Spunding 101

Foam has long captivated lagerphiles with its rocky meringues and the promise of hop and malt bouquets caught in its colloidal web. It sets beer apart from other drinks and, when created and presented properly, rewards the drinker in a way a headless beer cannot. For others less zealous about a beer’s fizz, it’s often been overlooked, but it deserves more consideration in process and recipe formulation. Let’s consider an older approach to natural carbonation: spunding. It saves on CO₂ costs and could shorten brite tank residency, but it also can improve the aroma, appearance, and texture of your beer. Aggressive force carbonation, on the other hand, can strip foam precursors and lead to a carbonic bite.

Spunding: Bubbles the Old-Fashioned Way

Spunding is a very straightforward concept. Bung your tank toward the end of fermentation to trap some of the natural CO₂ your yeast has produced and force it back into solution to create natural carbonation. It’s a method that’s been around at least since the start of the Reinheitsgebot, which forbade German brewers from adding things other than water, barley, and hops to create beer, and came about well before force carbonation was a thing. 

Initially it was as simple as sealing a cask and letting the yeast finish their job, but this method lacked precision. More modern spunding valves (spundapparat, German for ​“bunging apparatus”) refine the process. They’re adjustable pressure relief valves that can be dialed in to a specific pressure, and allow excess pressure to be released gently.

The spunding practice became less common compared to force carbonation, but it’s worth revisiting. Slower, natural carbonation avoids the larger, coarser bubbles that can be created by injecting CO₂. Some brewers will argue that it may be possible to create equal or superior foam through force carbonation, but spunding could be an easier (and cost effective) path if you pay attention to your beer. 

Why Spund?

Spunding is about being gentle on your beer. Forced carbonation can increase carbonic bite, and agitation during the fermentation process through transfers and forced carbonation can lead to foaming in the tank. The thing about beer foam is once it’s created, it’s not coming back. Many of the various foam positive elements that create beer foam are one and done, so if it’s foaming in your cellar, it won’t be foaming in your glass. 

Capturing natural carbonation removes a potential opportunity to rough up your beer, but it’s not a cure all. You still need to transfer slowly with counter pressure to prevent agitation. It’s also important to lower the temperature of your brite tank to be closer to the temperature of your beer. Spunding may also lower your residency time in brite tanks waiting for a beer to get its fizz via force carbonation. 

Choosing spunding over force carbonation also reduces the need for CO₂ from outside sources. Not only does this lower CO₂ costs, it also cuts out pinch points for cross-contamination from other equipment or potential oxygen pickup during forced carbonation.

From a physics perspective, carbonation is determined by the factors of temperature and pressure. The colder the beer, the more readily it will absorb and retain dissolved CO₂. Bunging the beer toward the end of fermentation before it’s dropped to a lagering temperature allows the beer the opportunity to absorb that natural CO₂ that’s been trapped in the tank after bunging.

Many brewers recommend closing the spunding valve toward the end of primary fermentation, once the beer is about 1°P away from its final gravity. The additional CO₂ that builds up through the rest of fermentation, along with the residual CO₂ already in solution, should land the beer close to your target carbonation, depending on style.

Timing is essential. If a tank is capped too soon, excess pressure can stress out the yeast, which can lead to incomplete fermentation and set the stage for autolysis. It may also trap sulfur compounds that can lead to off-flavors that would otherwise make their normal exit from the tank through a blow off tube. Also make sure the beer had a successful diacetyl rest with a VDK test before stepping down to a lagering temperature (or consider trying ALDC or a DKO strain and mark that worry off the list).

Spunding also helps beer retain sulfur compounds and delicate hop aromatics that would otherwise be sent into the atmosphere in an unsealed fermentation.

Often brewers will need to add a bit of forced carbonation prior to packaging to hit their specs after lagering is complete. Getting the natural carbonation level right via spunding takes time, repetition, and careful notetaking to really dial in. How can we get closer to the mark with natural carbonation?

Hitting your carbonation spec naturally is a matter of measuring and recording fermentation data, then using those numbers to determine how soon to close off the tank before final gravity is reached. Fermentation curves for flagship beers and others in regular rotation should provide reliable end points to calculate how far a beer is from its final gravity. For new and revised recipes, perform a forced fermentation test to get a grasp on how your yeast will perform.

In their book Modern Lager Beer (Brewers Publications, 2024) Jack Hendler and Joe Connolly of Jack’s Abby lay out their process for determining when to spund beers based on a calculation using how much more CO₂ will be produced during fermentation and what’s already in suspension (see box below).

It’s also worth considering that dry-hopping can lead to additional attenuation that can be difficult to predict, especially in new recipes. Spund the tank as normal and allow the valve to off-gas any additional CO₂.

Once the beer’s gravity reaches the desired spunding point, just close the spunding valve to bung the tank, set the device to the desired pressure, and let the bubbles build. If the fermentation exceeds the set pressure level, the valve will slowly release CO₂ until it returns to the set point.

After primary fermentation wraps up, lager and transfer to the brite tank as normal (slowly, gently). Check the final carbonation level prior to packaging to make sure the beer is hitting the right note. Even with the most careful of calculations and timing, some carbonation may be lost in the process.

Spunding is a historical technique that holds modern value, and it’s equally applicable to home and professional brewers, and can be used on lagers and ales alike. If you’re interested in giving it a try, might we suggest a dry-hopped rice lager?