Troubleshooting - Acetaldehyde

What is Acetaldehyde?

Acetaldehyde is an off-flavour common in beers with stressed fermentations.

Classically acetaldehyde tastes like green or under-ripe apples. however, this is only common in light beers. In other beers, the flavour impact of acetaldehyde can range from “Fresh Cut Pumpkin”, Avocado, fresh grass, overripe fruit and more. At higher concentrations, it can come across as paint thinner or solvent-like. Acetaldehyde is always present in low quantities in beer but below detectable thresholds in healthy fermentations.

Acetaldehyde molecule

How is Acetaldehyde Formed and Reduced?

Acetaldehyde is formed as a natural part of alcohol fermentation being the precursor to ethanol. It is always produced by yeast but in a healthy ferment, it is converted to ethanol.

For acetaldehyde to be broken down, the cell must have available NADH to reduce (react with) the acetaldehyde to form ethanol. NADH stands for "nicotinamide adenine dinucleotide (NAD) + hydrogen (H)." It occurs naturally in the yeast cell and plays a role in generating energy.

The reduction of acetaldehyde is catalyzed with the enzyme Alcohol Dehydrogenase (ADH1), which requires zinc for proper function. If NADH levels are low OR zinc is not found is appropriate levels, acetaldehyde will persist and become the off-flavour.

Anaerobic respiration of glucose to Ethanol. Alcohol Dehydrogenase (ADH1) is catalyzes the reduction of Acetaldehyde to Ethanol, consuming NADH. Source:
The active site of ADH1 requires Zinc and NADH in order to properly function and degrade acetaldehyde into ethanol. Source:

Unlike many off flavor compounds, the cell will always produce acetaldehyde, the key is to maximize its conversion to ethanol.

How can I maximize Acetaldehyde reduction?

  1. Ensure sufficient NADH is available
    1. Yeast accumulates excess NADH when they are exposed to oxygen (wort aeration)
      1. Sufficient wort oxygen levels are key to ensuring reduction of acetaldehyde
      2. See our article on Best Practices for Wort Aeration.
    2. Proper FAN (free amino nitrogen) and nutrition will ensure available NADH is not consumed through other pathways, preserving it for acetaldehyde reduction.
  2. Adequate Zinc levels ensure ADH1 is functional
    1. ADH1 is a zinc metalloenzyme, meaning, it requires zinc to function properly.
    2. Most worts are deficient in zinc if they are not supplemented properly.
    3. Inorganic zinc (ZnSO4-7H2O) added to the chilled wort or organic zinc (Yeast extract like Yeast Lightning) added at the end of whirlpool supplementation will ensure proper levels are present
      1. See our article on Best Practices for Zinc Additions

When else do we see Acetaldehyde?

  • Many Brewers experience acetaldehyde issues with dry-hopped beers due to hop creep since additional fermentation occurs. If the yeast does not have the resources (zinc and NADH) to fully metabolize acetaldehyde to ethanol, this flavour may persist.
    • Anecdotally, this issue can be remedied with proper wort zinc levels.
  • Pressurized fermentation and taller fermenters have been shown to contain more acetaldehyde in finished beers.

Root Causes of Acetaldehyde in Beer

  • Insufficient wort aeration resulting in a lack of available NADH.
    • Will often be coupled with Diacetyl, fusel alcohols and lower viability.
  • Insufficient Zinc levels.
    • Will often be coupled with slight to moderate under attenuation.
  • Odd fermentation temperature control and large temperature swings.
  • Overpitching results in each cell having less access to oxygen and nutrients and can lead to acetaldehyde issues, especially with Ale yeasts. In particular, Kölsch yeasts are susceptible to overpitching-induced acetaldehyde.

Questions for Troubleshooting an Acetaldehyde Issue

  • 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 Lightning?
  • Has this issue been present since the start of the yeast use or is it worsening with subsequent generations/yeast repitches?
  • Does the acetaldehyde occur at the end of fermentation or only after dry hopping?
  • Do you experience it only with certain yeasts (e.g. certain brands of yeast) or with all yeasts?

More Resources

MBAA Podcast - Troubleshooting Acetaldehyde

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