Why Oxygen is Bad in Your Home Brewed Beer

by Brad Smith on June 18, 2015 · 5 comments

Beer bottle and glass with fresh green hop and wheat on rustic wooden table still lifeCommercial brewers go to extreme lengths to reduce the amount of oxygen in their finished beers, and as a home brewer you should be concerned about oxygen as well. Oxygen can spoil the long term stability of your beer flavor and clarity and lead to a variety of off flavors.

Oxygen in Fermentation versus Finished Beer

Let me start by saying that before fermentation, oxygen is generally a good thing. In fact oxygen is needed to allow healthy yeast growth during fermentation, which is why many brewers aerate their beer before they pitch the yeast. Unless you use pure oxygen it is difficult to over-oxygenate your wort before fermentation. In early stages of yeast growth, the yeast will actually scrub all of the oxygen from the beer and use it to grow and expand.

After the yeast has started fermentation, however, oxygen is considered a contaminant. Many of us who rented party kegs or beer balls with a hand pump on them in our college days saw first-hand how oxygen spoils beer. These keg hand pumps put oxygen directly in the keg which meant that the beer would spoil within a very short period, often giving it a stale flavor within 24 hours. So the kegs needed to be consumed rapidly to compensate.

Oxygen, even in very small quantities is bad for finished beer. Not only does it rapidly spoil your beer, it can also damage the long term flavor stability of your beer even in small quantities.

Clarity also suffers in beer with free oxygen as the oxygen will interact with polyphenols and tannins in the beer to create chill haze and eventually a permanent haze in the beer.

Once fermentation is complete, a layer of carbon dioxide, which is heavier than air, sits above the beer and provides a protective layer in your fermenter. If you don’t disturb the beer, that layer does a good job of protecting your beer as it ages.

For home brewers, oxygen is most often introduced when transferring beer from one vessel to another and also during the bottling or kegging process. Excessive splashing during transfer, small leaks in your siphon or kegging system or a poor seal on your bottle caps or keg can lead to excessive oxygen.

Avoiding Oxygen in Beer

One strategy is simply to avoid transferring your beer as much as possible. Many brewers now skip the secondary fermentation entirely and bottle directly from their primary fermenter. Commercial brewers make use of conical fermenters, which let them remove excess yeast and sediment without transferring their beer.

Another important point is to use proper oxygen barrier containers if you are fermenting or storing your beer for any significant length of time. Glass and metal are good oxygen barriers, so use a glass or stainless fermenter if you plan to store your beer for an extended period. Plastics are all air-permeable to some degree, so they should not be used for extended storage (i.e. months) or aging of your beer.

Splashing during transfer and bottling is a large source of oxygen. Auto-siphon devices with a poor seal also often will pull oxygen in through the seal when siphoning. This can be seen as bubbles near the seal when siphoning. If you have a leaky auto siphon you should discard it or add sterile water above the seal so it pulls water and not air if needed.

When kegging you need to minimize splashing and do a good purge with CO2 after filling the keg. You can do this by standing the keg upright and simply releasing the pressure relief valve several times while the gas is turned on. The heavier CO2 will displace the oxygen in the keg, protecting your beer.

For bottling, you should try to keep your headspace to a minimum (generally an inch or so is sufficient), avoid splashing your beer when filling and use oxygen absorbing bottle caps if you have access to them. Also be sure to carefully adjust and check your bottle capper on some bottles as even a small leak will leave your beer undercarbonated and also stale.

Hopefully these simple tips will help you to be more aware of oxygen and some of the ways you can minimize oxygen in your finished beer. Thanks for joining me on the BeerSmith Home Brewing Blog. Be sure to sign up for my newsletter or my podcast (also on itunes…and youtube…and streaming radio station) for more great tips on homebrewing. Also check out the How to Brew Video series I shot with John Palmer if you want to learn more about all grain brewing.

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{ 5 comments… read them below or add one }

Vernon Jenewein January 1, 2018 at 8:18 pm

If you have access (own) a Soda Stream carbonator, OR if you have regular CO2 cylinder system with dual gauges make use of it. I have purged the air out of the receiving vessel before filling with beer with the Soda Stream and with the CO2 keg system. I bought an adapter on Amazon that lets me attach to the IN (gas side) of the CO2 cylinder and the adapter has threads for regular soda bottle thread, PLUS, it has a center SS barbed nipple that I have used a common drinking straw to attach and fit through the drilled fermentation bung on my MiniBrew Conical 6.5 gallon fermenter. Point it so use CO2 purging AND to purge the headspace of both the receiving and source container. Remember, as you draw out liquid in racking it has to be replaced with air, so purging the air out with CO2 while emptying is a good idea.

Justin Thyme April 5, 2019 at 1:35 pm

If you bottle your beer it has not finished fermenting.Yes, perhaps in the primary stage, but now, in the bottle it has been mixed with priming sugar which needs to ferment out – with oxygen and yeast – and produce CO2 to fizz it up.
“Plastics are all air-permeable to some degree, so they should not be used for extended storage (i.e. months) or aging of your beer.” That’s also complete nonsense, I’m afraid. I have matured brews up to 14 months in clear PET bottles. I would have tried longer but it tasted too good. If you store in a dark cool place you’re fine.

Jarel January 11, 2020 at 3:47 pm

After the fermentation process is complete, how long do I co2 a 5bbl fermenter to successful illy carbonate

Mark Van Ditta December 12, 2020 at 2:19 pm

To compare filtered kegged commercial beer with non-filtered amateur brewer-made beer demonstrates a fundamental lack of understanding of yeast behavior. Filtered beer stales quickly because there are no viable yeast cells in suspension to scrub O2. Now, compare that filtered beer reality to properly cellared, beer engine drawn, cask-conditioned beer, which has a much greater resistance to staling due to the live yeast culture within it. Yes, cask beer stales, but nowhere near as quickly as filtered kegged beer when exposed to O2. While fermentations above the Crabtree threshold, follow the lag phase -> exponential phase -> stationary phase -> quiescence pattern, they will continue to take up O2 any time it is introduced. It is this yeast behavior that affords bottle-conditioned beer a longer shelf life than filtered, force carbonated, bottled beer. Amateur brewers need to stop chasing the O2 boogyman.

Andy December 17, 2020 at 1:30 pm

> Once fermentation is complete, a layer of carbon dioxide, which is heavier than air, sits above the beer and provides a protective layer in your fermenter. If you don’t disturb the beer, that layer does a good job of protecting your beer as it ages.

Sorry but that is completely wrong. Gases do not stratify, they diffuse evenly over time due to the fact that the individual molecules are never at rest and are in fact zooming around randomly at several hundred m/s. This myth of the “CO2 blanket” needs to be debunked once and for all.

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