Keep in mind that all the flavor, aroma, IBUs, carbonic acid, and alcohol—including any fusels/hot alcohols—will be concentrated in the finished eisbock. High-quality ingredients and clean fermentation are key.
All estimates here are based on using half-barrel kegs. Much of our process for making Northbound Eisbock is based on SWAG—scientific wild-ass guesses—which is simply inferring variables we can’t know based on the science we do know.
We made the first batches of Eisbock 12 years ago by placing two kegs of flat Doppelbock—with at least 10 percent head space for ice expansion—outside, overnight, at –11°F (–24°C). That froze about half the beer in 12 hours. Nowadays, we can’t count on having enough sub-zero winter days, so we use a chest freezer at about 0°F (–18°C). This freezes about half of the beer in 30 to 36 hours.
Our Northbound Doppelbock is 8.5 percent ABV. The freezing point of this solution is roughly 26°F (–3°C). However, it’s not that simple: As the water freezes, the concentrated sugars and alcohol lower the freezing point. This is where SWAG comes in. We first assume that any ice formation is virtually all water, with only trace amounts of isomerized alpha acids, residual sugars, and alcohol.
We aim to freeze and remove half of the water from the doppelbock to produce the 16–17 percent ABV eisbock—but that’s an estimate. We haven’t tested its ABV.
Freezing the Beer
Start with flat doppelbock, or a flat version of whatever beer you’re freezing. Freezing will concentrate any dissolved CO2 in the finished beer, resulting in over-carbonation and difficulty racking.
Take out the keg and record the beer’s weight in pounds, minus the weight of an empty keg. You can estimate the beer’s volume in gallons by dividing its weight by eight. For example, if the beer’s weight is 32 pounds, you can estimate a volume of four gallons. (Meanwhile, one liter of beer weighs slightly more than one kilogram.)
The beer slowly freezes from the keg walls inward over 30 to 36 hours. To get an even freeze, use a rubber mallet and whack the outside of the keg wall every eight to 10 hours, to break the ice from the walls. That allows more of the concentrated beer to contact the sub-zero walls. Shake the keg to confirm that the ice has floated to the top—it should sound slushy.
These freezing times and volumes are approximate—though we aim to freeze half the water, we rarely hit exactly 50 percent. Sometimes we miss it, freezing off only 30 to 40 percent. In those cases, we place the frozen keg back in the freezer for another 12 hours. Shoot for over-freezing. If you overshoot those 30 to 36 hours, you’ll freeze more than 50 percent. In that case, give the ice 15 to 30 minutes to thaw. Rack and repeat until you hit 50 percent—or just keep your super-concentrated eisbock.
If you overshoot and freeze the doppelbock solid, don’t worry. Just draw a little at a time as it thaws. It won’t hurt the finished beer—in fact, that’s what we did with the North Eisbock that won 2024 GABF gold. All the alcohol should still be liquid trapped in ice, and it releases rather quickly.
Rack the Eisbock
After 30 to 36 hours, whack the keg walls with the mallet one last time. At this point, the bottom of the keg stem will likely be frozen, blocking the flow of finished beer—making it impossible to rack.
So, before racking, we put the keg in a tub filled with a couple inches of hot water for about five minutes. Also, we pour some hot water on the top of the keg, to ensure unobstructed CO2 flow. Connect the frozen keg to a clean, sanitized one via couplers and clear, vinyl hosing.
Determine the Volume
There’s really no way to know exactly how much of the beer has frozen inside the keg until you’ve siphoned off all the concentrated beer.
To estimate the volume, we use the difference in weight of the unfrozen doppelbock and the finished eisbock. Weigh the empty keg to get the tare weight. Place the frozen keg on the scale, including the coupler for transfer. Tare the scale and, very slowly, transfer to an empty keg until you start to draw bubbles.
For simplicity, we estimate that eight pounds of ice or liquid equals one gallon. Subtract the remaining weight of the frozen keg from the original beer weight. Divide that by eight to get the volume of water left behind. That will allow you to estimate the ABV and IBUs of the finished eisbock.
Calculating ABV and IBUs
Because the freezing point of ethanol is –173.5°F (–114°C), we can assume that all of the alcohol is liquid (minus trace amounts trapped in the ice).
So, if we start with 13 gallons (104 pounds) of dopplebock at 8.5 percent ABV, that’s 1.105 gallons of alcohol in solution (because 13 gallons x 0.085 = 1.105 gallons). If you freeze off half (6.5 gallons), the remaining eisbock volume is 6.5 gallons. To calculate your ABV, divide the volume of alcohol by the finished total volume (1.105 / 6.5 = 17 percent ABV).
The same goes for the IBUs. Again, we haven’t had it tested, but we assume the IBUs are concentrated in the finished beer in the same proportion. In this case, Northbound Doppelbock is 18 IBUs, so we estimate the Eisbock at 36 IBUs.
Remember that head space: One year, one of the brewers failed to leave that space before freezing the kegs. The kegs were destroyed, and Eisbock leaked everywhere.
