What is acetaldehyde?
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?
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.
- 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;
- 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?
How to remove acetaldehyde in finished beer?
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?
- 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.
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