NOTE: In the 2015 BJCP Style Guidelines, Eisbock has been moved to Style 9B in Category 9 Strong European Beer which contains more strongly flavored and higher alcohol lagers from Germany and the Baltic region. Most are dark, but some pale versions are known.
Eisbocks gain their strength from being frozen near the end of conditioning. Because water freezes before alcohol, the almost frozen beer is drained off the ice crystals that form in the conditioning vessel. The beer will lose about 7% to 10% of its water content. In the resulting beer, the alcohol concentration increases to near 10% ABV, about twice as much as a typical German lager. Because they are still bockbiers, Eisbocks have all the characteristics of a typical strong bockbier. They are much maltier and smoother than even the Dopplebock biers.
In Germany, you will find this beer is exemplified by G’frorns, which are brewed in the northern Bavarian Kulmbacher AG, (the reputed brewery of origin of the Eisbock style). These beers are best when sipped in a brandy snifter like a Sherry, Port, or Madeira. Eisbocks can be made as a lager with barley, like the Reichelbräu G’frorns, or as an ale with wheat, called weizeneisbock. The weizeneisbock that is most often available in North America is Schneider Aventinus Weizeneisbock, which has a 12% ABV.
These beers have a beguiling history with many different stories about how the first Eisbock was made. One of the most popular is that a brewery worker was told to roll all the doppelbock barrels to another building before he finished his shift for the day. Being the lazy worker that he was, he simply rolled them outside, intending to finish in the morning. After a bitter cold night, all the barrels froze solid and burst open. Supposedly the brewmaster made the boy drink the thick dark liquid in the middle of the ice as punishment. And the beer style we now know as Eisbock was born, or not.
To brew a good eisbockbier, you can’t just brew a strong doppelbock and then concentrate it by freezing. The resulting beer would be way too sweet and cloying with super strong caramel and bready malt flavors. When you brew a doppelbock that is on the upper end of the style in alcohol content, the resulting beer may turn out to taste like sweet cleaning fluid. According to Jamil in Brewing Classic Styles 80 Winning Recipes Anyone Can Brew, you must reduce the caramel and Munich malts and use a yeast which attenuates well for the beer to come out balanced. It’s best to keep the water loss around 10% and never more than 25% to keep the alcohol smooth and not harsh or burning.
It is key to first brew a Doppelbock that is in balance with subtle flavors. Once the beer is freeze-distilled, any flaws will become abundantly evident. You will most likely need a long conditioning period for the alcohol and flavors to mellow. Follow these guidelines and you should be able to make a great Eisbock.
References: Information for this page was adapted from the 2008 BJCP Style Guidelines, the page on Bockbier from The German Beer Institute, The German Beer Portal for North America, and Brewing Classic Styles, 80 Winning Recipes Anyone Can Brew, by Jamil Zainasheff and John J. Palmer.