Back to Ask the Brewmaster.

Question: My beer sometimes has an green apple-like solventy taste and smell. What is it? What can I do about it?

Answer: A green apple-like smell and flavor is usually due to a specific chemical compound called acetaldehyde. The compound has the chemical formula CH3CHO. You may notice that this is not a lot different than ethanol, which has the formula CH3CH2OH. Acetaldehyde, like a lot of other compounds, is almost always present at trace levels in beer. Under certain circumstances, the levels can rise up above perception levels. Some pale American lagers such as Budweiser for example, have low levels as part of their flavor profile. In some homebrew cases, however, the amount gets out of control and the flavor is so strong the beer becomes undrinkable.

Acetaldehyde is usually caused by late-stage oxygenation of beer. Ethanol can pick up an oxygen atom and split off a water molecule according to the following reaction:

CH3CH2OH + 1/2O2 --> CH3CHO + H2O.

When I detect acetaldehyde in a competition beer, I usually notice that the bottle has been filled by a counter-pressure filler and has a low fill level. This combination of factors allows oxygen from the air to get into finished beer and trigger the oxidation process. Occasionally, I'll notice it in a keg of beer that has lost its pressure seal. Another time I've found acetaldehyde is in beers that have been racked into carboys with too much head space. One particular variation on this is beers that are dry hopped. When adding hops to the secondary, by necessity you have to leave some room for the hops, which are light and airy. I find that Hop Ottin' often has a bit.

You can minimize the generation of acetaldehyde by following these tips:
1) Oxygenate your beer only before fermentation and avoid it at all later stages. (Oxygen is needed early for yeast reproduction).
2) Keep the air lock filled with liquid. (I use cheap vodka.)
3) Minimize head space during racking. I typically run primary fermentation in a 6 ½ gallon carboy and rack into a 5 gallon carboy.
4) Don't allow air bubbles into the racking cane or bottling bucket.
5) Fill bottles and kegs as full as possible.
6) If excess headspace in your keg or fermenter is unavoidable, purge it out with CO2.

So, despite all the precautions, you might occasionally find some acetaldehyde. Is there any hope? Yes, you can reduce the acetaldehyde back to ethanol, as long as the yeast is still active. Just add a bit of priming sugar and the yeast will be able to run the reaction in reverse. You should note that this is exactly what happens during a natural-carbonation bottling session. This explains why acetaldehyde is pretty rare defect in the first place. If you have a keg with acetaldehyde and still has yeast, add some priming sugar and it works there too. Just make sure you check the pressure occasionally and bleed it off if it gets too high. Make sure the pressure doesn't drop all the way to zero, though, or you'll just get more oxygen back in and start the process all over again. With repeated exposure, the beer will eventually get infected beyond repair.

If you are the adventurous type and want to make up a sample of acetaldehyde, such as for beer judge training, try this out: Next time you are bottling beer, deliberately underfill some of the bottles. Try one half-way, another ¾, and a third to the base of the shoulder. Wait about a month or so, and make sure the main batch has time to reach full carbonation. Then, pop one open and see if you can detect the aroma first, and then the flavor. Can you detect it in all three samples?