Beer Sensory Skills 101 | Yeast | Cidery

This article is part of a series of lessons and exercises that focus on enhancing your sensory skills to better recognize and address brewing problems. By training your sense of smell, taste, mouthfeel, etc., you will have a better understanding of identifying off-flavors/aromas and their causes, which can greatly assist you when looking for solutions to these problems. This blog series will come in many parts dealing with Yeast, Hops, Malt, Water, Infections/Contaminations, and Miscellaneous issues. 
Each lesson includes an exercise in a separate tab to assist you in developing your palate and other senses to better detect these flavors, aromas, etc.

Lesson 1.2 - Yeast: Acetaldehyde/Cidery - I'm Making Beer, Not Cider!


Have you ever taken a sip of a fresh homebrew only to find it reminiscent of crisp green apples or a tart glass of cider? If so, you may have encountered the distinctive flavor of acetaldehyde, a compound produced during fermentation that can impart cidery notes to beer. In this lesson, we'll delve into this cidery off-flavor and explore the science behind its formation, how to detect its presence in your brews, and strategies for preventing or embracing its unique character in your homebrews. 

Beer tasting cidery, or exhibiting flavors reminiscent of cider, can be caused by several factors, often related to fermentation and yeast management. Here are some common reasons why beer may taste cidery:


• Yeast Strain: Certain yeast strains, particularly some ale yeasts, can produce fruity esters during fermentation that resemble the flavors found in cider. If the yeast strain used in brewing is known for producing high levels of esters, such as isoamyl acetate (which has a fruity, banana or apple-like aroma), it can contribute to a cidery taste in the finished beer.

• Temperature Control: Fermenting beer at too high a temperature can stress the yeast and lead to the production of fruity esters. Elevated fermentation temperatures can encourage yeast to produce more esters, including those with apple or pear-like flavors commonly associated with cider.

• Incomplete Fermentation: If fermentation is incomplete, residual sugars may remain in the beer. These sugars can contribute sweetness and fruity flavors reminiscent of cider. Inadequate attenuation, where the yeast fails to fully ferment all available sugars, can lead to a sweet, cidery taste in the finished beer.

• Poor Sanitation: Contamination by bacteria or wild yeast due to poor sanitation practices can lead to off-flavors in beer, including those resembling cider. Proper cleaning and sanitation of brewing equipment, fermenters, and packaging materials are essential to prevent contamination and ensure the quality of the finished beer.

• Acetaldehyde Production: Acetaldehyde is a natural byproduct of yeast fermentation, arising from the conversion of sugars to alcohol during the brewing process. It is an intermediate compound in the metabolism of yeast, and under normal conditions, it is further metabolized into ethanol. However, if fermentation conditions are suboptimal or if yeast is stressed, acetaldehyde may not be fully converted, leading to its accumulation in the finished beer. It is this compound that we will be focusing on in today's lesson.


Acetaldehyde can be acceptable in certain beer styles where its presence contributes to the desired flavor profile or character. For example, in some Belgian ales or experimental fruit beers, brewers intentionally incorporate acetaldehyde to create a cider-like character, enhancing the overall complexity of the brew. Similarly, in sour beer styles like Lambics or Gueuze, a moderate presence of acetaldehyde can contribute to the tartness and complexity of the beer. Additionally, in certain traditional lager styles or experimental beers, brewers may tolerate or appreciate a subtle presence of acetaldehyde, adding unique fruity or funky characteristics to the brew. However, it's important to manage acetaldehyde carefully to ensure it enhances the beer's flavor without overpowering other elements.


The Chemistry of Acetaldehyde:

The creation and formation of acetaldehyde in beer involve complex biochemical reactions during yeast fermentation. Under normal fermentation conditions, acetaldehyde is rapidly converted into ethanol by alcohol dehydrogenase, minimizing its accumulation in the beer. However, factors such as yeast stress, suboptimal fermentation conditions (e.g., high temperatures), or insufficient yeast health can disrupt this conversion process, leading to the buildup of acetaldehyde in the finished beer.

Acetaldehyde (CH3CHO) is an intermediate compound in the metabolism of yeast and is produced through the following steps:


• Glycolysis: Yeast cells metabolize glucose (a sugar present in wort) through a process called glycolysis, which breaks down glucose into pyruvate. This step generates energy for the yeast cells to perform essential functions.

• Decarboxylation: Pyruvate undergoes decarboxylation, a chemical reaction in which a carboxyl group (COOH) is removed, resulting in the formation of acetaldehyde and carbon dioxide (CO2). This reaction is catalyzed by the enzyme pyruvate decarboxylase, which is present in yeast cells.

• Reduction to Ethanol: Acetaldehyde is then converted into ethanol (alcohol) through a reduction reaction, where it gains hydrogen atoms from the coenzyme NADH (nicotinamide adenine dinucleotide, reduced form). This reduction reaction is catalyzed by the enzyme alcohol dehydrogenase.


The overall reaction can be represented as follows:

Pyruvate → Acetaldehyde + CO2
Acetaldehyde + NADH → Ethanol + NAD+

In summary, acetaldehyde is created as an intermediate metabolite during yeast fermentation, primarily through the decarboxylation of pyruvate. It is then converted into ethanol through a reduction reaction catalyzed by alcohol dehydrogenase. However, factors that interfere with this conversion process can result in the accumulation of acetaldehyde in beer, leading to off-flavors and aroma reminiscent of green apple, cider, or fresh cut grass.


Detecting and Preventing Acetaldehyde:

Acetaldehyde can be detected by both smell and taste. In terms of aroma, it often presents as a green apple-like scent, although it can also exhibit a slightly sharp or pungent character. When tasting beer with acetaldehyde, you may notice flavors reminiscent of green apple, sour apple, or even freshly cut grass. The intensity of the acetaldehyde flavor can vary depending on factors such as the level of contamination and the overall beer recipe.

Preventing the formation of acetaldehyde in beer requires careful attention to fermentation conditions and yeast management. Here are some strategies to minimize the risk of acetaldehyde formation:


Healthy Yeast: Ensure that yeast cells are healthy and viable before pitching into the wort. Proper yeast handling, including hydration, rehydration, and pitching rates, can help prevent yeast stress and minimize the production of acetaldehyde. The addition of yeast nutrients can also promote yeast health and prevent yeast stress.

Optimal Fermentation Temperature: Maintain appropriate fermentation temperatures to promote healthy yeast metabolism. Avoid excessively high or low temperatures, as they can stress the yeast and lead to off-flavor production, including acetaldehyde. inconsistent variations in temperature can also stress the yeast. Always try to keep your fermenter in a location with a consistent temperature appropriate to the yeast being used.

Complete Fermentation: Ensure that fermentation is allowed to proceed to completion, with all available sugars fully metabolized by the yeast. Monitor fermentation progress with hydrometer readings or other methods to confirm that the beer has reached its target final gravity.

Adequate Oxygen: Oxygen is required for yeast health and fermentation, but excessive oxygen exposure can lead to the production of acetaldehyde. Oxygenate the wort appropriately before pitching yeast, but avoid introducing excess oxygen during fermentation and bottling/kegging.

Proper Sanitation: Maintain strict sanitation practices throughout the brewing process to prevent contamination by bacteria or wild yeast strains that may contribute to off-flavor production, including acetaldehyde.


By implementing these preventive measures and carefully monitoring fermentation conditions, brewers can minimize the risk of acetaldehyde formation and ensure that their beer maintains its desired flavor profile and quality.


Yeast autolysis stages


Detecting Acetaldehyde



Green Apple Scent: Acetaldehyde often presents with a distinct green apple-like aroma. When assessing the aroma of beer, particularly during fermentation or conditioning, pay attention to any noticeable fruity or sweet-smelling notes reminiscent of green apple.

Fruity or Floral Notes: In addition to green apple, acetaldehyde may contribute to other fruity or floral aromas in beer. Be on the lookout for unwanted aromas that resemble pear, melon, or even floral scents, as these can also be indicative of acetaldehyde presence. 

Intensity and Persistence: Take note of the intensity and persistence of fruity aromas during aroma evaluation. Acetaldehyde-related aromas may be more pronounced in beers with higher levels of the compound, and they may linger in the aroma longer than desired.



Green Apple or Sour Flavor: When tasting beer, particularly during sensory evaluation or quality control assessments, pay attention to any flavors reminiscent of green apple or sour apple. These flavors can indicate the presence of acetaldehyde in the beer.

Sourness or Tartness: Acetaldehyde may contribute to a slightly sour or tart flavor profile in beer, resembling the acidity found in green apples. Be alert to any perceived tartness or sourness in the beer's flavor profile, as this can be a sign of acetaldehyde-related off-flavors.

Paint Thinner/Solvent: In some cases, at higher concentrations, acetaldehyde can contribute to off-flavors that resemble paint thinner or solvent. This is especially true when acetaldehyde levels are elevated beyond the threshold of perception and become more pronounced in the beer. When acetaldehyde reaches such high levels, it can manifest as harsh and solvent-like notes that detract from the beer's overall flavor profile and quality.

Overall Balance and Harmony: Assess the overall balance and harmony of flavors in the beer. Acetaldehyde-related flavors can disrupt the balance of the beer and detract from its overall drinkability. If the beer exhibits an unpleasant or distracting green apple-like flavor, it may indicate the presence of acetaldehyde.


The presence and intensity of these aromas and flavors can vary depending on factors such as yeast strain, fermentation conditions, and aging process. When balanced and integrated into the beer's flavor profile, the flavors resulting from autolysis can contribute positively to its complexity. However, excessive autolysis leading to overpowering or off-flavors is generally considered undesirable in beer.


Sensory Training:

• Compare to Known Standards: Familiarize yourself with the aroma and flavor characteristics of acetaldehyde by tasting beers intentionally spiked with the compound or using off-flavor kits. This will help you recognize and differentiate acetaldehyde-related off-flavors in your own beer.

Practice Sensory Evaluation: Regularly practice sensory evaluation techniques, such as blind tastings or triangle tests, to hone your ability to detect acetaldehyde and other off-flavors in beer. Consistent practice will improve your sensory skills over time.

Seek Feedback: Share your beer with experienced brewers, beer judges, or sensory experts for feedback on aroma and flavor characteristics, including the presence of acetaldehyde. Their insights can help you refine your palate and improve the quality of your beer.

By paying attention to aroma and flavor characteristics, comparing to known standards, and practicing sensory evaluation techniques, you can develop your ability to detect acetaldehyde in beer and maintain the desired quality and flavor profile of your brews.

Recommended Beers For This Lesson

While acetaldehyde is generally considered an off-flavor in beer due to its green apple-like aroma and flavor, there are some beer styles, such as Belgian ales and sours, where its presence may be acceptable or even desirable in moderation. And in some styles, like Czech Pilsners or Helles Lagers, a subtle presence of acetaldehyde may be tolerated or even appreciated as part of the beer's flavor profile. Here are a few examples of styles where acetaldehyde can be acceptable:

It's important to note that while acetaldehyde may be acceptable or even desirable in certain beer styles, the key is moderation. Excessive levels of acetaldehyde can result in off-flavors and detract from the overall quality of the beer. Brewers must carefully manage fermentation conditions and yeast health to ensure that acetaldehyde levels remain within acceptable ranges for the intended beer style.