Secondary Fermentation In Kegs: Save Your CO2

Secondary Fermentation In Kegs: Save Your CO2

Love kegging, but getting sick of trekking across town for co2 refills? Before pressurized gas was readily available, brewers had to think up ways of keeping the natural co2 produced by fermentation in the beer. Nowadays, more and more homebrewers are reviving old practices, in an effort to improve their beer, while saving time and money.

Naturally carbonated beer is said to enjoy a softer carbonation compared to force carbed beer, with smaller bubbles forming a tighter head. Some styles gain a lot of flavour and other characteristics from being naturally carbonated, in particular English cask ale, traditional lagers and several Belgian styles.

A keg is the ideal vessel for naturally carbonating your beer, as it can withstand high pressure, is easy to transport and can also be used to serve the finished beer from. There are a number of methods today’s homebrewer can use to carry out secondary fermentation in a keg, all guaranteed to cut down on co2 usage.

Using a Spunding Valve


A spunding valve is the thrifty brewer’s best friend and presents the easiest way to ensure perfect, naturally carbonated beer, brew after brew. Spunding allows the brewer to make use of the natural co2 that is created during primary fermentation to carbonate their beer to a pre-set level. You will require a fermenting vessel that can ferment under pressure, like a keg or the fermentasaurus for this method.

A spunding valve is basically an adjustable pressure release valve, which allows the brewer to set the pressure at which the beer will remain. The best practice is to keep the pressure low for the first 3 days of fermentation, as high pressure can kill yeast cells. Aim for around 8 to 10 psi for the first 3 days, raising it up to 25 to 30 psi for the end of the fermentation. This will generally allow the yeast to attenuate as much as possible, while providing a well carbonated beer as soon as the primary fermentation is over.

There are several methods you can use when naturally carbonating your beer with a spunding valve. The entire process can be completed in one keg, though a more popular practice is to transfer to a secondary keg for the end of the fermentation, raising the pressure in the second vessel. A fermentasaurus fitted with a spunding valve works great in this instance, with the spunding valve then being fitted to keg at an increased pressure.

If you do choose to use a spunding valve, you will want to avoid any beer getting into it. Use FermCap or another anti-foam agent to reduce the krausen, especially if you’re using a keg as a fermenter.

Early Transfer


This is a useful method for brewers who are not fermenting their beer under pressure, but want to carry out an effective secondary fermentation in the keg. The brewer must know at what point the yeast will attenuate to. To figure this out, you must either carry out a forced fermentation test, or be extremely familiar with your recipe. A forced fermentation test simply involves taking a sample of your wort and pitching a large amount of yeast. Within 36 to 48 hours, it should be fully fermented. Take a gravity reading and you’ll know where your beer will finish.

You will need to use a calculator to figure out the exact numbers for your brew, but generally, wort produces 2 volumes of co2 per °P. You’ll need to factor in the fermenting temperature in order to discern how much co2 will be retained in the beer, and have an idea of what your final serving carbonation will be. From this, you can work out how much co2 you’ll need to trap in the secondary vessel and at what point to make the sealed transfer. For example:

  • Desired serving carbonation = 2.4 volumes of co2
  • Fermentation temperature = 10°c, meaning 1.15 volumes of co2 will be retained
  • Required volumes of co2 to reach serving carbonation of 2.4 volumes = 1.25
  • Transfer to sealed keg when 0.63 °P away from final gravity (use a calculator to figure out)

Of course, you don’t have to be entirely accurate, especially if you’re using a spunding valve on the keg. With recipes you’re familiar with, you’ll learn at which point to make the transfer. By transferring the beer into a sealed keg before it has fully fermented, the last volumes of co2 will remain within the keg and be absorbed into the beer, naturally carbonating it. This method can be difficult at first and it can take a few attempts to get it right.

Priming Sugar


Using priming sugar is perhaps the easiest method of carrying out secondary fermentation in a keg, and the one that requires the least investment in new parts. It works in the same way as bottle conditioned beer, in that the brewer must use a priming sugar calculator to determine how much sugar is required to produce the desired level of carbonation. Many brewers advocate that you’ll need slightly less sugar for the same volume of beer in a keg, compared to bottles, due to the smaller headspace.

Once a sugar solution is added to the beer, the yeast wakes up and starts converting it to alcohol, producing excess co2. Simply add your sugar solution to a sanitized keg and transfer the beer onto it. Seal the keg, hit it with a little bit of pressure from a co2 tank, around 30 psi should be enough and allow the pressure to build as the secondary fermentation in the keg gets underway. The reason for injecting the extra co2 is to ensure your keg is tightly sealed, preventing the natural co2 from escaping.

This method will increase the strength of your beer, and is also the most likely to introduce oxygen. On the flip side, it is the easiest to carry out.

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1 comment

Your heading: PRIMING SUGAR

I do my secondary fermentation in a keg in pretty much the same way you have described, including “hitting it with a little bit of pressure from a co2 tank.” You suggest the downside is that it will introduce oxygen into the beer. I get rid of the oxygen by ‘burping’ my keg:
- I introduce CO2 until a pressure of around 10psi is reached
- pull the pressure release valve on the keg to release the CO2/air mixture in the head space
- repeat the process two more times (total of 3 times)
- and leave the keg charged with about 10psi to seal the lid (Corny keg)

I know from my days as a medical microbiologist growing anaerobic bacteria (to which oxygen is poison) that this works as a way of eliminating oxygen from the fermentation vessel. The only oxygen left in the keg is that dissolved in the beer, and the yeast will consume that early in the piece, and benefit from it.

When the keg, having been kept at around 20-25 degrees C, reaches a pressure of 25-30 psi, and that pressure is stable for 2-3 days, I know secondary fermentation is over, and that’s when I put it in my six-keg keezer. After 48 hours in the keezer the temperature of the keg of beer is down to 2-3 degrees C and the pressure of the head space CO2 has dropped to something like 10-15 degrees C (pouring pressure). I adjust the pouring pressure of each keg, using a manifold and individual inline pressure regulators, to the ideal pressure for that style of beer, and I’m done! I usually leave it another 24 hours before testing the pour and making any final adjustments.

Cheers, Jim

James Elliott (Jim)

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