Spunding Beer During Fermentation

Getting your Trinity Audio player ready...

Today I want to talk about spunding beer during fermentation. Reason being; it’s discussed amongst brewers regularly online. Especially when lagers are the topic of discussion…

So, What Is Spunding?

Spunding is the method where brewers “naturally” carbonate their beers. A device called a spunding valve (pictured below) is attached to your tank. A spunding valve allows a brewer to set the head pressure of a fermentation vessel to predetermined level.

When you ferment a beer; CO2 is created as part of the process. Now, some of the CO2 in the head space is absorbed by the wort/beer. Whilst some escapes out of the spunding valve.

What A Spunding Valve Looks Like

When the pressures too high; the spunding valve allows the excess C02 to escape the tanks. During the fermentation; wort slowly picks up carbonation, naturally absorbed from the headspace.

Brewers will tell you this method produces smaller/finer bubbles for a “smoother” carbonation compared to forced carbonation later in the brewing process.

I’ve been spunding beers for most of my 25-year brewing career. But, I don’t spund all beers during fermentation. For instance; I don’t spund when I’m using a hop cannon later in the brew.

However, for the majority of beers I do cap the beer during fermentation. The reason being is…

The Advantages of Spunding Beer During Fermentation

Cost Savings

You’re essentially “capturing” some of the CO2 given off during fermentation to carbonate the beer. If you carbonate your beer after fermentation then you’re paying for the CO2 you’re adding. You can fully carbonate your beer naturally by spunding.

Cleaner Beer

During fermentation there are many complex reactions taking place. When you ferment under pressure many undesired by-products are suppressed. Such as, limiting ester and fusel alcohol production.

Ester production usually leads to fruity aromas plus flavors in beer. For lagers, these “fruity notes” are not true to style. So, fermenting under pressure is preferable.

“Again, it was found that both esters and fusel alcohol concentrations decreased as a result of increasing pressure, which the authors said was “partially caused by the inhibition of growth of metabolically active biomass.” Specifically, concentrations of ethyl acetate at the end of fermentation were approximately around 30 mg/l @14 psi, 13 mg/l @29 psi, and 9 mg/l @43 psi (converted from bara).”

Taken from Renger, R. S., Hateren, S. H., & Luyben, K. C. (1992). The Formation Of Esters And Higher Alcohols During Brewery Fermentation; The Effect Of Carbon Dioxide Pressure. Journal of the Institute of Brewing

Less Headspace Needed

When you ferment under pressure you reduce the krausen. The “foam” on top of the fermentation is reduced. For brewers this is desirable; as you can fit more wort into the FV without the krausen coming out of the tank.

Increased Hop Aroma/Flavor

If the CO2 is allowed to escape during fermentation, it can drive off the hop oils with it. A role of a brewer is to showcase those volatile hop oils in many beer styles. Therefore, capping your fermentation helps keep more of these the volatiles in the beer.

Boiling at Altitude and Spunding Beer During Fermentation

Fermenting at Higher Temperatures

As we have written above; when you ferment under pressure you suppress the formation of off-flavours. If you don’t ferment under pressure plus have a high fermentation temperature; it can produce solvent like off-flavours.

When you ferment under pressure; you can ferment at a higher temperature without the risk of these off-flavours being present in the final beer in many instances.

The Disadvantages of Spunding Beer During Fermentation

There are some disadvantages spunding beer during fermentation. Although they’re more like processes and issues to look out for. Anyway, here they are:

What Beer Styles Are You Brewing?

In some beer styles ester formation is desirable. For instance; with hefeweizens, witbiers and some IPA’s the fruity flavors from esters are true to style.

So, fermenting under pressure means you lose some of this “fruity” aspect from the beer. In this instance some brewers still cap the fermentation, but later in the process. Around 1 to 2 points before final gravity, then let the head pressure go to 30-40 PSI to natural carbonate the beer.

Stressed Yeast

If the pressure during fermentation gets too high; it can stress the yeast in some instances. This can lead to lower yeast growth and the brewer not reaching desired terminal gravity. Some brewers believe when you cap the fermentation, you shouldn’t go above 15 PSI.

The Dreaded Dry Hop Volcano – Safety Issues

Adding hops to beer already carbonated; can cause a dreaded dry-hop volcano. When this happens, the added hops act as a nucleation point causing the CO2 to break out of solution and surge. It’s a similar reaction to when you add mentos to cola.

Equipment and Costs

If you ferment under pressure you need a vessel than can hold said pressure. So, having a PRV (pressure relief valve) on your tank is recommended. So, if for some reason the pressure in the tank gets too high it will be released automatically.

Pressure Relief Valve Brewing - Spunding Beer During Fermentation
Pressure Relief Valves (PRV’s)

Pressurised tanks are more expensive than non-pressurised tanks. Furthermore, the cost of the PRV and spunding valve also adds to the costs. However, over time some of the costs can be recovered from the money saved on CO2.

How Do You Spund Your Beer?

Everyone has their own method of spunding beer. With the fermentation being capped at different set pressures and stages of attenuation. This depends on beer style, plus what the brewers trying to achieve with the brew.

There are two points of order about the amount of CO2 dissolved by a liquid. These are pressure and temperature. The colder a liquid; the more readily it will absorb carbon dioxide. The higher the pressure the more quickly CO2 is absorbed too.

I’ve adapted my spunding technique throughout my career with some changes coming more recently. I was lucky to brew with an old Czech brewer two years ago. I did a brewhouse installation in Armenia for a wonderful brewery called Dargett.

Dargett had bought their equipment from Slovakia; we ended up doing the first few brews on the new system with the Czech brewer. He had some great tips on how and when to spund. Which I’ve incorporated into this article after doing more research myself.

Spunding Beer During Fermentation – When to Cap

I start spunding when I know the beer is actively fermenting. Usually when then beer has dropped 2-3 Plato and/or there’s visible active fermentation. As I say; some brewers cap earlier, or even directly after pitching.

The pressure I keep on the tank is dependent on tank geometry and how much headspace there is. I’d suggest 10 PSI as a good starting pressure when spunding for the first time. However, you can start lower; then increase pressures over time finding you find your “sweet spot’.

As I’ve said; some brewers don’t go above 15 PSI, believing you might stress the yeast. Again, this is personnel preference. As is the point you decide to cap fermentation depending on attenuation reached.

Six Rock Saison - Fat Fat Beer Horse
Six Rock Saison – Fat Fat Beer Horse: Made with Sweet Potatoes

I’m Not Going Getting Too Specific

I am not going to tell you what’s the perfect method. As in a room of 10 brewers; you’ll have 10 different opinions! All I know is what works for me.

Also; I’d like to note: Everyone I’ve given spunding tips to; have been extremely pleased with the results. As they’ve been able to produce super clean drinkable beers.

I’ve known brewers cap late. Say 1 or 2 Plato before target gravity, setting the pressure of the spund valve to 30 PSI and making great beer with good natural carbonation.

I like to start spunding first thing in the morning if I can (if the beer is actively fermenting). Then set target pressure for the spund valve.

If I’m lucky the desired pressure in tank is reached before the end of the work day. It allows me to make minor tweaks to the spund valve if the pressure is slightly off (set too high or low).

If the tank hasn’t reached the desired pressure by the days end. I usually set it deliberately on the low side, allowing me to set it to the correct pressure the day after. So, I never over pressure a tank for safety reasons.

You keep the set pressure for the rest of the fermentation, whilst you’re recording the gravity daily. Different beer styles have various desired carbonation levels depending on the style. Wheat beers usually have higher carbonation than say a pale ale.

Please click on the image below for our FREE one-page PDF reference guide for beer styles and their recommended carbonation levels.

Linkedin Cheat Sheet - Android and Spunding Beer During Fermentation

Spunding Beer During Fermentation – What Beers to Spund?

So, as we said before you don’t want to spund all beers. If you’re looking to dry-hop a beer; then in most instances you don’t want to spund. There are devices to dry-hop a tank under pressure so, some brewers do spund and dry-hop.

I’m an old school brewer; so, believe you only have “one chance” to form a good head which stays, giving good lacing on the glass too. If I dry hop, I carbonate after adding the dry hop addition. So, I’m not degassing the beer when hops are added, as they can act as nucleation point for CO2 breakout.

Carbon dioxide is added either by cranking up the head space pressure for the beer to absorb the CO2 or using a carbonation stone after the dry hop addition.

Carbonation Stone Brewing
Carbonation Stone for Brewing

Beers not capped, include wheat beers, saisons or certain IPA’s. Usually, any beer where increased extra ester production is desirable.


For a good clean lager, I also like to FREE rise my fermentation. For more on lager brewing please download my handy FREE guide. When brewing lager, I let the fermentation rise in temperature at around 50-60% of attenuation.

Please note, I mostly use dried yeast when making lager. I’ve a lot of love for W34/70 by Fermentis. You can make great lagers with this yeast. I have brewed and exported pilsners made with this yeast to multiple countries.

When you free rise a fermentation; you’re allowing the beer to naturally increase in temperature. When beer is fermenting, heat is produced. You usually control the temperature of the fermentation using cooling jackets on the FV.

FREE Rising Lagers

When you free rise you turn off the cooling jacket or set to a higher temperature say 18°C (can go as high as 20°C). It will slowly rise by a few degrees per day.

The idea being, allowing the beer to “warm ferment” at this stage means yeast will reabsorb and breakdown unwanted chemicals like diacetyl (butterscotch off-flavor) and acetaldehyde (green apple).

You keep the higher set point until the fermentation is complete. You may see the temperature drop a little towards the end of the fermentation if the ambient temperature is lower.

I then wait for the yeast to clean itself up, after the end of fermentation (around 3-4 days). Whilst dumping the dead yeast every 24 hours.

Then I begin to cold crash the beer at increments of 5°C until the beer is at 0°C. When the maturation process begins.

I usually lower the temperature by 5°C per day however; but do hold it at 10°C for 48 hours, because it helps the yeast flocculate when using W34/70 yeast. I’ll dump or re-pitch the yeast when the beer has reached 0°C for 24 hours.

This is personal preference; it’s process I’ve developed over the years. It’s also the method I teach my clients when they’re paying for my services (now I’m giving it away for free!).

I’m giving away my trade secrets for FREE!

Spunding Beer During Fermentation – Conclusions

So, these are my thoughts on spunding beer. I know some brewers will have their own take. As it’s still a hot topic of debate. I’ve used my own research and years of experience to put this article together.

It works for me. Plus, the clients/employers I’ve taught these methods too over the years, have all taken on and still use my processes. Anyway, let’s finish with some take-aways:

  • Begin spunding when you’ve definite active fermentation (drop of 2-3 Plato)
  • Set the spund valve to 10 PSI as a starting point and tweak with more experience
  • Spund till the end of the fermentation
  • Free rise the beer if appropriate at 50-60% of attenuation
  • Always record as much data as possible for each brew; it’ll allow you to finesse your technique more easily.
  • If the CO2 level is too low; you can increase headspace pressure adding CO2 (from bottle/reserve) or use a carbonation stone

Spunding beer during fermentation can be beneficial producing an improved final product. As well as saving you money on CO2 costs.

Always think about safety: Having a PRV valve on a pressurized tank is always recommended. Working with pressure presents danger, so always be on alert.

Anyway; thanks for reading my article and if you’ve any feedback please leave a comment below or email me at:


I would love hear other people’s methods and thoughts on spunding too. For now, have a great day and happy brewing.



Asian Brewing Consultant - Neil Playfoot Mashing In

Neil Playfoot

Neil is a brewer with 25 years international brewing experience. Based out of China (first came in 2010) he works as a brewing consultant helping brewers with their projects and brewing processes. To find out what services Neil can provide your brewery please click here. If you'd like to contact Neil you can email at neil@asianbeernetwork.com.
4.7 3 votes
Article Rating
Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Newest Most Voted
Inline Feedbacks
View all comments
Chris McEntyre

Great article! I’m just getting into pressure fermentation and even though this appears to have been a thing for about three years (when I first heard of it anyways) there is not a lot of content out there. Very informative, thank you,n

marc gorfinkle

Hi Neil, great article! I have a question about the PSI setting the spunding valve during active fermentation. Per a carbonation chart wouldn’t 10 PSI be way to low to achieve proper carbonation? This point always confuses me. Per a carbonation chart 10 PSI @ 65 F degrees (for an ale) would only yield 1.4 volumes of CO2 if left to “set it and forget it” for ~10 days. Wouldn’t you need to set the spunding valve at ~30 PSI @ 65 F degrees for 10 days to yield fully carbonated beer of ~2.5 volumes Keep in mind the batch… Read more »

Last edited 2 years ago by marc gorfinkle

Thanks for the Article Neil.
Does a co2 carbonation chart exist? I’m looking into investing into isobaric fermenters


Hello Neil, Thank you very much for this article. It’s the most complete one I found, included professional forums. The spending procedure you described is more or less what we do in our brewery. We spun dale and lagers (we use “clean” yeasts) after 24-36 hours for 1,2 bar, 19-21 Celsius for ales, 12 Celsius for lagers (free rise at 16 Celsius). During lagering, after yeast dumpings, we adjust at 0,75 bar for 1,5 Celsius by realising the remaining pressure. Our unitanks are up to 4000 L, often used filled up to the CIP ball (we don’t waste fermentation capacity… Read more »