeped-grain recipe. Instead, brewers without the facility to mash a grain bill can simply use bock-style malt extract. There are a couple options available, including a hopped liquid extract from Muntons, but my preferred choice is Weyermann’s Bavarian Maibock extract. You could make an extract brew by using an 8.8-lb. (4-kg) jerrycan of the golden-brown, unhopped extract, which is produced from a two-step infusion mash of 80% Vienna malt, 10% Carafoam®, and 10% Carared®. It has a sugar (soluble solids) content of 72 to 78%. If diluted to a wort with an original gravity of 1.052 (13 °P), the can of extract produces a wort with a color value of 8 to 19.3 °L (20 to 50 EBC). 

From the start of the boil onward, proceed as described in the following paragraphs both for the all-grain and extract versions of the recipe.

The Boil

When brewing a bock you should boil the wort for 90 minutes. One of the key reasons for the long boil is the initiation of the so-called Maillard reaction (see sidebar below for more on this), which produces plenty of red-brown and aromatic, bready-, toffee-, sour-, and caramel-tasting melanoidins. These are wort compounds that form during an extended kettle boil at temperatures above 194 °F (90 °C), in complex reactions between amino acids derived from grain proteins, on the one hand, and reducing sugars derived from grain starches, on the other. Melanoidins are an essential element in classic bockbier flavors because they contribute to a deep maltiness in the finished beer; and the human detection threshold for the molecules produced by the Maillard reaction is very low. 

The sugars involved in melanoidin reactions include the two plentiful wort monosaccharides; glucose and fructose. Because amino acids and sugars are both products of enzymatic degradation in the malt house and in the brew house, the amount of melanoidins that can be produced in the kettle depends on the extent of protein modification and starch conversion of the malt prior to and during mashing. The Maillard reaction is also known as non-enzymatic browning, because it darkens the wort slightly.

Finishing the Wort

Add the bittering hop 15 minutes into the boil. Add the flavor hop 85 minutes into the boil. Add the aroma hop at the start of whirlpooling. For an authentic Bavarian bock, you can use any of the four traditional German hop landraces (Hallertauer Mittelfrüh, Hersbrucker, Spalter, and Tettnanger), and their modern genetic descendants. The selection for the recipe I have provided below  relies on the slightly grassy-citrusy Tettnanger for bittering and the classic noble-herbal Mittelfrüh for both flavor and aroma. 

Heat-exchange the wort to the selected yeast’s recommended fermentation temperature (usually about 50 °F/10 °C). Aerate well and pitch about twice as much yeast as you would for a “regular” brew. Select a lager strain with a high attenuation potential to keep the residual sweetness inbounds after the freeze-distillation has concentrated the brew. Among the yeast candidates suitable for Maibock are Wyeast 2124 (Bohemian Lager), Wyeast 2206 (Bavarian Lager), White Labs WLP830 (German Lager), White Labs WLP833 (German Bock), and Fermentis SafLager S-23. Ferment the Maibock for 3 weeks, then rack and lager it close to freezing for 3–5 weeks.

Turning a Finished Maibock into an Eisbock

For freezing, commercial breweries keep their well-lagered bocks in jacketed, cylindroconical tanks — a luxury rarely available to homebrewers, who generally have to rely instead on ambient cooling in a freezer to achieve the same effect. Therefore, rack the finished Maibock into a plastic or stainless steel fermentation bucket that has been purged with CO₂ to minimize oxygen pickup. Place the vessel into a freezer for freeze-distillation. If a single bucket does not fit into your freezer space, it may be necessary to split the batch into separate, smaller buckets. Always leave some headspace in the bucket(s) because freezing liquids expand. Also, avoid glass carboys, which might burst under expansion pressure. 

The bucket(s) should have a spigot at the bottom. This makes it easy to drain the bucket(s) because frozen slush rises to the top of the liquid. To “harvest” the eisbock, simply place the bucket(s) on a table, attach a sanitized tube to the spigot, and drain the liquid into a receiving vessel (such as a Cornelius keg) that has been purged with CO₂. Add CO₂ to the top of the discharging bucket(s), too, as the headspace increases during draining. This also reduces the chance of oxidation. For the same reason, the transfer tube should be long enough to reach the bottom of the receiving vessel to reduce splashing.

Unfortunately, there is no simple way to determine when enough ice has formed during freeze-distillation. At a constant freezer temperature, however, the freezing process should take no longer than two or three days. For maximum strength, it is better to leave the eisbock a bit longer than necessary rather than draining off the ice too soon. If you find that you drained it too early and left too little frozen water behind then you can always put the result back in the freezer for a second freeze-distillation. Or, if too much ice was left behind, you could allow more to thaw at room temperature and continue collecting it until you hit your target. Use the volume formula given earlier to calculate your finished beer’s alcohol content, however the best way to determine if you separated enough water from your eisbock is likely to do a taste test.

An Alternative for Brewers Living in the Snowbelt

If you happen to live in a part of the world with severe winters, you can also use a “natural” way of freezing your eisbock. Leave your plastic or stainless steel container of finished Maibock outside overnight in low freezing temperatures, or bury it for a few days (or longer if you wish) in a mound of snow. Retrieve the vessel on a particularly cold day (or night) and drain the eisbock as described earlier. It is best to do this outside in the cold to ensure that the icy slush does not melt before the bucket is fully drained.

Filtration

After you have racked the freeze-distilled eisbock, it is probably slightly turbid. Commercial eisbocks, therefore, are usually filtered. Homebrewers without a filter may wish to allow the finished beer to sediment for a few days and then rack it again.

Priming

After the elaborate freeze-distilling process, there may not be enough viable yeast cells left to metabolize the priming sugar, especially in a cold, high-alcohol environment. If you filtered the beer, there are no yeast cells left, of course. Therefore, to condition the finished beer via the priming method, add not only some priming sugar but also some fresh yeast to the brew. Alternatively, if you collect the finished beer in a Cornelius keg, you can force-carbonate it with CO₂ from a canister with up to 2.4 volumes (about 4.8 g/L) of CO₂ for an attractive, creamy head.

Sensory Comments

One of the main differences between freeze-distilling and heat-distilling as a means of raising the alcohol content of a fermented beverage is the amount of flavor elements that are transferred into the concentrate. During heat-distilling more flavor elements remain in the non-recoverable residues. Alternatively, during freezing more flavors are extracted into the concentrate. In eisbock, therefore, more malt aromas are preserved in the concentrated beer than, for instance, in a beer that is heat-distilled into whiskey. Also during freezing, some of the brew’s cold trub, which consists of several compounds including hop resins, tannins, and proteins, adheres to the ice crystals or sinks to the bottom of the container. This enhances the exceptionally soft and smooth, bready and caramel-like maltiness of an eisbock. The aromatic hoppiness of the original Maibock is still noticeable, but it is very subdued in the eisbock’s velvety finish. Visually, a Maibock-based eisbock is sparkling and brilliant with subtle ruby hues. 

If properly carbonated, the head is off-white to ivory and moderately stable. An eisbock is best served in an elegant Cognac snifter. This allows the beer’s almost Bourbon-like bouquet to shine. In the finish, an eisbock pleases with complex, fruity aspects of raisins, molasses, plums, dates, and, in the darker versions, even of mild chocolate.

And Eisbock Legend: Tale or Tall Tale? (sidebar)

Nobody knows exactly where, when, and how the first eisbock was made, but there is an amusing story about its origin, which is probably more fiction than fact. 

According to this tale, eisbock was “discovered” in Kulmbach around 1890. On an ice-cold winter evening, an apprentice brewer was apparently too tired to roll a bunch of casks of finished doppelbock that had been filled during the day, from the brewery yard into the cellar, as he had been instructed by his brewmaster. The fellow assumed that the casks would be fine if he left them outside for a night.

However, that turned out to be a colossal mistake, because the thermometer dipped that night so low that the doppelbock froze to ice and burst the casks’ staves. When the brew crew returned to work the following morning, it seemed that, because of the apprentice’s negligence, the entire lot of doppelbock was ruined. 

When the brewmaster inspected the disaster, he noticed a brownish liquid that had accumulated in the center of each cask. He obviously did not realize that this aqueous solution was mostly the beer’s alcohol and maltiness, which had migrated gradually to the center as the beer froze from the outside in. To mete out the most severe punishment he could think of, the brewmaster demanded that the young fellow drink the nasty brown puddles from inside each cask. The apprentice took his first sip with great trepidation, but then took one after the other with ever-increasing delight. Finally, he let his fellow brewers share in his “punishment.” Thus, the legend goes, the eisbock was invented.

How much of this tale is based on truth can’t be proven. We do know, however, that Kulmbach brewers, at the beginning of the 20th century, started to make it a practice of rolling a few casks of doppelbock out into the open, whenever the prospects for a starry, cold night looked good. They then collected the liquid content from the casks the following morning, which explains why an eisbock from Kulmbach is still sold under the brand name of G’frornes, which means “frozen stuff” in the local vernacular.

Melanoidin Formation: A Complex Chain Reaction (sidebar)

The Maillard reaction is named after its discoverer, the French chemist Louis-Camille Maillard, who first described it in 1912. The reaction is a complex, multi-stage, high-temperature chemical chain of events that occurs not only in sugary mashes and worts, but also in many foods, where it is responsible for the signature “browned” flavors and aromas of such things as seared steaks, bread crusts, grilled sausages, fried onions, and roasted coffee. 

At the start of the Maillard reaction, fractions of amino acids and sugars form a new, intermediate compound called the Amadori rearrangement. It becomes stable only after a molecular restructuring. It is named after the Italian organic chemist Mario Amadori who discovered it in 1925. Further reactions of the Amadori product with nitrogen atoms — called the Strecker degradation — finally form the brown and highly aromatic melanoidins as the end product of the Maillard reaction. This final degradation is named after the German chemist Adolph Strecker, who researched the subject as early as the 1850s.

The Maillard reaction comes into play when brewing bock as an extended 90-minute boil creates melanoidins that are an essential element in classic bockbier flavors because they contribute to a deep maltiness in the finished beer.

An Alternative Way to Eisbock (sidebar)

On a recent BYO bike, hike, and brewery trip through Germany led by Publisher Brad Ring and including 18 BYO readers, the group stopped in for beers at Giesinger Bräu of Munich and came across a truly unique approach to making eisbock. Giesinger Bräu is a rather new brewery considering that all of its competitors in the city are centuries old. The brewery is modeled after a modern brewpub with a full-menu restaurant section. Its beer portfolio includes about half a dozen all-year brews, as well as several seasonals.

One of the brewery’s regular brews is a classic, golden, malt-dominant Märzen with restrained notes of hops and a standard ABV of 5.7%. Because the brewery does not filter its beers, this Märzen is slightly yeast-turbid. What makes this beer interesting, however, is the way the brewery uses it to make a unique adaptation of an eisbock. Instead of freezing the beer in the fermenter, as is normal for a commercial eisbock, this brewery pours the finished beer into a special glass jug called an Essence of Beer made by Bukanter and freezes it upright for about a day until it is almost solid. 

The brewery then serves this special eisbock in its frozen state, upside down, over a small decanter, at the table! To enjoy the brew, the patron must wait for the concentrate of alcohol and dense malt essence to drip slowly into the pitcher, from where it can then be transferred in increments into a small eau-de-vie glass. The brewery claims that the almost liqueur-like eisbock “drippings” have a staggering ABV of 30%. But, obviously, as some of the frozen water melts at room temperature, the ABV of the collected beer eventually declines. Since the eisbock is consumed without any additional steps, the beer had little to no remaining carbonation, however with the deep malty flavors and high alcohol, it drinks more like an aged Port wine than a beer, and the carbonation was not missed.

Giesinger Bräu was founded in 2006 as a small garage operation. Since 2012, it operates a commercial brewhouse that is reportedly capable of producing about 12,000 hectoliters (slightly more than 10,000 US barrels) annually.

Homebrewers can replicate what Giesinger Bräu does by purchasing their own Essence of Beer, which makes for a fun center of conversation at a social gathering. To learn more about Bukanter and the Essence of Beer, visit https://www.bukanter.de/en/

Eisbock Recipe

(5 gallons/19 L, all-grain)
OG = 1.067  FG* = 1.014
IBU* = 28  SRM* = 10.6  ABV* = 7%

* The base recipe, and specifications listed above, are for my favorite Maibock recipe before freeze-distillation. Note that once freeze-distilled, these values will change, however to what effect is determined by how much ice you decide to leave behind.

Ingredients
12 lbs. (5.4 kg) German Pilsner malt
0.46 lb. (0.21 kg) melanoidin malt
0.4 lb. (0.18 kg) Carapils® malt 
0.4 lb. (0.18 kg) Weyermann Caramunich® I malt
0.4 lb. (0.18 kg) Weyermann Carahell® (10 °L)
0.4 lb. (0.18 kg) Weyermann Carared® (20 °L)
0.23 lb. (0.10 kg) Weyermann Caraaroma® (150 °L)
3.6 AAU Tettnanger hops (85 min.) (0.9 oz./26 g at 4% alpha acids)
3.8 AAU Hallertauer Mittelfrüh hops (5 min.) (0.9 oz./26 g at 4.25% alpha acids)
1.4 oz. (40 g) Hallertauer Mittelfrüh hops (0 min.) 
Wyeast 2124 (Bohemian Lager) or Wyeast 2206 (Bavarian Lager) or White Labs WLP830 (German Lager) or White Labs WLP833 (German Bock) or SafLager S-23 or Mangrove Jack’s M76 yeast
Lallemand CBC-1 yeast and 2⁄3 cup corn sugar (if priming)

Step by Step
Mash-in at 126 °F (52 °C); rest 15 minutes; raise temperature to 144 °F (62 °C); rest 25 minutes; raise temperature to 162 °F (72 °C); rest 20 minutes; raise temperature for mash-out to 172 °F (78 °C). Lauter as normal.

 Boil 90 minutes, adding hops as indicated. At the end of the boil add the aroma hops and whirlpool for 15 minutes. Cool to 55 °F (13 °C). After the beer reaches terminal gravity (should be approximately 1.014 after 7 to 10 days), reduce temperature by 4 °F (2 °C) per day to about 32 °F (0 °C). Package about four weeks after brew day.

Up to this point you have brewed a Maibock. To turn it into an eisbock, purge your fermentation bucket (ideally one with a spigot at the bottom) with CO₂ to minimize oxygen pickup and then freeze the beer. 

When enough of the beer has frozen to your liking (likely 2–3 days — the longer you freeze it the more concentrated the eisbock will be), drain the higher-concentrated beer into a receiving vessel (such as a Cornelius keg) that has been purged with CO₂. Add CO₂ to the top of the discharging bucket(s), too, as the headspace increases during draining. This also reduces the chance of oxidation. For the same reason, the transfer tube should be long enough to reach the bottom of the receiving vessel. Let the beer settle for a couple of days and then rack it again to help clarify (or filter the beer).

Rack to a keg and force carbonate, or add fresh CBC-1 yeast and priming sugar before bottling.

Eisbock

(5 gallons/19 L, extract only)
OG = 1.067  FG* = 1.014
IBU* = 28  SRM* = 10.6 ABV* = 7%

This extract recipe assumes you can find Weyermann Bavarian Maibock malt extract. If this extract is unavailable in your region of the world, you can steep all the specialty grains in the all-grain recipe and swap out the Pilsner malt with 8 lbs. (3.6 kg) Pilsen liquid malt extract.

Ingredients
8.8 lbs. (4 kg) Weyermann Bavarian Maibock malt extract
3.6 AAU Tettnanger hops (85 min.) (0.9 oz./26 g at 4% alpha acids)
3.8 AAU Hallertauer Mittelfrüh hops (5 min.) (0.9 oz./26 g at 4.25% alpha acids)
1.4 oz. (40 g) Hallertauer Mittelfrüh hops (0 min.) 
Wyeast 2124 (Bohemian Lager) or Wyeast 2206 (Bavarian Lager) or White Labs WLP830 (German Lager) or White Labs WLP833 (German Bock) or SafLager S-23 or Mangrove Jack’s M76 yeast
Lallemand CBC-1 yeast and 2⁄3 cup corn sugar (if priming)

Step by Step
Start heating 5 gallons (19 L) of water. As the water warms, remove from heat and stir in all the extract until fully dissolved. Return to heat and bring to a boil.

 Boil 60 minutes, adding hops as indicated. At the end of the boil add the aroma hops and whirlpool for 15 minutes. Cool to 55 °F (13 °C). After the beer reaches terminal gravity (should be approximately 1.014 after 7 to 10 days), reduce temperature by 4 °F (2 °C) per day to about 32 °F (0 °C). Package about four weeks after brew day.

Up to this point you have brewed a Maibock. To turn it into an eisbock, purge your fermentation bucket (ideally one with a spigot at the bottom) with CO₂ to minimize oxygen pickup and then freeze the beer. When enough of the beer has frozen to your liking (likely 2–3 days — the longer you freeze it the more concentrated the eisbock will be), drain the higher-concentrated beer into a receiving vessel (such as a Cornelius keg) that has been purged with CO₂. Add CO₂ to the top of the discharging bucket(s), too, as the headspace increases during draining. The transfer tube should be long enough to reach the bottom of the receiving vessel to reduce oxidation. Let the beer settle for a couple of days and then rack it again to help clarify (or filter the beer).

Rack to a keg and force carbonate, or add fresh CBC-1 yeast and priming sugar before bottling.

Written by Horst Dornbusch