Purging Kegs of Air

Maintaining proper sanitation in your home brewing equipment is vital to producing delicious, well-crafted beer. Many home brewers use sanitized airs and gases to maintain a sterile environment during fermentation, racking, kegging and aging home brew. This is due to the great probability that there are organisms in the air that could potentially contaminate a future batch of beer. By using a purified gas to purge your kegs of the various airs that end up inside them, you remove one more factor that could cause beer spoilage.

It is always a possibility that unwanted visitors could enter into your brewing space at any time. This is especially true if you or someone nearby keeps any sort of vinegar cultures, such as Kombucha, or if there are any fruit trees growing nearby. Fruit trees and grape vines often harbor wild yeasts on their fruit. If these yeasts are successful at getting to the sugary fruit then wild vinegar bacteria is sure to find root there too. Both will eventually become airborne in search of new food supply. Wherever there is sugar in nature you will find yeast consuming that sugar. After the yeast has produced alcohol you will next find vinegar bacteria - the nemesis of beer brewers everywhere.

On the other hand, wild yeast can also provide an interesting opportunity to experiment with the flavors imparted by different yeasts - will the yeast that grows on a plum provide a better flavor than one than grows on an apple or in your brewer's beard? For the adventurous home brewer, wild yeast cultivation and experimental brewing can be quite interesting. I find that most, but not all, home brewers are more interested in reproducing a reliable great tasting beer rather than experimenting with wild yeast.

Wild strains of yeast or other microorganisms that might enter your brewing area are uncontrollable.  We can control this no more than we can control the same organisms from being in the air we breathe.

A glass carboy being purged of sanitizer and air using CO2 gas.

Closing off your brewing system as much as possible is advisable.  Even if you do an excellent job of cleaning and sanitizing there is always a chance of wild yeast, mold or bacteria like Lactobacillus, Pediococcus, Zymomonas spp., Pectinatus spp. or one of the various Enterobacteriaceae wafting into your primary fermenter just as you are pitching the yeast. (Side note: Always sanitize scissors used for opening smack packs of yeast as well as vials of yeast before pitching.)

Those are just some of the bacteria that could contaminate your beer, causing off flavors. There are also the unfavorable yeasts that cause beer haze and differences in the intended alcohol level – for good or bad.  Mold can sometimes enter into beer and contaminate it, but proper cleaning and sanitation should eliminate this factor for the most part.  If you can smell mold in your brewing area, it is probably something you should take care of before brewing your next batch.

A healthy active yeast culture will help prevent your beer from becoming contaminated as well.  The more aggressive your chosen yeast culture is, the less likely it is that any other yeast which might sneak into the wort will be able to proliferate.  Informing yourself on the best techniques for activating and pitching yeast is advisable.

When cleaning and sanitizing your kegs, I recommend doing a purging of the airs as the last step before storing them. This will help to ensure that the cleaning job you did will keep and the kegs will be well sanitized until you are ready to use them again.  To facilitate the purging of your keg, all you need to do is to hook up your CO2 tank to your closed off keg, set it on 15 PSI or so and pull the pressure release valve to allow the air to escape. Hold it open for about five seconds.  Let the pressure build and repeat the process five times or until it seems like the CO2 has pushed out all the air.

When your wort has finished boiling and cooling and you have pitched your yeast into the mixture, use CO2 gas to purge the headspace in the fermenting vessel or keg so that your brew can get started in a sanitized CO2 environment.  If you are using a glass carboy, you can attach a CO2 line to a carboy cap to force air out and displace it with CO2 before adding an airlock.

CO2 closed transfer between glass carboys and homebrew kegs.

It is a bit easier to do this sort of displacement with a soda or Sanke keg that you are using as a primary and/or secondary fermenter.  It does require some specialized tooling and attachments for the kegs. Once this tooling is acquired it is possible to keep fermenting wort under pressure in a closed transfer system. See more on closed transfers.

Some home brewers even go so far as to keep a closed transfer system in operation from the time their beer has been pitched with yeast.  This also makes it easy to carbonate beer naturally. Keeping the beer in a CO2 pressure positive environment for the entire life of the beer, although much of the CO2 comes from the beer itself, some CO2 from a canister is needed to purge the headspace of air and to push the beer from the primary fermenting vessel to the secondary and/or serving keg.  This means the beer isn’t totally “real ale” by some people’s definition, but this process of carbonating is far more natural than force carbonating or even than bottling with priming sugar.

The process of purging kegs of air can add some security and peace of mind to the home brewer worried about contamination.

Christian Lavender is a home brewer in Austin, TX and founder of Kegerators.com and HomeBrewing.com.

Related Articles:
How a Cornelius Keg is Reconditioned - How to check your homebrew soda kegs for dents, punctures and inspect the rubber seals and replace if they show signs of wear.
Rebuilding a Used Soda Keg - Learn how rebuilding used soda kegs is the most efficient way to prepare your home brew for consumption, in terms of both time and money.
Homebrewing with a Closed Transfer System - Different types of closed transfer systems to keep your beer away from unfiltered air.
Ball-lock vs. Pin-Lock - Differences between the two popular styles of Cornelius kegs.