Off-flavor Spotlight: Acetaldehyde

Acetaldehyde

Perceived As: Green Apple, Bruised Apple, Latex Paint, Pumpkin Guts, Unripe Avocado

Flavor Threshold: 1-15 ppm

What is Acetaldehyde?

Acetaldehyde is a naturally occurring flavor compound commonly found across a number of different industries.  In addition to beer, it is prevalent in ripe fruits, yogurt, coffee, wine, tobacco smoke, and perfume.  Acetaldehyde is also the byproduct of the body’s processing of alcohol and, notably, the key compound in the feeling of a hangover.  The flavor and aroma of acetaldehyde may seem familiar because it is present in all beers to some degree. Its flavor threshold can be style dependent, and is largely tied to the interaction with flavors from the other ingredients present.  For example, it may be much more noticeable in a pale lager beer than an IPA or stout, even if the levels are the same.  Because of this, understanding where acetaldehyde comes from and how it develops is an important step in recognizing the difference between normal background complexity and a true flavor defect.

How is Acetaldehyde Perceived?

One of the most valuable skills in sensory analysis is learning how to translate what you perceive into the common language used throughout the brewing industry.  Developing this vocabulary helps bridge the gap between subjective sensory impressions and the technical brewing processes that create them. The more fluent you become in connecting aroma and flavor descriptors to their underlying compounds, the easier it becomes to identify root causes, communicate with other industry professionals, and improve beer quality consistently.

This skillset is vitally important with a flavor compound like acetaldehyde, that has such a wide range of commonly accepted descriptors.  The most common descriptors are green apple or bruised apple.  However, other tasters note a latex paint or chemical quality that stands out the most.  Still others identify the flavor as having more green or vegetal elements like the inside of a pumpkin or underripe avocado.  You can improve your ability to perceive acetaldehyde by identifying the descriptors that resonate with you most through continued exposure and familiarization.

How is Acetaldehyde Produced?

Acetaldehyde is often referred to as a ‘green beer’ flavor suggesting that the beer is young and not fully matured. The compound is produced by yeast during their growth phase early in fermentation and serves as the last step on the pathway to alcohol production from wort sugars.  This reaction is facilitated later in the fermentation process by the action of the enzyme, alcohol dehydrogenase. 

Fermentation parameters that encourage yeast growth can lead to greater accumulation of acetaldehyde during the fermentation process.  Under healthy fermentation and maturation conditions, many of these same parameters can lead to rapid removal of acetaldehyde.  However, issues can arise if conditions become suboptimal after higher levels of acetaldehyde have been produced, or if yeast is removed from the process prematurely. 

Some factors that can lead to higher acetaldehyde production by yeast:

• Higher pitch rates
• Higher wort aeration/ oxygenation
• Higher fermentation temperature
• Increased FAN levels

How is Acetaldehyde Controlled or Prevented?

Proper reduction of acetaldehyde takes sound management of fermentation parameters, healthy yeast activity, and time.  Elevated levels of acetaldehyde in finished beer are commonly a result of separating the beer off the yeast in the fermenter too early or not allowing the yeast the time to reduce it to alcohol later in fermentation or during maturation.

Other suboptimal fermentation parameters can also lead to elevated levels of acetaldehyde.  When fermentations conditions change rapidly, and yeast becomes unhealthy or stressed, they can lose their ability to process acetaldehyde into alcohol, leaving a surplus of acetaldehyde behind in beer. 

One commonly overlooked element that affects acetaldehyde is zinc levels in wort.  Zinc is an essential for alcohol dehydrogenase to function efficiently.  When wort is low in zinc, it can lead to an accumulation of acetaldehyde in beer. This can be of particular concern for beers brewed with high levels of adjunct grains. However, wort of all types can typically benefit from the supplementing of zinc levels as zinc provides several other benefits in fermentation as well.

Minimizing post fermentation oxygen exposure, particularly in packaging, is also crucial in minimizing acetaldehyde levels in beer.  Over time, alcohol can also oxidize back into acetaldehyde.  This means that acetaldehyde can be an age-related flavor in old or stale beer.  Additionally, oak used to make beer barrels is naturally porous, and oxygen ingress during this process is common.  This makes following barrel-aging best practice vitally important to prevent the occurrence of unpleasant levels of acetaldehyde.    

Finally, acetaldehyde can be a result of bacterial contamination, so maintaining proper hygiene and cleaning procedures is a crucial step in acetaldehyde prevention.  This is most often an issue with cask ale, where Zymomonas sp. bacteria can contaminate the beer.  This commonly occurs when priming the cask with sugar or as a result of unsanitary filling conditions when simple sugars are present.