Avoiding Oxidation

Jester Goldman offers tips and techniques for avoiding oxygen exposure when you transfer from one vessel to another.

Jester Goldman May 12, 2017 - 8 min read

Avoiding Oxidation Primary Image

Timing is everything. Aeration offers great benefits for your yeast at the start of fermentation, but shortly after, oxygen becomes the enemy. A great beer can turn lifeless as hops character quickly fades and stale flavors such as cardboard, sherry, and rotten fruit take over. Since most homebrew is enjoyed immediately in smaller batch sizes, merely following good habits, such as minimizing splashing during transfers, can keep oxidation at bay through the end of the keg. But if you’re making a mead or a big beer that will need to age, you may need to take some sterner measures. Heavily dry-hopped beers, such as New England–style IPAs, are also fairly vulnerable to losing their fresh hops intensity.

The greatest oxidation threat comes when you move the beer from its safe cocoon in the fermentor to another vessel. Let’s break it down.

Transferring the Risk

When you rack your batch to secondary or to a keg, there are three components: the starting carboy, the destination carboy or keg, and the connection between them. Each one offers a chance for oxygen exposure.


Before you begin the transfer, your beer is usually under a blanket of CO2 from fermentation. As the beer is siphoned out, air is pulled in to fill the space. This air blends with the CO2 and can expose the top surface of the beer to oxygen. The good news is that this bit of potentially oxidized beer is often left behind, and there’s usually not very much turbulence on the source side of the siphon.


Nevertheless, some brewers consider it worth the effort to avoid this relatively minor risk. The easiest method is to use a carboy cap with two ported openings. The racking tube goes through the center, down into the beer, and then a CO2 line is attached to the other via a barbed adapter. Three or four PSI of CO2 is sufficient to push the beer out of the carboy and over to its next home. To reduce the chances of blowing out the cap or damaging the carboy, don’t rush it with higher pressure.


By far, the biggest risk for oxidation comes when the beer is delivered into the destination carboy or keg. This is inherently turbulent and, although some CO2 will come out of solution, if the keg is full of ordinary air, the beer will be exposed. Then, once the beer is racked, the headspace will provide a lasting air reservoir.

There are couple of ways to handle this, which vary slightly depending on whether you’re going into a keg or a carboy. The simplest is run CO2 into the key or carboy and hope it drives the air out. For a keg, this can be done by attaching the gas to the keg and adding CO2 at 30 PSI or so, then venting off the excess pressure. Since a carboy doesn’t have a gas inlet, you can use a gas line without a fitting on the end and run a lower pressure for 30 seconds or so. Adding CO2 like this works, but you have to take dilution into account: the incoming CO2 doesn’t uniformly force out the air; they blend together and merely reduce the percentage of oxygen present.

A better alternative is to fill the keg or carboy with sanitizer and push the liquid out with CO2. Since the vessel doesn’t have any air in it to begin with, the incoming CO2 can fully displace the contents. This is easy with a keg; just dispense the solution through the tap with some applied CO2 pressure. For a carboy, follow the same purging steps for pushing your beer out of a carboy.


In Between

It’s easy to forget that, as the beer moves through the racking cane and tubing, it is exposed to air within. In general, this is washed away by the initial flow of beer. Once the line is full of beer, any bubbles you see are probably CO2. You should still keep an eye out for persistent bubbles where the tubing connects to the racking cane, though. A poor fit here can leave a leak that aerates all the beer as it passes.

If you are concerned about the initial bit of air, the hose and cane can be purged by attaching the tubing end to a sanitized hose barb and running a brief burst of CO2.

Playing it Safe

You can combine these steps to radically reduce the chance of oxidation.

  1. Fill the receiving vessel with sanitizer.
  2. Attach CO2, either to the gas post on the receiving keg or the second port of the carboy cap. If you are using a keg, open the dispensing tap. Otherwise, use a racking cane placed in the center port of the carboy cap. Push out the sanitizer with about 3 PSI of CO2.
  3. Once the receiving vessel is empty of sanitizer, place a racking cane through the center port on the carboy cap on your full carboy. Don’t let it dip into the beer yet.
  4. Attach a hose barb to the second port and connect your CO2. Purge the racking cane and tubing by running the CO2 for 5–10 seconds at 15 PSI.
  5. Turn off the CO2 and insert the end of the transfer tubing down to the bottom of the receiving vessel. Lower the racking tube near to the bottom of your full carboy. Try to avoid sucking up the trub.
  6. Lower the CO2 pressure to 3 PSI and start racking your beer.

This may seem like a lot of effort, but oxidation can trash all your other hard work. I don’t do this for every beer I brew, but I am very careful when it comes to my meads and beers I expect to lay down for months. Protect your investment!

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