The issue of peculiar odors in beer has always been a problem for brewers, despite certain beer styles having characteristic odors. However, acetaldehyde is a special case because it can lead to hangovers, which are detrimental to health.
What is Acetaldehyde?
Acetaldehyde is a naturally occurring compound found in coffee, fruits, and beer. It is produced similarly to ethanol during the fermentation process. Healthy yeast strains normally convert acetaldehyde (C2H4O) into ethanol. This usually removes acetaldehyde from the beer, but excessive oxidation can cause it to reappear.
Acetaldehyde is a highly reactive and toxic substance that can damage cells and genomes in the human body. Drinking is the main source of acetaldehyde, so reducing its content in beer is necessary. When acetaldehyde levels exceed the threshold (5-15 mg/L), the beer develops a green apple taste.
Why is Acetaldehyde Produced?
Saccharomyces cerevisiae produces acetaldehyde as an intermediate compound during the conversion of glucose to ethanol. Therefore, acetaldehyde is present in every bottle of beer during the primary fermentation process. In a healthy fermentation, most acetaldehyde compounds are converted into alcohol. Any remaining residues are typically below the flavor threshold. However, if the fermentation process is not ideal, incomplete conversion can result in excessive acetaldehyde in the beer.
Additionally, acetaldehyde is produced through the oxidation of ethanol. For example, when beer is exposed to oxygen after the initial fermentation, ethanol may be oxidized to acetaldehyde. If bacteria are present, they can convert acetaldehyde into acetic acid, exacerbating the problem.
In general, the existence of acetaldehyde usually has the following three root causes:
– The beer is not fully fermented.
– Unhealthy yeast strains with poor performance.
– Excessive oxidation reactions, often during the bottling process.
Other factors to consider:
– The choice of yeast strain. Some strains produce more fermentation by-products, including acetaldehyde.
– Using a yeast starter. Insufficient yeast population can lead to greater acetaldehyde production during the growth phase.
– Fermentation temperature. Certain yeast strains produce more acetaldehyde at lower temperatures.
– Temperature fluctuations during the first few weeks of fermentation. Fluctuations can affect yeast strains and lead to premature flocculation.
– The percentage of auxiliary sugar used. Additives exceeding 20% do not provide enough nutrition for yeast, resulting in more peculiar odors.
– Proper aeration of the wort. Insufficient oxygen content can hinder yeast reproduction and result in off-flavors.
How Does Acetaldehyde Taste?
Acetaldehyde has a distinct taste, resembling the sourness of a green apple. In some cases, it can make beer taste like a very dry cider. While certain beer styles may require a slight apple flavor from acetaldehyde (e.g., Biere de Garde, certain American lagers), it should generally be eliminated during the conditioning process. Excessive acetaldehyde during fermentation can lead to unpleasant off-flavors.
How to Remove Acetaldehyde in Finished Beer?
If you detect acetaldehyde in your beer, there are methods to salvage it:
– Extend the fermentation time by adjusting the beer’s temperature. This allows yeast strains more time to convert acetaldehyde to alcohol. If your beer tastes like a green apple or has not finished fermenting, giving it more time can help.
– If the airlock stops bubbling but the acetaldehyde off-flavors persist, try krausening. This German technique involves brewing a second batch of the same recipe and adding it to the first batch during peak fermentation. The lively second batch revitalizes fermentation in the original brew, leading to further acetaldehyde conversion.
– If your beer still strongly exhibits green apple flavors even after krausening and full fermentation, it is best to start over with a new batch. However, there are steps you can take to prevent acetaldehyde off-flavors in future brewing.
How Can I Avoid Acetaldehyde Overdose in Future Brewing?
Since most of the odor is caused by yeast during fermentation, the primary goal is to promote yeast health:
– Add an adequate amount of yeast and use a starter when necessary.
– Ensure your yeast is healthy and active before use.
– Aerate the wort with air or oxygen before fermentation.
– Control the fermentation temperature.
– Allow fermentation to complete before bottling or adding clarifying agents.
– Minimize exposure to oxygen once fermentation has started.
– Maintain proper hygiene with brewing equipment.
