Identifying Off-Flavours in Beer: Oxidation

This could be the most common fault found in beers, both commercially and in the homebrew community. Unless you are dealing with an elegantly aged Lambic or barley wine, where light oxidation can bring out a complex wine-like character, oxidation can and will always be considered a flaw.

Oxidation is described as tasting of cardboard, wet paper, sherry, almond-like, or simply stale, depending on what compounds oxygen molecules bonded or reacted with. There are numerous routes that O2 and Dissolved Oxygen (DO) can enter your batch throughout various stages of the brewing process. Knowing these will not only ensure a better tasting final product, but can also help increase shelf life.


The malting and kilning process creates the essential diastatic enzymes and starches we depend on for a proper fermentation along with the flavours, aromas, and colours that are critical in determining our target style. While fairly innocuous, there are a handful of degrading enzymes and oxidases (thiol oxidase, ascorbate peroxidase, ascorbic acid oxidase, etc.), along with oxygen-sensitive melanoidins that are also formed.

Thankfully, most of these beer-degrading enzymes are mitigated through proper aging and storage before malt is packaged and shipped out. The few enzymes that make it through are only activated in more alkaline environments which your target pH in the 5.2–5.6 range handily avoids, and higher temperatures that you can avoid by staying below 70°C (158°F).

As for those oxygen-sensitive melanoidins, you will need to eliminate as much aeration in your mash regimen as possible.

Mashing (hot-side aeration)

To reduce hot-side aeration, you should aim to eliminate excessive stirring of your mash, and limit the splashing during your recirculation phase. Most brewers will know that messing with your mash too much is bad for the grain bed, but any excessive splashing can and will increase the amount of DO in your wort.

Something worth noting: the rate of oxidation accelerates as the temperature goes up, while the solubility of O2 goes down. This is great news when it comes to the boil, because no new O2 molecules can be introduced. But any O2 that was introduced during your mash will quickly bond with those oxygen-sensitive melanoidins mentioned before, resulting in sherry or almond-like flavours in the final product. 

Post-Boil Aeration

It should also make sense that the rate of oxidation goes down significantly when the temperature drops, as does the solubility of O2. This is why it is generally considered safe to introduce O2 at this stage, but also why you need to ensure you get that temperature down as quickly as possible post-boil. Allowing your brew to slowly cool down leaves it in a critically sensitive range for oxidation and oxygen exposure, not to mention leaving it vulnerable to unwanted yeast and bacteria.

If you are oxygenating by shaking or other form of agitation, or even an air-compressor that aerates with the help of a HEPA filter, there is little chance you can over-oxygenate. If anything, you run the risk of under-oxygenating, so don’t be shy. You don’t want to have a stalled or stuck fermentation.

If you are oxygenating with pure O2 or O2 blend, then it is certainly possible to over-oxygenate. Be sure to research what oxygenation rates are best for your system size. Any O2 not consumed by yeast during fermentation will remain in the form of DO to the final product and will impact shelf life and staling.

Any aeration after this stage is classified as cold-side aeration.

Racking and Transferring

Just as excessive splashing can aerate your wort during the mash, the same holds true during this stage. It is virtually impossible to eliminate O2 exposure 100% when performing these tasks, and the best tool you have is to go slow, steady, and avoid splashing.

Most home brewers have eliminated the stage of transferring to a secondary vessel from their regimen. Unless you are aging your beer on oak or some fruit that you didn’t want in the primary, then there is no need to rack to a secondary vessel before bottling. It unnecessarily exposes your beer to more O2 than is reasonable.

In commercial settings fermentors and brite tanks, as well as the transfer lines that connect them are fully purged of air with the aid of CO2 and bleed valves, respectively. If you have the ability to do this on your system, then wonderful, but most of us won’t have these capabilities. If you have the ability to blanket a fermentor or tank with a layer of CO2, that is also wonderful, but be very careful not to put pressure on glass carboys or wood barrels, and be mindful of the risks related to being exposed to high concentrations of CO2.

Dry Hopping

Dry hopping can introduce unwanted O2 to your beer. While hops are a great preservative and can increase shelf-life, this fact is somewhat nullified when dry-hopping. Try your best to eliminate splashing, how long you have your lid off, etc. Any beer that is dry hopped will not be a good candidate for long-term aging and should be consumed within three months before strong oxidative flavours have the chance to take hold.

Bottling, Canning, or Kegging

Just as with racking and transferring, it is virtually impossible to eliminate O2 exposure completely during the packaging process. In commercial settings bottles, cans, and kegs are purged with CO2, but you probably won’t have this option as a homebrewer. If you do keg, do your best to purge with some CO2.

If you are bottling, do your best to reduce splashing and gurgling during the fill. Leave only about 2 cm of head space — too much head space can lead to oxidized, flat beer, and too little head space can result in a bottle bomb. Chances are you will be carbing your beer with a little added dextrose here, and the yeast will happily reduce the amount of O2 that is trapped.

DO and O2 in the Final Product

As DO sits in your beer or new O2 finds its way into your packaged product, it will slowly degrade lipids causing the cardboard, wet-paper, and overall stale flavours. You can minimize this process by keeping your beer cold. Commercial breweries aim to store beer as close to 1° C (33/34°F) as possible without going below the freezing point. You likely won’t have cold storage capabilities of this nature, so store your beer in the fridge or a cold basement or closet.

Homebrew competitions typically require entries to be submitted in bottles, so it is worth noting that bottle caps aren’t as efficient at keeping O2 out as cans or kegs (part of the reason many breweries are switching to cans – they are safer to package into and the product is more shelf stable). Knowing this, you should always enter a freshly brewed batch and avoid submitting something older than 3 months.

It is impossible to eliminate O2 completely, even in a commercial setting. So just do your best to mitigate exposure and drink you your brew within 3-6 months. As always, take notes and refine your process where applicable. Happy brewing!

David Boggs has been homebrewing for 15 years and has worked as a professional brewer in Denver, CO, and Edmonton, AB. He holds a certificate in Applied Craft Brewing and is a Recognized BJCP Judge. He is a fan of Belgian ales and anything barrel-aged.

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