By using a simple spunding valve, you can naturally carbonate your lagers and ales without worrying about building dangerously high pressure.
In order to either speed up production, maintain a natural process, achieve a certain mouthfeel, follow their interpretation of Reinheitsgebot or simply spend less money buying carbon dioxide gas (CO2), some brewers opt to naturally carbonate their beer before packaging. This differs from America’s much more common “forced carbonation” method of forcing the proper amount of purchased or self-collected CO2 into the beer.
Because an unregulated natural carbonation requires a brewer to keep active vigil over her ferment, lest it develop head or fizz that’s too fervent or too lackadaisical, this potentially dangerous technique can leave too much to chance. As a way to control the amount of gas absorbed or released, some brewers turn to “pressurized” fermentation, otherwise known as spunding.
What is Spunding and How Does it Work?
Spunding directly translates from German to English as “bunging.” This method of natural carbonation involves carefully monitoring your present gravity and sealing off the tank after the aggressive initial stages of fermentation have finished. Once your wort ferments to near your targeted final gravity (too many variables keep us from recommending how near) and you’ve closed off all orifices, you set the spunding valve you’ve attached to your tank to your desired hold-pressure. (Though the pounds per square inch depend heavily on the style and amount of carbonation you’re after as well as the actual temperature of the beer in the tank, this number will almost always hover in the single digits so that you don’t risk exceeding the safe limitations of your tank.)
The valve’s attached gauge monitors PSI (or the display could read in metric pressure units such as “bar” or “kPa” units of measurement), and any extra gas emitted above the set level triggers the variable pressure relief valve to open automatically. Once the pressure falls back down to the desired setpoint, the valve closes.
Why Should I Practice Spunding?
“Carbonation achieved via the old, proven German practice of spunding is one approach to creating smallest-possible bubbles and creamy mouthfeel in beers,” says Jaime Jurado, Production VP at Ennoble Beverages and past president of the Master Brewers Association of the Americas. “I am mystified as to how this might be explained. There is no apparent technical reason why my perception of these finer carbonation bubbles compared to force-carbonation exists. But vaya con Dios!”
Though the effect is unproven, other words sometimes used to describe the bubbles and/or mouthfeel generated by naturally carbonated beer are “soft,” “round,” “fine,” “bright,” and “consistent.”
But one word you won’t likely hear associated with spunding is “pungent.”
Because pressure can suppress the expression of volatile compounds that form esters, pressurized fermentation can suppress unwanted esters from certain yeasts, leading some advocates to proclaim that the technique can create clean “lager-like ales” in less time and at higher temperatures than it would take to brew an actual lager.
“You can also cut out the end time it takes to add CO2,” says Ingrid Epoch, a brewer at New Jersey’s Eight & Sand Brewing.
Can I Ferment Both Ales and Lagers This Way?
Most armchair brewers associate spunding with lagers because the tradition started in Germany where brewing began long before you could buy CO2 containers, and though German brewers typically consider this a non-issue, some “purists” argue that Reinheitsgebot’s list of four approved ingredients does not include external gas.
Today, most American craft brewers who ferment under pressure do so with lagers. However, some do find success spunding ales. While fermenting an ale under closed conditions can create dangerously high pressure levels, a properly functioning spunding valve should stop these levels from rising to an unsafe point. Note: To reduce capex investment, brewers generally choose tanks that are rated for lower operating pressures, though they can purchase fermenters rated for higher internal pressures to support such robust natural carbonation.
How Do I Dry-Hop Wort in a Closed System?
How do you infuse hops into a fermenter when it’s sealed? Easy. Hook a hop-filled brink to the fermenter and set it to a PSI lower than the tank, say five PSI to the tank’s eight. This lets the brink suck in wort, to which you can add hops to create a hop slurry. Now, set the brink to 12 PSI so the slurry pushes back into the fermenter. Voila, dry-hopped beer.
How Do I Clean Up Off-Flavors?
Epoch shaves days off her brew by letting the yeast clean up diacetyl and other off-flavor-producing compounds during the last third of fermentation instead of waiting until it’s done to give the beer a diacetyl rest. To do so, she removes the cooling jacket and lets the temperature free-rise.
“After the yeast has aggressively reproduced there’s not enough food left to create a bunch of off-flavors,” she says. “So when you cut off that temperature control, as a last resort they’re eating those unwanted chemical compounds.”
What Precautions Should I Take When Spunding?
As detailed earlier, fermenting under pressure can prove dangerous if you don’t pay close enough attention to the gas build-up in your tank. As such you’ll want to take two important precautions to ensure your spunding valve works as designed.
- Don’t start spunding too early, as krausen can form (primarily in ales) and clog your valve.
- Some spunding valves measure in bar even though many American brewers calculate pressure in PSI. You can buy gauges that display both but if yours don’t, do your conversions carefully. With 1 PSI equaling .08689 bar, says Epoch, “The difference is staggering. You want to be under one bar, maximum.”
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About the Author
Tara Nurin is the beer and spirits contributor to Forbes, the drinks columnist for New Jersey Monthly, a co-host of the weekly What’s on Tap TV show, and a writer for publications like Food & Wine and Wine Enthusiast. The certified beer judge teaches a for-credit university beer class and leads beer seminars for institutions like the Smithsonian. The former broadcast news reporter has won two first place awards from the North American Guild of Beer Writers, founded NJ’s original beer education group for women and volunteers as the archivist for the Pink Boots Society for women in the beer industry. She’s currently writing a book about the history of women in beer for publication in spring 2021.