5 Off Flavours Caused By Beer Yeast (And How To Prevent Them)
As a brewer, the last thing you want to hear is a complaint about off-flavours in your beer.
We all strive to create the perfect pint. However, sometimes things don't go as planned, and we end up with off-flavours that can ruin our hard work. While there are many factors that can contribute to these flavours, yeast management is often a culprit.
In this article, we will discuss the five most common off-flavours caused by yeast in beer:
- Diacetyl
- Acetaldehyde
- Sulfur
- Phenols
- Fusel alcohols
We'll also provide suggestions on how to prevent and fix these issues.
To learn more about any of these, we recommend digging into our Knowledge Base, Blog, and YouTube channel!
Diacetyl
Diacetyl is a buttery or butterscotch-like off-flavour that can be caused by a few different factors, including yeast. This flavour is often associated with lagers and English ales but can appear in any beer style. Diacetyl is a byproduct of yeast metabolism, specifically during the process of fermentation. The yeast produces diacetyl as a precursor to other compounds, which are then used to create the characteristic flavours and aromas of beer. However, if the yeast is stressed or the fermentation temperature is too low, diacetyl can remain in the beer and cause off-flavours.
To prevent diacetyl, it's important to ensure that your yeast is healthy and has enough nutrients. Consider using a yeast nutrient or oxygenating your wort to promote healthy yeast growth. Additionally, it's crucial to monitor your fermentation temperature and avoid any sudden drops or fluctuations. If you do end up with diacetyl in your beer, a common solution is to perform a diacetyl rest. This involves raising the temperature of the beer slightly towards the end of fermentation, allowing the yeast to consume any remaining diacetyl.
Note that different people have wildly different sensitivities to diacetyl. A forced diacetyl test can help you if you are not sensitive to diacetyl.
For more information on diacetyl including troubleshooting diacetyl in beer, check out our Knowledge Base article on diacetyl as well as our YouTube video on diacetyl.
Acetaldehyde
Acetaldehyde is a green apple-like flavour that can be caused by yeast. Like diacetyl, acetaldehyde is a byproduct of yeast metabolism during fermentation. However, in this case, acetaldehyde is created as an intermediate step in the conversion of sugars to alcohol. Normally, the yeast will consume the acetaldehyde quickly, but if there is an insufficient amount of yeast or the fermentation temperature is too low, acetaldehyde can build up in the beer.
To prevent acetaldehyde, ensure that your yeast is healthy and has enough nutrients. Also, ensure that the fermentation temperature is appropriate for your yeast strain and that there is sufficient oxygenation. If you do end up with acetaldehyde in your beer, the best solution is to allow the beer to age longer. The yeast will eventually consume the acetaldehyde, but it may take a few extra days or weeks.
It is known that nutrient supplementation, especially addition of zinc, helps with reduction of acetaldehyde in beer. Zinc plays a key role in yeast metabolism because it is a cofactor in key fermentation enzymes. In particular, the alcohol dehydrogenase enzymes help convert acetaldehyde to alcohol require zinc as a cofactor.
Sulfur
Sulfur is a common off-flavour in beer and can be caused by several different factors, including yeast. The yeast produces sulfur compounds during fermentation as a byproduct of metabolism. These compounds can give beer a distinct aroma, often described as rotten eggs or burnt matches. While small amounts of sulfur can be desirable in some beer styles, excessive amounts can be unpleasant. Remember the early days of hard seltzers, and all the sulfury seltzers on the market? This was likely due to a lack of nutrients for the yeast.
To prevent sulfur off-flavours, ensure that your yeast is healthy and has enough nutrients. Additionally, ensure that your fermentation temperature is appropriate for your yeast strain and that there is sufficient oxygenation. If you do end up with sulfur off-flavours in your beer, there are a few different options. One solution is to allow the beer to age longer, as the yeast will eventually consume the sulfur compounds or they may otherwise flash off. Another solution is to use a yeast strain that produces fewer sulfur compounds. A third option is to use an oxidizing agent such as a small piece of copper pipe during the transfer of the beer.
A key way to reduce the risk of sulfur is to match the FAN and nutrients in your wort to your yeast. You can use our FAN Utilization data here as well as our guide to troubleshooting sulfur in order to optimize this for each yeast strain you use.
Phenols
Phenols are a group of compounds that can give beer a range of off-flavours, including medicinal, band-aid, and clove-like flavours. These off-flavours are often caused by yeast and are produced during fermentation. Phenols can be produced by yeast when they are exposed to chlorine (in water), and also in response to the availability of phenol precursors from malt or other ingredients.
To prevent phenolic off-flavours, ensure that your yeast is healthy and has enough nutrients. Additionally, ensure that you are not exposing your yeast to chlorine or other halogens, as these can cause phenolic off-flavours. Finally, ensure that you are fermenting your beer at an appropriate temperature for your yeast strain. If you do end up with undesirable phenolic off-flavours in your beer, it can be sometimes addressed through aging or through masking by other ingredients such as fruit.
Check out our Knowledge Base article on malt precursors to phenolic compounds for more information.
Fusel Alcohols
Fusel alcohols are a group of compounds that can give beer a range of off-flavours, including hot and solvent-like flavours. These off-flavours are often caused by yeast and are produced during fermentation. Fusel alcohols can be produced by yeast when they are exposed to too much heat, when they have access to too much nutrient (FAN), or when they are stressed.
To prevent fusel alcohol off-flavours, ensure that your yeast is healthy and has enough (but not excessive) nutrients. Additionally, ensure that you are fermenting your beer at an appropriate temperature for your yeast strain and that you are not exposing your yeast to too much heat. If you do end up with fusel alcohol off-flavours in your beer, the best solution is to allow the beer to age longer. However, depending on the severity of the off-flavour, it may be best to dump the batch and start again. If you consistently get fusels from a certain recipe, we recommend reducing the FAN content of the wort by substituting some of the malt for sugar (sucrose or dextrose).
Conclusion
Yeast is a crucial component in beer production, but it can also be a cause of off-flavours if not managed properly. By ensuring that your yeast is healthy and has enough nutrients, monitoring your fermentation temperature, and avoiding exposure to certain compounds, you can prevent many of the common off-flavours caused by yeast. However, if you do end up with an off-flavour in your beer, don't panic. With a bit of patience and some careful attention to your brewing process, you can often fix the issue and produce a delicious, high-quality beer.
Do you want to learn more about beer fermentation? Escarpment Labs prioritizes creating and sharing our knowledge. Check out these resources:
- Our blog (www.escarpmentlabs.com/blog), with more than 100 informative posts including how to do a Forced Diacetyl Test and how to ensure success with Kveik yeast.
- Our Knowledge Base (www.escarpmentlabs.com/knowledgebase) with over 80 entries covering topics including Yeast Oxygen Requirements, Yeast FAN (free amino nitrogen) Requirements, and many Troubleshooting articles.
- Our YouTube Channel (www.escarpmentlabs.com/youtube) with more than 20 hours of educational brewing content and brewer interviews including our Yeast Basics series (Part 1 and Part 2) with more information on diacetyl, acetaldehyde, and fusel off flavours.
Yeast Nutrients help reduce the risk of off flavours
We encourage the use of Yeast Lightning Nutrient, which is specifically formulated for the unique nutrient needs of beer yeasts. Yeast Lightning Nutrient will help reduce the risk of off-flavours by ensuring a complete fermentation and healthy yeast. 
