Acetaldehyde: How To Prevent That Green Apple Off-Flavour

Does your beer taste frustratingly like green apple or pumpkin?

Acetaldehyde is a common off-flavour in beer, typically associated with a “green apple”-like character. Acetaldehyde (and aldehydes in general!) are a tricky group of compounds to understand from a taste perspective as they often taste different to different people. You often hear about acetaldehyde tasting like “green apple” but throughout several tastings I have run where I increase the dosage of acetaldehyde in a set of light lagers, the flavours range from the iconic “green apple” to rotting/over-ripe fruit, smoke, artificial sweetener, a sharp mouth feel, avocado, fresh cut grass, fresh cut pumpkin, and more. These perceived flavour changes occur both by the individual person as the concentration increases and between different people tasting the same concentration.

Aldehydes, in general, are not talked about enough as beer flavour compounds, so the goal today is to shed some light on them so that we can all better understand and identify their presence in beer.

(We are going to get a little technical) - Don't want to read this whole article? Let's take a short cut.

The summary:

• If you have an off-flavour, but you are not quite sure what it is, then you likely have acetaldehyde or other aldehyde issues.
• Aldehydes can occur during primary or secondary fermentation, especially with hop creep.
• Best course of action: add zinc to your wort in the form of zinc sulphate heptahydrate or as a complex yeast nutrient like Yeast Lightning.
• If issues persist with zinc addition, you are likely not supplying enough oxygen to your wort.
  • See our knowledge base article on wort aeration here.
  • If the issue still persists, reach out to your yeast supplier, as you may have wort FAN deficiency or other limiting factors that can be addressed.

If you want to get to the nerdy stuff, keep scrolling!

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What is Acetaldehyde?

Acetaldehyde is an off-flavour common in beers with stressed fermentations. Acetaldehyde is always present in low quantities in beer but below detectable thresholds in healthy fermentations. Classically acetaldehyde tastes like green or under-ripe apples. However, this is only common in light beers. At higher concentrations, it can come across as paint thinner or solvent-like.

 

Acetaldehyde is formed as a natural part of alcohol fermentation being the precursor to ethanol. This means it is always produced, however, it is not always fully broken down!

For acetaldehyde to be broken down, the cell must have available NADH to reduce (react with) the acetaldehyde to form Ethanol. The reduction of acetaldehyde is catalyzed with the enzyme Alcohol Dehydrogenase (ADH1) which requires zinc as a cofactor for proper function.

Anaerobic respiration of glucose to Ethanol. Alcohol Dehydrogenase (ADH1) catalyzes the reduction of Acetaldehyde and Ethanol consuming NADH. 

If NADH levels are low OR zinc is not found is appropriate levels, acetaldehyde will persist and become an off-flavour.

Unlike many off-flavour compounds, the yeast will always produce acetaldehyde. The key is to maximize its reduction.

 

How can I maximize Acetaldehyde reduction?

Maximizing aldehyde reduction comes down to two key factors:

1. Ensuring sufficient NADH is available.

2. Ensuring you have enough zinc.

To better explain why this is, let's use an analogy.

ADH1 is your machine for breaking down aldehydes!

NADH is your electricity or fuel to run ADH1. If you run out of fuel halfway through a job then the job will stay half done, that is it. Proper wort aeration and FAN levels will ensure you have enough fuel to run your machine.

Zinc is the oil for ADH1. If the machine does not have oil then it will consume more fuel to complete the same work using the fuel less efficiently and likely end up damaging itself through use - neither of which are good. If we have zinc, the machine works smoothly, and its consumption of fuel (in the form of NADH) is reduced for each aldehyde molecule it breaks down.

For more detail on how and why this works, see our Knowledge Base article on Troubleshooting Acetaldehyde.

The active site of ADH1 requires Zinc and NADH in order to function and degrade acetaldehyde into ethanol properly.

When else do we see Acetaldehyde?

• Many brewers experience acetaldehyde issues with dry-hopped beers due to hop creep and pressurized fermentation. Both can typically can be remedied by having proper zinc levels in the wort, ensuring ADH1 is functional.

• Large temperature swings can result in aldehyde issues mainly due to the NADH running out as it is used to help adjust the cell membrane composition in response to the temperature change.

• Serial overpitching will result in aldehyde issues mainly due to nutrient limitation. When we pitch twice as much cells, each cell is going to have half as much oxygen and nutrients available to it, resulting in less NADH and zinc per cell.

Questions to ask to help troubleshoot an Acetaldehyde issue:

• How do you aerate your wort? (How long, what pressure, what flow rate)
• How do you clean your O2 stone? (it’s worth digging into this to ensure O2 transfer is efficient)
• Do you supplement fermentations with zinc or other yeast nutrients such as Yeast Lightning?
• Has this issue been present since early yeast generations or is it worsening with subsequent generations?
• Does the acetaldehyde occur at the end of fermentation or only after dry hopping?

Want to learn more?
Acetaldehyde is not the only aldehyde found in beer - far from the only. Other aldehydes are produced in a similar mechanism in fusel alcohol production. Other aldehydes can produce cucumber, melon, or pumpkin-like aroma in beer. If you want to learn more about those aldehydes, see our knowledge base article on Fusel Alcohols.