Making an eisbock is a relatively simple procedure, and according to the German Beer Institute web site it goes a little something like this:
"Because water freezes before alcohol, the chilled brew can be drained off the ice crystals that form in the tank. During this process, the beer loses about 7 to 10% of its water content. As a result, the alcohol concentration in the beer increases, usually to about 10% by volume, about twice as much as the 4.5 to 5.5% of a regular German lager."
I've thought about doing this for a long time now. The actual doing part didn't happen until recently because of an unfortunate blockage in my mind. Because I was focused on accomplishing the task with five gallons of beer, I just assumed that I would need to somehow get an entire corney keg in a freezer and somehow remove the ice from that container and then carbonate the remaining concentrate left in the keg for dispensing. I know it seems strange, but it never occurred to me until the other day, that I could simply dispense a small amount from the keg and do the process on a small scale. The following is the process I used to increase the alcohol percentage of my weizenbock by extracting the frozen water content by about 50%. First let me get out of the way the fact that an eisbock is traditionally made from a lager or bock beer but in my case I will be using a dark weizenbock (dunkel weizenbock?) that has an alcohol by volume of 7%. In my case this is ale. Again, from the German Beer Institute:
"Eisbock can be made as a barley-based lager, like the Reichelbräu G'frorns, or as a wheat-based ale, called Weizeneisbock. The Weizeneisbock most readily available in North America is the Schneider Aventinus Weizeneisbock, which is brewed and then frozen to a strength of 12% alcohol by volume."
Secondly, even though this is not a distillation but rather a concentration of alcohol, it is still prohibited by law. So if you're reading this, don't tell on me. Anyway, I drank the evidence.
The following is one way to make an eisbock and how I did it this time.
I began by dispensing two litres of beer from the keg into a large pitcher and placed this container in the freezer. It took several hours for the beer temperature to drop to the point of freezing but I kept a close eye on it so that I didn't freeze the surface area too deeply. Once I saw that freezing was beginning I started the process of removing the ice crystals from the surface of the container.
The other way to do this is pouring the unfrozen portion off and placing this back in the freezer but I choose my method because it seemed easier at the time.
I used a perforated spoon to scoop out the ice and then I held the ice over the pitcher in order for the unfrozen liquid to drain before completely removing the ice. I also saved the ice as the process continued and after it melted I drank this beer because it still contained some remnants of the beer flavor and alcohol.
I repeated these steps about a dozen times and this took many hours to complete. I would recommend starting early in the morning to finish in one day. I started too late in the day and ended up placing the beer in the fridge until the following day to complete the task.
Finally, after reducing the volume of beer by 50% I stopped. I ended up with a one litre soda bottle pictured here. I placed a carbonating cap on the bottle and hooked up a co2 line from my kegerator to carbonate.
The results were great. The beer was an amped up version of its old self with intense malt character but with a smooth rich dark, dried fruit flavor. I didn't get any alcohol presence in the nose or taste like Aventinus.
Unfortunately, because it was such a small amount of beer, I didn't take a sample for a hydrometer test and so I don't know the final alcohol content.
The up side to this experiment is that it is pretty easy to do and so I'll be taking a sample of my Belgian tripel which has an 8.75%abv. (tripel bock?) and reducing it in the same manner. I'll make sure I take a reading with that one and report back here with the results. Cheers!