Does your beer taste frustratingly like cheap booze or have a hot or sharp alcohol flavour?
Fusel Alcohols are a common group of off-flavour compounds produced not by choice, but by necessity, often tasting like cheap booze associated with a burning sensation and headaches. Like many other off-flavours, yeast cells do not like fusel alcohol and aldehydes. THESE COMPOUNDS ARE TOXIC TO CELLS! Similar to us, if our bodies have to produce something toxic or negative, our bodies will find a way to compensate for it by either making it tolerable or removing it.
When we have a Fusel Alcohol flavour in a beer, it means 2 things:
- We placed the cells in a situation where they had to make them (controlled via malt and nutrients selection)
- We did not provide the cells with the resources to compensate or remove the fusel alcohol (controlled via nutrient and wort aeration)
Today we are going to explain why yeast cells produce fusels, how they can be avoided, and how we can maximize their removal.
We are going to get technical today - Don't want to read this whole article - it's all good.
If you are having Fusel Alcohol issues in your beer here is what you should do:
- Increase your wort dissolved oxygen- will provide the cells with more energy to help clean Fusels up and eliminate other associated off flavours
- Add yeast nutrients (like Yeast Lightning) - will provide more building blocks to the cells circumventing the fusel alcohol production process
- Ferment colder to slow fermentation reducing fusel production
- Add more yeast - fusel alcohols occur primarily due to cell growth. More initial cells = less cell growth = fewer fusel alcohols
- Make sure you remove the trub (hot and cold break) - especially in lighter beers
Now that you know what to do to solve most fusel alcohol issues, let's dive deeper into the why. It can be a little complicated but you are smart — you can do it!
What are Fusel Alcohols & How are they Formed?
Fusel alcohols (also known as Higher alcohols or Fusel Oils) are an off-flavour common in beers with stressed fermentations, especially those with either high or low levels of Free Amino Nitrogen (FAN). FAN is the Amino Acid source found in the wort mainly contributed by the malt. Fusel alcohols taste like hot or burning alcohol commonly associated with cheap spirits. Fusel alcohols are an important precursor for esters, a positive fermentation flavour, however, when unreacted they negatively impact flavour. Fusels are always present in low quantities in beer - below detectable thresholds in healthy fermentations.
Fusel alcohols are a group of larger carbon-chain alcohols that are normally described are harsh or burning-like in flavour in moderate concentration. At higher concentrations, they can come across as paint thinner or solvent-like.
Fusel Alcohols are formed as a natural part of yeast growth and fermentation mainly being a byproduct of the degradation or biosynthesis of Amino Acid needed for cell growth and repair. Fusel alcohols are formed through a pathway known as the Ehrlich pathway.
Before we explore this topic, some background.
Amino acids are the building blocks (think Lego or your builder brick of choice) for making proteins (cellular machines) that allow the cell to do anything it needs. Each protein will require a different set of Amino Acids which the cells gains by either absorbing them from their environment (FAN) or creating them internally (biosynthesis). Depending on the wort composition (FAN content of the wort) and needs of the cell line, one or both of these pathways will be utilized to gain the needed Amino Acids. Additionally, the nitrogen found in Amino Acids can be removed and repurposed to make not only proteins but DNA, RNA, and other essential components within the cell.
The Ehrlich Pathway
Amino acids provided to the cell are frequently stripped of their nitrogen (deaminated) to allow the cell to produce other needed Amino Acids, DNA, or other essential components. The problem is that the resulting waste product (alpha-keto acid) needs to be dealt with and broken down into something either less toxic or ideally more useful to the cell.
Alpha Keto Acids are broken down in two steps
- Decarboxylation(CO2 removal): produces a Fusel Aldehyde which is toxic to both yeast cells and humans. (note: one of the causes of hangovers in humans is aldehydes)
- Reduction: The Fusel Aldehyde is reduced via NADH producing a Higher or Fusel Alcohol. NADH stands for "nicotinamide adenine dinucleotide (NAD) + hydrogen (H)" and is found abundantly in properly aerated yeast cells. In cells that are poorly aerated, NADH is limiting, reduction will not occur resulting in the more toxic and negative-tasting Fusel Aldehyde remaining. This is the same methodology as how Acetaldehyde is reduced: see our article TROUBLESHOOTING - ACETALDEHYDE
The Ehrlich pathway is also driven by Amino Acid biosynthesis or the creation of Amino Acids from sugar (glucose) through Alpha-Keto Acids. The reasons the cell does this are unclear, however, the prevailing hypothesis is that it is done to increase ester production (very important to species survival in the wild) in the absence of Amino Acids.
Depending on the Amino Acid broken down or synthesized, we will see different fusel alcohols be produced having more or less a negative impact on the finished flavour of the beer.
How are Fusel Alcohols Broken Down or Esterified?
In Saccharomyces, Esters are typically produced through the reaction of an alcohol (ethanol OR any Fusel Alcohol) and Acetyl-CoA (or its derivatives).
To maximize the esterification of Fusels, we need to maximize the amount of Acetyl-CoA.
Acetyl-CoA is an essential molecule used within the cell mainly associated with the TCA cycle which is maximally activated when the cell has access to oxygen. If no oxygen is present, Acetyl CoA levels will stay low and the negative-tasting fusel alcohols will remain.
Pyruvate Oxidation into Acetyl CoA. This reaction primarily occurs when Oxygen is present within the cell
In order for us to esterify any fusels present - we need acetyl CoA to break them down and the best way to increase Acetyl-CoA is to ensure proper wort aeration levels.
💡 Fewer Fusels = More Acetyl-CoA = More Oxygen
To find out more about esters see here: Troubleshooting Esters (coming soon).
How to Control This Pathway?
We have 2 options for control
Minimizing the creation of fusel alcohols
- If the cell has the Amino Acids via FAN it already needs, then this pathway is diminished.
- If the cell has too little FAN, this pathway lights up to enhance Amino Acid Synthesis (biosynthesis-driven fusel alcohols)
— In my experience, most Fusels are due to a lack of available FAN or specific Amino Acids
— Adding yeast extract like Yeast Lightning will fulfill the requirements of the cell and reduce Fusels
- If the cell has too much FAN, this pathway will light up to enhance growth (Deamination driven fusel alcohols
— If the beer is over 7% ABV, then you might need to reduce the FAN through the use of sugars
- It's a Goldilocks situation
Maximizing Esterification of Fusels into Esters
1. Provide the cells with more oxygen to enhance Acetyl-CoA levels and enhance esterification of the produced fusels
— See our resources on Wort Aeration
Anything else to note about Fusels? Any edge cases?
There is one large edge case or oddity to note.
Scenario: Your beer sometimes tastes boozy but sometimes just strangely fruity but not in a good way (almost spoiled). It changes from batch to batch. The strangely fruity batches also tend to give more hangovers.
Explanation: This is an example of poor wort aeration and likely zinc deficiencies. The “strangely fruity” flavour is not esters but aldehydes associated with the above. They are variably being broken down due to a lack of NADH (lack of wort aeration) resulting fusels sometimes and aldehydes the other. Adding more Zinc helps the enzyme associated with aldehydes fully break them down into fusels which can then be esterified with proper wort oxygen levels. See TROUBLESHOOTING - ACETALDEHYDE for more information (same mechanics apply).
Questions we will ask to help troubleshoot a Fusel issue:
- What is the malt profile of your brew?
- How do you aerate your wort? How long, what pressure, what flow rate
- How do you clean your O2 stone
- Do you supplement fermentations with zinc or other yeast nutrients such as Yeast Lightening?
- Has this issue been present since early generations or is it getting worse with subsequent generations?
Want to learn more?
Fusel alcohols are a complicated topic as they involve multiple systems all working together.
For more information see our knowledge base articles here
Hope this was helpful and happy brewing!