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Beer Aeration Oxidation and Mitigation

Saturday, August 22nd, 2009

Oxidation is not the same as aeration. Aeration is the process by which oxygen is introduced to your beer or your wort, oxidation is the outcome, it is what happens to your wort or beer when exposed to oxygen at the wrong point in the process. Aeration of COOL wort (<80F) prior to fermentation beginning, is the only safe type of aeration.

Aeration of HOT wort (>80F): Aeration of hot wort will cause oxygen to bind with various wort compounds. Over time, these compounds will break down, releasing oxygen into the beer and oxidize hop compounds and alcohol, leaving off flavors. These flavors taste like wet cardboard or Sherry.

Aeration during fermentation: Likewise, introducing aeration after fermentation has begun can cause yeast to create more diacetyl in your beer (buttery flavor or smell).

Aeration of finished beer: Aeration after fermentation is complete will cause more rapid staling of your beer, loss of long term stability.

Below we will discuss where in the process aeration and subsequent oxidation can occur and how to mitigate these dangers.

How can aeration/oxidation occur?

  • Whipping up a froth while stirring your hot mash (>80F)

  • Whipping up a froth while chilling your wort after the boil (>80F)

  • Pouring hot wort from one vessel to another causing splashing (>80F)

  • Transferring hot liquids through spigots without tubing to reduce splashing (>80F)

  • Agitating the fermenter AFTER primary fermentation has begun (any temperature)

  • Agitation after fermentation is COMPLETE (any temperature)

  • Bottling, leaving too much head space and O2 in the bottle

  • Not purging the head space in the keg when kegging beer

How to mitigate the danger of aeration/oxidation:

  • Stir your mash gently

  • Stir your cooling wort gently, create a whirlpool effect

  • Do not pour hot wort from one vessel to another, it is dangerous AND can cause oxidation

  • Use high temperature tubing to transfer hot wort from one vessel, to the bottom of another

  • Do not agitate fermenting wort

  • Do not agitate or splash fermented wort, use a siphon or a spigot and tubing to transfer beer

  • Leave as little head space as possible when bottling (1” usually)

  • Use O2 absorbing caps when bottling

  • Purge kegs with CO2 prior to racking beer into them, and purge the head space with CO2 after the keg is sealed.

  • Use spigots, siphons and hoses to allow gentle movement of hot or fermented fluids

Mentioned above is the issue of beer becoming oxidized through the bottling process. This can be caused by improper (turbulent) transfer of the finished beer from the fermenter to the bottle, or by leaving too much head space in the bottle. One way to mitigate the oxidation danger of head space in your bottles is by the use of O2 absorbing bottle caps (called Oxycaps).

These caps have a special liner in the underside of the cap which absorbs and sequesters O2 that is left in the bottle after capping. These caps are easy to come by and are only slightly more expensive than standard bottle caps ($1-$2 more per gross) Oxidation from bottling may not be an issue in bottled home brew if you consume your beer in a timely manner. However, for extended aging and storing these caps are cheap insurance to preserve your creation over time.

  1. 3 Responses to “Beer Aeration Oxidation and Mitigation”

  2. So I have a quick question. I thought when oxygen was present yeast produced CO2 (aerobic respiration) and when oxygen was absent yeast produced alcohol(anaerobic respiration). So if the beer has finished fermenting there should be little to no O2 present, wouldn’t you want to/ need to aerate the beer a little before the bottling process in order to produce CO2?

    By Eric on Aug 1, 2011

  3. According to, yeast make CO2 in both modes of respiration. According to this fermentation equation, O2 is not involved in the breakdown of glucose (sugar) into alcohol and CO2:

    C6H12O6 → 2 C2H5OH + 2 CO2

    What I am sure of is that exposing beer to air is bad for it and leads to oxidation problems. Do not aerate your beer before packaging it! It will taste like cardboard in a month.

    By Larry on Aug 1, 2011

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