Beer Odor Guide: Acetaldehyde

The peculiar smell in beer has always been a headache for brewers, although some peculiar smells are characteristic in some beer styles. But acetaldehyde is a special taste, because it may cause you to have a hangover, which is very detrimental to your health.

What Is Acetaldehyde?

Acetaldehyde is a naturally occurring compound, usually found in coffee, fruits, and beer. The production of acetaldehyde is very like the production of ethanol (alcohol), both of which are naturally produced by-products during the fermentation process. During normal fermentation, healthy yeast strains will convert acetaldehyde (C2H4O) into ethanol (alcohol). This usually removes the acetaldehyde in the beer, but the more oxidation reaction will cause the acetaldehyde to reappear.

Acetaldehyde is a highly reactive and toxic aldehyde substance. Acetaldehyde can cause damage to the human body at the cell and genome levels. The main source of acetaldehyde is drinking, so it is necessary to cut the content of acetaldehyde in beer. When the presence of acetaldehyde exceeds the threshold (5-15 mg/L), acetaldehyde will make the beer taste like a green apple.

Why Is Acetaldehyde Produced?

Saccharomyces cerevisiae produces acetaldehyde and acts as an intermediate compound for the conversion of glucose to ethanol. So, acetaldehyde is present in every bottle of beer you make, at least during the primary fermentation process. But, in a healthy fermentation, the yeast strain will completely convert most of the acetaldehyde compounds into alcohol. Also, after a healthy fermentation process, any residues are below the flavor threshold. If the fermentation process is not ideal, the conversion of acetaldehyde to alcohol may be incomplete, and eventually, too much acetaldehyde will remain in the beer.

Also, the oxidation of ethanol (alcohol) produces acetaldehyde. For example, when beer after the initial fermentation is exposed to oxygen, ethanol may be oxidized to acetaldehyde. When bacteria need oxygen to grab acetaldehyde and convert acetaldehyde into acetic acid, the problem becomes more serious.

In general, the existence of acetaldehyde usually has the following three root causes:

• The beer is not fully fermented;

• Yeast strains are unhealthy and have poor performance;

• Excessive oxidation reaction occurs, allowing ethanol to be oxidized to acetaldehyde. This usually happens during the bottling process;

Also, you need to think about other issues:

• What strain of yeast are you using? Some yeast strains produce more fermentation by-products than other yeast strains. Also, some yeast strains flocculate before completing fermentation, resulting in incomplete fermentation and production of acetaldehyde.

• Did you make a yeast starter? A smaller number of yeast strains puts pressure on the yeast, which means that they produce more acetaldehyde during the growth phase of their life cycle.

• At what temperature do you ferment? The low temperature of certain yeast strains also causes them to produce more acetaldehyde.

• In the first few weeks of fermentation, have you experienced temperature fluctuations? Temperature fluctuations can affect the yeast strain and make it flocculate prematurely.

• What percentage of auxiliary sugar do you use? Any additives exceeding 20% can not provide enough nutrition for yeast strains, so they will produce the more peculiar smell.

• Have you fully aerated the wort? When the oxygen content in the wort is low, the yeast strains cannot fully reproduce during the growth stage and produce peculiar smells.

How Does Acetaldehyde Taste?

The taste of acetaldehyde is easy to find, it tastes like the sour taste of a green apple. In some cases, acetaldehyde can make beer taste more like a very dry cider. In beer styles such as Biere de Garde and certain American lagers, a little apple flavor of acetaldehyde is usually required. But in most cases, it should be removed from your beer during the conditioning process. If there is too much acetaldehyde remaining in the fermentation process, unpleasant off-flavors may occur.

How To Remove Acetaldehyde In Finished Beer?

Once you detect the presence of acetaldehyde in beer, you may want to know if the beer can be preserved. The good news is that you can save your beer by trying some methods.

In some cases, the problem can be solved by adjusting the temperature of the beer for a longer period of time. Because most acetaldehyde is converted to alcohol by yeast strains during the fermentation process, extending the fermentation time allows healthy yeast strains more time to complete the fermentation. If your beer contains a green apple flavor or has not yet finished fermentation, you can choose to give your yeast strains a period of time to let them complete the task.

If your airlock stops bubbling and the off-flavors of acetaldehyde are still presenting themselves, the next thing to try is a German technique called krausening. Though the process is not terribly complex, it does involve brewing a second batch of the same recipe you started with. A few days into fermentation, the thick head of bubbles and foam that fills the top of your fermenter is called krausen. When the krausen reaches its peak on the second batch, add it to the first batch. By boosting your beer with the lively second batch, you can reawaken fermentation in your original brew which will, in turn, ferment out more acetaldehyde.

If your beer still tastes strongly of green apple even after krausening and a full fermentation, it’s best to start over with a new batch. The good news is that there are concrete steps you can take to prevent acetaldehyde off-flavors the next go-round.

How Should I Avoid Acetaldehyde Overdose In Later Brewing?

Since most of this odor is caused by yeast during the fermentation process, our main goal is to promote yeast health:

• Add enough yeast to the beer you are brewing, and use the starter when needed.

• Make sure your yeast is healthy and active before serving.

• Aerate the wort with air or oxygen before fermentation.

• Control the fermentation temperature.

• Let your fermentation complete before bottling or adding clear liquid.

• Once the fermentation has started, do not expose the beer to more oxygen.

• The brewing equipment maintains perfect hygienic habits.