It’s amazing how often the good things in life are the results of happy accidents.
Seeing a particularly nice sunrise over the mountains while on a long road trip… Running into a friend you haven’t seen in a long time because you happened to stop for lunch at the same place… Adding the wrong spice to a dish but voilà, it’s the new secret ingredient for a winning recipe…
And, of course, accidentally leaving a barrel of lager out in the freezing cold, thus creating a new brewing tradition… or so the story goes.
We’re not going to fret here about whether eisbock’s foundational myth has any basis in historical fact. Instead, let’s consider what may have been a happy accident as a technique worth deploying and honing—that is, fractional freezing as a way to concentrate flavors (and alcohol). This method isn’t without its risks—and please, dear readers, check on the legality in your own jurisdictions—but in the best cases, the results speak for themselves. A great eisbock is fun, delicious, and impressive.
So, let’s dig into how to do it and the kinds of beers the method can produce—and let’s think more about how we might use freezing to our advantage in the brewery, guided by folks who have produced some of the best eisbocks in the world.
Might you end up with an exploded keg or an experiment gone awry? Sure. Happy accidents may court unhappy accidents, but with proper know-how we can avoid the worst of the latter to enjoy the best of the former.
The Method, Explained
Fractional freezing is fundamentally pretty simple: Beer is mostly made of water, so when we lower the temperature of a keg or barrel—do not try this in glass—to below 32°F (0°C), that water begins to freeze.
As ice forms, the liquid it leaves behind is a less watery version of the beer—so, the more we freeze, the more concentrated that beer gets. This creates an opportunity to enhance, evolve, and accentuate the flavors and textures of that beer—not to mention the alcohol. See? Simple.
However, to get a better idea of what the result will be, we’ll need to employ what the technical experts refer to as a SWAG—that is, a scientific wild-ass guess. Or so I’m told by Jamie Robinson, president and brewer at Northbound Smokehouse and Brewpub in Minneapolis. Northbound Eisbock, incidentally, won the gold medal at the 2024 Great American Beer Festival.
The rudiment is correct: Make it cold and wait. However, calculating what temperature and how much time you need to get how much beer at whatever strength—that’s not linear.
“As the water freezes, the concentrated sugars and alcohol lower the freezing point,” Robinson says. “This is where SWAG comes in.”
Northbound has put in the time to improve its wild-ass guesses. Robinson and his team made the first batches of Eisbock 12 years ago by leaving two kegs of flat doppelbock—that “flat” part is important, stay tuned—out overnight in the Minnesota winter, at –11°F (–24°C). That froze about half the beer in 12 hours. Today, they use a chest freezer at about 0°F (–18°C), which takes 30 to 36 hours to freeze about half the beer.
Iterate Your Eisbock: Try It, Then Adjust
Because there are many variables at work here, this is very much a “try then trust” situation.
Case in point: The first time I brewed an eisbock, I set an alarm clock every couple of hours for what ended up being a very long night—I had no idea just how long it would take. And you definitely want that beer to be flat: There will be natural expansion when ice crystals form, and you’ll exaggerate that expansion effect—and potentially damage or blow out your keg—by freezing a carbonated beer. It can be done, but it’s safer to start with a flat beer.
So, you want to approach fractional freezing systematically and intentionally. We also should think about just how the freezing is happening.
“The beer slowly freezes from the keg walls inward,” Robinson says. “To get an even freeze, every few hours you should break the ice from the walls and let it float to the top, allowing more of the concentrated beer to have contact with the sub-zero walls. We do this by hitting the outside keg wall with a rubber mallet. Then, we shake the keg to confirm that the ice has floated to the top. It should sound slushy.”
There’s an added benefit to this method: You get a somewhat better idea of just how far your freezing has gone. The shaking should give you a sense of how much has frozen on the walls of your keg. Once you think it feels just about right—again, SWAG—it’s time to separate the beer from the ice.
“Prior to racking,” Robinson says, “we place the keg in a tub filled with a couple inches of hot water for five minutes. [We] pour some hot water on the top of the keg, too, to ensure unobstructed CO2 flow.”
If that sounds counterintuitive, don’t worry: You won’t melt much of the ice, but you will make it much easier to transfer the beer.
So, how do you know what to expect in terms of volume? Well… you don’t. Not exactly.
“There is really no way to know exactly how much of beer has frozen inside of the keg until you have siphoned off all the concentrated beer,” Robinson says. “To estimate the volume, we use the difference in weight of the unfrozen doppelbock and the finished eisbock.”
In other words, weigh equal volumes of the doppelbock (before freezing) and the eisbock (afterward), and compare. That should give you a decent sense of just how much you’ve concentrated your beer.
Eisbock and Eis-Not-Bocks
As the name indicates, the most common application for this method is to turn a bock—usually but not necessarily a doppelbock—into an eisbock, which BJCP guidelines describe as “a strong, full-bodied, rich, and malty dark German lager, often with a viscous quality and strong flavors. Even though flavors are concentrated, the alcohol should be smooth and warming, not burning.”
There may be a reason that bocks get this treatment: Without much in the way of hop bitterness or roasted notes, bocks hold up well to concentration without creating a beer that’s far out of balance.
Florian Kuplent, brewmaster at Urban Chestnut in St. Louis and in Wolnzach, Germany—where they occasionally brew the excellent Frigus Eisbock—explains: “Traditionally, it’s a well-crafted doppelbock recipe—decent attenuation, limited number of specialty malts, no roasted malt.” Why no roast? “To me, the risk to get a burnt note is too high.”
It’s a base that lends itself to intensification because it’s already a clean, simple flavor profile with limited IBUs and a dryish (but malty) finish.
That’s not to say that other styles can’t benefit from the technique. At Smog City Brewing in Los Angeles County, owner-brewer Jonathan Porter says he’s experimented with “icing” a wide range of recipes.
“I ended up doing small fractional freezing tests on just about every style of beer that came out of the pub,” Porter says. “The craziest one was probably a bourbon barrel–aged rye bock. … I served it at an anniversary party, and it had so much beta-glucan from the rye in it that you could push the surface of the beer with your finger, and it would bend before your finger went into it. It was as viscous as half-cooled Jell-O.”
Sebastian Sauer, founder of the Freigeist Bierkultur brand based in Stolberg, Germany, is another brewer who’s tried it with a variety of other beers. “Over the years, we produced a range of different styles of eisbock,” he says, “with the base styles varying from doppelbock, barleywine, adambier, mango gose, imperial stout.…”
The results tend to be impressive. Even ciders and perries can get into the game—though it may be useful to be push those with nitrogen instead of CO2, to avoid making a sparkling product.
Given that concentration of flavors, however, fractional freezing isn’t going to improve every style of beer.
“Blonde ale and lower-ABV lagers would probably taste better if you just brewed them stronger,” says Porter at Smog City. “Higher-ABV and maltier styles turned out the best because the process accentuated those characteristics. There’s a reason eisbock is so tasty.”
Advanced Design
These are challenging beers to make, so it’s important to keep a few things in mind as we prepare, begin, and proceed.
First, if there’s one thing to consider when deciding on a recipe or base style for fractional freezing, it’s this: It all concentrates.
“It is important to keep in mind that all of the flavor—desirable or not—aroma, IBUs, carbonic acid, and alcohol—including any fusel/hot alcohols—will be concentrated in the finished eisbock,” says Robinson, “so high-quality ingredients and clean fermentation are key.”
That’s also valuable to remember if you suddenly notice an off-flavor not usually present in the base beer: Concentration can boost a range of flavors up into “detectable” range, where otherwise they would pass under our sensory radar.
It’s also a good idea to stick with the stronger base styles.
“You want to make sure that the base beer is already pretty strong, as otherwise the water content would be too high,” Sauer says. At the same time, you don’t want to start with a beer that’s overly sweet. “You need to keep in mind that all flavors will be very much intensified, so you want to make sure to keep a good balance, as the sweetness will be very present no matter how dry the beer is.”
Next, don’t worry too much about “over-freezing.” If you overshoot the mark, you can always thaw it a bit, and the water will simply go back into solution with no real side effects.
Finally, how do you measure what you have? This is a matter of estimation, but as a rule of thumb, we can safely assume that nearly all of what freezes (and is thus removed) is just good ol’ H2O because the freezing point of ethanol is about –173°F (–114°C). Practically, that means virtually all the ethanol and isomerized alpha acids that were in the un-concentrated beer are still there in the concentrated one.
Thus, you can simply divide the relevant statistic—such as ABV or IBUs—by the percentage of volume remaining to get a decent ballpark measure. For example: If we start with 20 gallons (76 liters) of doppelbock at 8 percent ABV and 18 IBUs, and we concentrate it by about a third to 13 gallons/49 liters (a 0.65 factor), we now have an eisbock of about 12.3 percent ABV and 28 IBUs.
Do a little pre-math to work out your targets, and that will inform both your base recipe and your final goal for concentration and volume.
Last thing: I’m not kidding about the expansion. Leave yourself at least 10 percent of head space—and I usually go further, to 30 or 40 percent. If you think bottle bombs are fun, wait until you explode your first corny keg.
Eis Is Nice
We have one clear advantage over that first mythical brewer who “accidentally” froze his bock: We know we’re going to be fractional freezing.
Choosing a recipe, managing our method, and assessing what we’ve produced are all components that we can “see” from the beginning of the process, and that greatly improves the odds of a positive result.
Whether it’s a traditional eisbock or something more avant-garde, this is a method that has great potential to produce creative and interesting beers that push the bounds of your brewing portfolio. And it’s one more reason to look forward to the chilly winter months.
