Identifying Off-flavours In Beer – Acetaldehyde

This article is the third installment of a mini-series on the most important off-flavours and compounds that a homebrewer should know. Last month we talked about DMS. This month, we discuss acetaldehyde.


Also known to chemists as ethanal, acetaldehyde (a-suh-tal-duh-haid) is a common organic compound that occurs naturally in foods such as coffee, ripened fruit, and bread, but also shows up in things like gasoline, car exhaust and cigarette smoke. As an off-flavour in beer, acetaldehyde will taste of green apples or notes of hard cider. It is easier to perceive in lighter coloured beers with lower ABV but can be harder to detect in darker beers that use larger quantities of roasted malts.

While green apple and cider notes don’t sound all that bad, in higher quantities acetaldehyde can be observed through other sensory perceptions in the form of headache, dizziness, or feeling the “hangover” effects too soon after only a moderate amount of alcohol has been consumed. The main reason for that is acetaldehyde is the very same hangover toxin that our body creates as it metabolizes and breaks down alcohol. All the more reason to eliminate it in our beer when possible. Who wants to feel crappy after just one beer?

Acetaldehyde can appear in your finished product through one of three ways so it is important to know each of these possibilities and solutions to eliminate its presence.

Acetaldehyde in green beer

Acetaldehyde is created naturally by yeast during fermentation as a precursor compound when they first start digesting the glucose around them. Later, they will come back and and finish converting it all into ethanol. The other compound that happens to be created during this stage is diacetyl, which should give you a clue to the solution for mitigating acetaldehyde at this stage.

To eliminate the risk of getting this off-flavour in your finished product, simply allow your beer to finish fermenting and perform the standard diacetyl rest of seven days once terminal gravity has been reached. Racking your beer out of primary too soon means the yeast can’t finish their job. Not only will you not get the full amount of alcohol conversion and the full, rich flavour of a mature beer, but you will probably end up with a cidery, butterscotch bomb that will likely give you an unpleasant headache. So don’t rush your process.

If you have the ability to do a forced diacetyl test as discussed in the earlier post on diacetyl, that would be the best way to ensure you have eliminated both compounds from your finished product. Otherwise simply plan on leaving your beer in primary for about two weeks from the moment you pitched your yeast. You’ll have about three to four weeks before you start having issues, so don’t fret and don’t rush.

Acetaldehyde from yeast autolysis

The second likely source of getting this compound in your finished brew is through the process of autolysis – dead yeast cells breaking down and releasing various compounds and off-flavours, including acetaldehyde. Green apple will be the least of your worries though, as other “yeast-bite” flavours such meaty, metallic, brothy, raw dough, sulphur, and even dirty diaper can appear. Needless to say, if you are perceiving green apples and cider along with any of these other flavours, then you will know what went wrong.

Where above we discussed what happens when you rush the fermentation process, yeast autolysis is what happens when you wait too long to transfer your beer. We’re talking four to five weeks (or more) in the primary sitting on the yeast cake. The easiest solution – just rack into your secondary or bottling vessel as soon as your diacetyl rest is complete.

If you are fermenting in a conical fermenter, you have a greater risk of having yeast autolysis occurring than a flat-bottomed vessel on account of the greater concentrated pressure on the yeast cells at the bottom of the cone, but realistically this only becomes a serious concern when dealing with large conical fermenters on a commercial scale. If you do ferment in a conical fermenter, and you can’t get your beer racked off the yeast once the diacetyl rest is complete, then you have the option to dump the yeast gunk (a.k.a. trub) from the cone’s bottom arm.

Acetaldehyde from dissolved oxygen

The last possible way this might creep into your brew is from an excess of dissolved oxygen (DO) occurring before and/or after fermentation. The typical oxidation weak points that homebrewers encounter include racking, transferring, and bottling, so be sure to avoid splashing during these steps and work quickly to minimize exposure time. Many brewers now go straight from primary to their bottling vessel, completely skipping the secondary vessel as a way to limit oxygenation and acetaldehyde development. If you have access to CO2 to purge your vessels and bottles that can help, but it is expensive and unrealistic on some systems, especially glass fermentors and barrels where it can be very dangerous if not handled correctly.

On a commercial scale, the risk of getting too much DO usually comes from over-oxygenating wort with pure O2. If you are experimenting with oxygenating with pure O2 on a homebrew system, be sure you are calculating your dosage rates appropriately.

You should note that acetaldehyde from DO doesn’t usually show up until later as it slowly breaks down ethanol back into acetaldehyde while the beer sits in bottles, cans, or kegs. Consuming your beer quickly at its freshest is the best way to mitigate this process. If you are aiming to age your beer, do your best to eliminate and reduce oxidation as much as possible before storing. If you are shipping bottles off for judging, make sure they are as fresh as possible.

David Boggs has been homebrewing for over 10 years and currently works as a professional brewer in Edmonton. 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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