The Great Purge - Does a Full Liquid Purge of a Keg Protect Hop Aroma Better

drew's picture

It's time to test out the validity of another of Drew's favorite techniques - keg purging.

Breweries spend a lot of time worrying over removing as much oxygen possible from their packages (bottles/kegs). Why? It's pretty well established both in tradition and science that oxygen is destructive to beer flavor and aroma pretty quickly. So packaging brewers do everything they can to reduce O2 levels after their intial injection of oxygen post chilling. 

On the homebrew side, we tend to be a bit more cavalier. We have the protection of knowing who's going to be handling our beer post packaging and we're fairly certain that person isn't going to do idiotic things like leave the beer out in the sun, etc. For the most part at least.

How far can careful O2 handling and storage take you? It can nab you Ninkasi awards. Jamil Zainasheff talked about winning medals with lower gravity beers that were over two years old. They still tasted fresh because of the care he took while bottling and the subsequent cold storage he kept them at. Drew's experience included a keg of his Gnome Is In The Details - a light weight Belgian blonde ale designed to express the Wyeast Ardennes yeast. He brewed it and served it at the first San Diego Homebrewer's Conference in 2011. The keg went into the deep freeze to be ignored because things and when it came time to get ready for the 2014 Conference in San Diego, there it was - still fresh as it had been 3 years prior. Drew claims it's because of the cold storage and his purging technique.

Drew's technique is fairly straightforward. Fill the keg (to the top, brimming) with your sanitizer. Push the sanitizer out with CO2 (preferably into another keg) and then fill the keg with your fresh beer. Chill, carbonate and store. The idea being that the CO2 will completely replace the liquid with minimal remaining oxygen.

The more popular technique with homebrewers involves multiple (~7) cycles of filling the keg with CO2 and then releasing the pressure. The operating principle says that doing this will cause CO2 to settle in the keg and displace the O2. Since O2 and CO2 are miscible, the multiple cycles are needed to repeatedly decrease the amount of O2 left behind. 

So which is better? Let's do a couple of experiments to find out:

1) Package with no purging / Package with Drew's Total Purge

2) Package with Cycled Purging / Drew's Total Purge

All the kegs will be stored cold. (Cold vs. warm storage is another test)

Since hops fade more quickly - the tests will be done using a hoppy recipe. We'll be using an open top transfer to allow IGORs who aren't capable of pressure transferring to participate.

Experiment Status: 


Proposed By: 



Does purging a keg with CO2 protect hop aroma over time compared to an unpurged keg.


Yes, the beers packaged in the purged kegs will demonstrate a noticeable difference in hop aroma

Brewing Sessions Needed: 



Special Equipment/Process: 

  • Sanitize 2-3 kegs (2 for only one leg, 3 if you're testing all 3 purge styles)
  • For the no purge keg, do no purge. Gently rack the beer in via the top covered with foil.
  • For the cycle purge keg, purge by completely filling the keg with CO2 at 25 psi. When filled, pull the pressure relief valve and completely bleed the pressure. Repeat 7 times. Gently rack the beer in via the top covered with foil.
  • For the full purge keg, fill the keg with sanitizer and push it out with CO2. Gently rack the beer in via the top covered with foil.

Special Observations: 

  • Note any color changes (post testing). The oxidized beer can appear darker
  • Any other aroma changes?

Experimental Procedure: 

  1. Brew enough volume of the target recipe to split the wort into two-three equal fermenters. Chill and prep the wort/fermenters exactly the same
  2. Ferment the batches in the same space and under the same conditions, especially temperature. Match the fermenters and other equipment.
  3. After fermentation subsides, record the length of fermentation and the final gravities.
  4. Package the two beers in kegs. One keg should be purged via Drew's method, the other not purged at all or cycle purged as decribed above. (If doing all three, prep all 3 styles of purge) Chill and force carbonate. Store cold. Wait one month before testing.
  5.  Perform a triangle test with samples of the full purged vs cycle/no purge and record the results. If doing all 3, please make sure to do two separate tests, but always with one of the test beers being the full purge.
  6.  Ask the testers for their observations on the samples. DO NOT Reveal the Difference between the samples.
  7. If creating all three styles of purge. Give your tasters, post triangle test, samples of all three and have them rank the beers.
  8. Discuss the results and record any further observations.

Experimenters Questions: 

1. Should any special procedures be used to rack from the fermenter to the keg to reduce oxygen intake?


2. How do you account for any liquid (sanitizer) left at the bottom of the keg?

7 CO2 purges

The effectiveness of the method of repeatedly "filling" the keg with carbon dioxide and emptying is entirely dependent on how much carbon dioxide you put in and possibly how long between fill and empty you wait. "Filling" with carbon dioxide could occur with only a single molecule of carbon dioxide gas because all gasses expand to "fill" the container they are in. You did specify the pressure on the web site, but you didn't do this on the podcast. Also I'm not certain that the oxygen in the tank will have completely mixed with the carbon dioxide without some--perhaps short--period of allowing the gases in the tank to mix. These variables could affect the results. In think that theoretically one fill/empty cycle at 25 psi (after equilibration of the mix of gases) would reduce the oxygen content to 1/25 (4%) of the original oxygen content. The next cycle would further reduce the oxygen content to 4% of the 4% level and so on for each fill/equilibrate/empty cycle, so 7 cycles would be very effective at reducing oxygen--you do the math, but it takes much more carbon dioxide to dilute oxygen than does the method of displacing the tank filled with liquid.

Fred L Johnson
Apex, North Carolina, USA

Rgrosholz's picture
CO2 Purge

This experiment comes at the perfect time as I've been working to reduce oxidation to improve my beer.  I've tried the purge method on the last 2 beers I've kegged and need a few weeks before I can make an assessment.  My concern with this method is the residual sanitizer (water) that's remains in the keg after the  purge. I was recently on a brewery tour and the brewer there felt it was important to dry bottles with sterile air after sanitizing to mimimize the beers contact with water.  Since water is part O2, this contact could lead to oxidation over time.

Should I be concerned about this when using the proposed purge method?


Don't fear the foam

The amount of sanitizer remaining in the keg minimal if you pressure transfer until your blowing CO2.  In the brewery case their bottles are right side up and I guess there could be residual sanitizer in a 12 oz bottle to skew flavor.  But thats in 12 oz not 5 gallons.  

M. Ellis

Artichoke's picture
Crack the lid after CO2 purge?

Hi, regarding: 

  • For the full purge keg, fill the keg with sanitizer and push it out with CO2. Gently rack the beer in via the top covered with foil.

I presume this means opening up the lid after displacing the sanitizer with CO2 and racking into the keg through the large lid opening. Is it not a concern that O2 will get in and mix with the CO2 using this method? 

- Artichoke

ejpejp77's picture
Siphon in through the out port

I think that's an unnecessary risk as well. To minimize the O2 when you can't use pressure to do the transfer, here's my proposed solution:

  • Put the fermenter on the counter and the keg on the floor
  • Connect the hose from an auto siphon to a output connection on the keg.
  • Open the pressure releif valve on the keg
  • Start siphon

Tried it once already and it works great.


Pressurized Racking

I agree, I have a set up similar to this one an push beer from my pressurized PET carboy to the clean, sanitized, and purged keg.

M. Ellis

Mr Mats
O2 during dry-hopping and cold-crashing

Another opportunity for O2 to sneak in, I guess, is between fermentation and packaging. For IPAs I usually do a two-phase dryhopping, and then cold-crash. I used to do all of that in my fermenter(s) before kegging. But I got worried about the O2 and in my last batch I cold-crashed in the keg. However, I guess that it would be better to also do the dryhopping in the keg? Or maybe the remaining yeast during both dryhopping sequences will produce enough CO2 to keep the O2 at bay?