Spunding: the good, the bad, and the bubbly
09/08/2022
By Ryan Hansen from Big Pop Brewing
Process and purpose of spunding:
The age-old tradition of spunding has been making a comeback. In short, spunding is the process of trapping the naturally occurring cO2 that is created during fermentation inside the fermenter so that it absorbs into the wort as it turns into beer. When done correctly you can have a perfectly carbonated beer that is ready for packaging without the need for outside carbon dioxide to force carbonation.
The original meaning of the word spunding directly translates to “bunging” in German which means this is how the practice is still used in some instances, however because of safety concerns and the invention of new technologies, we can safely do this at the home brew level. The old school way is to simply cap, or bung, the fermenter when there is a specific amount of degrees plato left then the remaining cO2 produced gets reabsorbed into solution. This math has quite a few variables to consider, so I don’t recommend it for homebrewers without proper research or mentorship because of the potential risk of damage to person or property. The technology to dial-in pressures and spund with a valve are easily purchased these days, making the practice much safer than sealing a pressurized vessel.
There are a lot of articles that discuss how spunding can suppress the creation of volatile ester producing compounds, meaning that it can effect the flavor of your beer. We won’t get into that here because the ale vs lager spunding conversation rages on and rarely contains real data. You can research that till your heart is content on your own. For what it’s worth, I do spund certain ales and have never experienced issues or off-flavors. Another hot topic is the idea that spunding can create a creamier or smoother mouthfeel compared to force carbing, but again, I haven’t seen scientific data or anecdotal evidence proving that that’s even scientifically possible so I’ll leave that to you to experiment with.
Equipment necessary + available options:
The two key pieces of gear needed for this (at the home brew level) are a pressure rated fermenter and a spunding valve. There are a handful of options for each. My setup includes a Spike Brewing Flex+ fermenter and a BlowTie spunding valve and analog gauge with PSI markings. If your valve has a readout in “bar” or “kPa” measurements, just make sure you’ve got a handy conversion cheat sheet.
Other pieces of equipment you’ll need are the connections and lines you would normally use for an oxygen free transfer into kegs: cO2 bottle, lines, sani, etc…
Spunding in action:
Brew day was very straight-forward and I hit all the numbers as expected with this recipe.
After chilling the wort down to pitching temps, I pitched the yeast and attached a blowoff tube. I allowed bubbling activity to start that first night. You can attach the spunding valve right after pitching, but because my valve is analog and reactive (meaning you can’t set it to a pre-calibrated pressure) I had no idea what it was set to. On day two, once the bubbles were consistent at about three bubbles per second, I removed the blowoff tube and attached the closed BlowTie Spunding valve and watched the pressure rise over the next hour. My fermenter has its own PRV that should go off at about 15PSI, but again, take care to not build up dangerous levels of pressure.
Once the pressure had reached my desired target of 12 PSI I turned the valve’s handle until I saw a slight pressure drop and could audibly hear carbon dioxide escaping. Because of the positive pressure I didn’t feel the need to add tubing for that cO2 to blow off into sanitizer. Although it would be nice to see the bubbles, I didn’t know if it would alter the pressure levels so I just left it.
The first five days maintained the perfect 12 PSI set point and then I noticed a slight drop to 10 PSI once primary fermentation had completed. It remained at 10 PSI until I cold crashed on day 10 and my FG was stable. During the two days of cold crashing I saw the expected drop down to 6 PSI. I had my cO2 bottle ready to add positive pressure in case it fell to 0, but it balanced itself out.
The transfer to a keg is a bit trickier than a usual oxygen free transfer because it’s important to balance the pressures to avoid foaming during transfer. To do this part I removed the spunding valve from my fermenter and put it onto my clean, sanitized, and cO2 purged keg. I opened the spunding valve until it was at 7 PSI and stable, then I attached my bottle of cO2 to the fermenter and raised the pressure to just under 9 PSI. This balance allows for smooth transfer into the keg without exposing the beer to dramatic pressure shifts that would cause the carbonation to want to leave solution and create foaming.
Transfer of all five gallons went very smoothly and once it was complete I removed all gas and transfer connections before placing the keg directly onto a tap line in the kegerator.
Without a carbonation tester I can only give a sensory impression that the beer was about 90% carbonated right out of the fermenter – great news for those of us who hate waiting a week for carbonation! After just one night at serving pressure I was satisfied with the carbonation. One more week in the chiller and this beer had become the brilliantly clear, crisp summer sipper I try to always have on tap.
Overall this is a cool technique. As long as you have the correct gear, it is one more tool in the toolbox for advanced homebrewers wanting to save time in their process, or who want to experiment with ester production on their beer recipes. Have fun and brew strong!
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