Troubleshooting - Fusel Alcohols (Fusels)

What are fusel alcohols?

Fusel alcohols (also known as Higher alcohols, fusels or fusel oils) are an off-flavor common in beers with stressed fermentations, especially those with inappropriate levels of Free Amino Nitrogen (FAN). FAN is the amino acid source found in beer wort, mainly contributed by the malt. Fusel alcohols taste like hot or burning alcohol ("boozy") commonly associated with cheap spirits. At high concentrations, they can come across as paint thinner or solvent-like. Fusel alcohols are an important precursor for many desirable esters. However, when unreacted, they can negatively impact flavour. Fusels are always present in low quantities in beer and are typically below detectable thresholds in healthy fermentations.

How are fusel alcohols formed?

Fusel Alcohols are formed as a natural part of yeast growth and fermentation. They are a byproduct of the degradation or biosynthesis of amino acids needed for cell growth and repair. Fusel alcohols are formed through a pathway known as the Ehrlich pathway.

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) must be dealt with and broken down into something less toxic or more useful to the cell.

Alpha Keto Acids are broken down in two steps:

1. Decarboxylation (CO2 removal): produces a Fusel Aldehyde which is toxic to both yeast cells and humans. A note: one of the causes of hangovers in humans is aldehydes!
2. 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. This means 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.

This 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.

How can fusel alcohols be broken down?

In Saccharomyces yeast, esters are typically produced through the reaction of an alcohol (ethanol OR any Fusel Alcohol) and Acetyl-CoA or a fatty acid (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 (respiration) which is 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.

In order for us to esterify any fusels present, we need Acetyl-CoA to break them down. The best way to increase Acetyl-CoA is to ensure proper wort aeration levels.

How do I control fusel alcohol production in beer?

1. Control FAN levels
   1. Too much FAN results in more Fusels as excess Amino Acids are stripped of their nitrogen and converted into new fusel alcohols.
      1. Solution: when brewing high gravity worts over 17ºP/1.065, swap some of your malts for sugar or lower FAN adjuncts. Increasing oxygen will also ensure more fusels are converted into esters.
   2. Too little FAN results in more fusels as a result of biosynthesis.
      1. These fusels tend to be more pungent as well as desirable Amino Acids are stripped of their nitrogen producing more pungent tones
      2. Solution: When brewing standard and low gravity worts under 17ºP/1.065, make sure to use a complete yeast nutrient like Yeast Lightning to add FAN as well as other essential cofactors for optimal yeast health.
2. Maximize Fusel Alcohol Esterification
   1. Ensure proper wort aeration to maximize Acetyl-CoA
      1. See BEST PRACTICES - OXYGENATION
   2. Use of Dextrose (Glucose) has also been shown to increase Acetyl CoA concentrations, decreases fusels, and increases esters.

What Else Impacts Fusel Alcohol Production?

1. Fermentation Temperature
   1. Higher temperatures result in more fusels. This is thought to be due to the faster rate of consumption of FAN and sugar resulting in the cell working less than optimally.
2. Trub presence in final wort
   1. Trub (hot or cold break) has been shown to result in faster fermentation and cause a slight increase in fusels. It is unclear if the increase in fusels is due to the faster fermentation or directly due to the presence of trub.
3. Too little or too much FAN
   1. If too little, add Yeast Nutrient like Yeast Lightening to your whirlpool to enhance FAN content
   2. If too much FAN, swap out high-protein grains for low-protein grains or sugars
      1. Only prominent in high ABV all-grain worts
4. Dissolved CO2 (e.g. pressurized fermentation, larger fermentors)
   1. Increased CO2 levels have been shown to decrease fusels alcohol production, the reason for this is unclear but likely due to a mixture of slower fermentation and less cell biomass creation.
5. Pitching Rate
   1. Low pitch rate = more cell growth = more fusels. However, some yeasts do not seem to be impacted such as kveik, which also have high capacity for esterification, likely mitigating fusel issues.
6. Insufficient Magnesium (Mg2+) can cause a decrease in pyruvate dehydrogenase (enzyme that produces Acetyl-CoA) resulting in lower esterification of fusels
   • This is also often coupled with decreases in attenuation and poor flocculation, especially in lager yeast.

How do I know I have Fusels?

1. Your beer tastes more boozy than it should (or than you want).
2. Your beer (often variably between batches) develops an over-ripe fruit or boozy characteristic.
   1. Fusel aldehydes can taste like certain fruit. For example, propanal and pentanal have a characteristic melon-like aroma. This happens when the cell does not have enough NADH to reduce the fusel aldehydes into fusel alcohols.
   2. Commonly occurs due to insufficient wort aeration, low pitch rate, very low FAN or Zinc deficiency (especially if this occurs post dry hop).

Questions for Troubleshooting a Fusel Alcohol Issue

1. What is the ABV of the beer and is the wort all grain or contain any adjuncts?
2. How do you aerate your wort? How long, what pressure, what flow rate
3. Has this issue been present since early generations or is it getting worse with subsequent generations?
4. What beers does this effect? Is it all beers or only a few?
   1. Are there any common ingredients or processes associated with the affected brews

More Resources

1. The Ehrlich Pathway for Fusel Alcohol Production: a Century of Research on Saccharomyces cerevisiae Metabolism
2. Factors affecting the formation of fusel alcohols during fermentation.
3. Diagnostic Key Classes of Off-Character Compounds Odor Impact Amino Acid Derivatives Found in Wine