Preventing Beer Oxidation in the Fermenter during Racking

By Andy Tipler and Pierre Margraff

This article expands upon the featured story “Carbon Captured” in the July/August 2023 Zymurgy magazine.

Even if one were able to remove all oxygen from a keg before filling it with beer, a potential upstream source of oxidation remains: the fermenter itself. As beer is racked from a fermenter to a keg, something must make up for the volume of beer removed from the fermenter. If we are not careful, we may expose beer in the fermenter to air as the beer exits.

The risk of oxidation from this exposure is much less than that in an unpurged keg, as the fermenter headspace will normally be only carbon dioxide at the start of the transfer. As the level of beer in the fermenter drops, air can be drawn in, but there will be much less turbulence or mixing than would be seen in the keg, so the rate of oxygen absorption into the beer will be much slower.

In other words, we should be able to get away without having to purge the fermenter headspace. It amazes us that the use of bottling buckets, still recommended by some brewers, appears to be successful—we would have thought that these would result in high oxygen exposure.

Adding Pressurized CO₂ to the Fermenter

For those brewers not willing to take the risk of oxidation in the fermenter during transfers, it is possible to use CO₂ to backfill the fermenter headspace as the beer drains or siphons out of it. We’ve used this technique with conical fermenters and carboys that can be semi-sealed. Instead of using a siphon, pressurized CO₂ is introduced into the top of the fermenter and the beer is pushed out through a spigot or dip tube into the previously prepared keg. The keg no longer needs to be located below the fermenter. In this way, no air should enter the fermenter during transfer. However, a significant volume of CO₂ will be needed to make this happen, roughly equivalent to the volume of the beer being transferred.

WARNING: If you use this method, be very careful about the pressure you use, especially with a glass carboy! 2 psi doesn’t sound like much pressure, but it represents a total force of almost a ton applied to the inner surface of a 6-gallon glass carboy. Use very low pressures (much less than 1 psi) and wide bore tubing to make such transfers.  

Closed-Loop Drain or Siphon

A second way to backfill a fermenter’s headspace with CO₂ while transferring beer is to drain or siphon the beer between a sealed fermenter and a sealed keg pre-filled with carbon dioxide. As the beer enters the keg, instead of venting the displaced CO₂ to the atmosphere, we can feed it back into the headspace of the fermenter. In this way, as the beer passes from the fermenter to the keg, CO₂ simultaneously passes from the keg back to the fermenter. The whole system is sealed, so no outside air contacts the beer during this process. An important benefit of this method is that the initial charge of CO₂ in the keg not only protects the beer in the keg from exposure to oxygen, but this same gas also protects the beer in the fermenter—a very economical way to use our valuable CO₂.

The first step is to make sure the receiving keg is located below the fermenter as we’ll use gravity to displace the beer. Next, we’ll use a gas-to-gas jumper to balance gas pressure between the fermenter and receiving keg. Depending on the fermenter configuration a liquid jumper will need to be assembled to connect the liquid outlet of your fermenter to a keg liquid disconnect—most pressure-fermenting brewers already own one of these jumpers.

At this point, the liquid post of the receiving keg is connected to liquid output of the fermenter, and both gas posts are also connected to a gas jumper. Prime the transfer by temporarily disconnecting the gas jumper from the fermenter and adding just enough CO₂ from a tank to allow beer to flow from the fermenter to the keg. Once the transfer is primed, replace the tank disconnect with your gas jumpers disconnect. Beer flowing into the keg will push gas back into the fermenter in this air-free closed loop.

One benefit of this method is that it can also be used to transfer beer that has been fermented under pressure or has already been carbonated.

Using an Air Pump to Improve the Closed Loop Transfer

While the closed loop siphon method works well, it does have a few caveats:

• The keg must be located below the fermenter: this is not always easy.
• The process can be very slow.
• Priming the siphon, if used, can be difficult.
• If siphoning is interrupted, it can be difficult to restart.

All these issues can be simply addressed by installing a miniature air pump in the return gas line from the keg to the fermenter. Miniature diaphragm air pumps can be obtained for very little cost which are fully sealed and can deliver up to 18 psi positive pressure or 10 psi of negative pressure (vacuum).

It’s recommended that an electronic speed controller be used to power the pump so one can (1) fine-tune the rate at which the keg fills and (2) prevent creation of a large vacuum in the keg, which would causing foaming of the beer and possibly physical damage. A vacuum gauge is connected to a tee-piece on the pump inlet to monitor the internal pressure in the keg to prevent this from happening. Once the system is assembled, it is a good idea to pressure check the plumbing with CO₂. This will also purge oxygen.

Born and raised on Tafelbier in Belgium. Pierre Margraff moved to the States 25 years ago and learned to homebrew shortly after tasting his first American beer, Andy Tipler grew up in England and moved to the United States 30 years ago for his job as a research chemist. He has been homebrewing (legally) for more than 50 years and is a certified beer, mead, and cider judge. Andy is active in competitions as a judge and as a contestant, and he enjoys talking and writing about brewing. He is a member of the Underground Brewers of Connecticut (the second oldest homebrew club in the USA). He would very much like to have an English pub next door.