Beer Sensory Skills 101 | Yeast | Diacetyl

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 1.4 - Yeast: Diacetyl - Buttered Popcorn Beer?

The Chemistry:

Diacetyl (2,3-butanedione) is a chemical compound that can develop during the fermentation process of beer. It is characterized by a buttery or butterscotch-like aroma and flavor. While small amounts of diacetyl can contribute to the complexity of certain beer styles, excessive levels can result in off-flavors that are considered undesirable by many consumers.

Diacetyl is primarily formed during fermentation as a natural byproduct of the metabolic activity of yeast. Specifically, it is produced during the conversion of alpha-acetolactate, an intermediate compound in the synthesis of amino acids by yeast, into diacetyl. This conversion typically occurs during the early stages of fermentation when yeast cells are most active.

Here's a simplified overview of the chemical reactions involved in the formation of diacetyl:


1. Formation of Alpha-Acetolactate: During fermentation, yeast cells metabolize sugars present in the wort (the liquid extracted from mashing malted barley) to produce alcohol and carbon dioxide. As part of this metabolic process, yeast           generates alpha-acetolactate, an alpha-hydroxy-ketone.

2. Conversion to Diacetyl: Alpha-acetolactate spontaneously decarboxylates to form diacetyl, which is the undesired compound responsible for the buttery or butterscotch-like aroma and flavor associated with diacetyl in beer.

3. Reduction and Reabsorption: Normally, yeast can reabsorb and reduce diacetyl to less flavorful compounds such as acetoin and 2,3-butanediol. This process, called a "diacetyl rest", typically occurs towards the end of fermentation and during conditioning. However, if fermentation conditions are not optimal or if yeast health is compromised, diacetyl levels may remain elevated in the finished beer.


It's important to note that while some level of diacetyl is acceptable and even desirable in certain beer styles, excessive levels can lead to off-flavors and negatively impact the overall quality of the beer. Therefore, brewers carefully manage fermentation conditions, yeast health, and maturation processes to control diacetyl levels and ensure that the beer meets desired flavor standards.


When Buttery Beer is a Good Thing:

Diacetyl is desired in certain beer styles where its presence contributes to the overall flavor profile and enhances the drinking experience. Although diacetyl is generally considered an off-flavor when present in excessive amounts, in certain beer styles and at appropriate levels, it can provide a pleasant complexity and mouthfeel. Here are some beer styles where diacetyl is intentionally incorporated:


• English Ales: Traditional English ales, especially those of the cask-conditioned variety (known as "real ale"), may exhibit low levels of diacetyl. It contributes to the characteristic smoothness and rounded flavor profile of these beers.

• Bocks and Märzen: Some traditional German lager styles, such as bocks and Märzen, may have subtle diacetyl notes. These styles often feature malty sweetness, and the presence of diacetyl can enhance the perception of richness and depth in the beer.

• Barleywines: Barleywines, known for their high alcohol content and complex malt character, can benefit from a touch of diacetyl. It can add a buttery undertone that complements the beer's intense maltiness and alcohol warmth.

• Scotch Ales: Scotch ales, particularly the stronger Wee Heavy variety, may exhibit low levels of diacetyl. The buttery notes can complement the beer's caramel and toffee-like malt flavors, adding to its overall richness.

• Some Belgian Ales: Certain Belgian ale styles, such as Belgian Strong Ales or Belgian Dubbels, may contain subtle diacetyl notes. These styles often showcase a wide range of complex flavors, and the             presence of diacetyl can contribute to the beer's overall character.


It's essential to note that the presence of diacetyl in these styles should be subtle and well-balanced, enhancing the beer's complexity without overwhelming other flavors. Brewers carefully control fermentation conditions and yeast selection to achieve the desired level of diacetyl in these styles while ensuring that it remains within acceptable limits. Additionally, preferences may vary, and what constitutes an appropriate level of diacetyl can differ between individuals.


Avoiding Diacetyl:

Avoiding diacetyl in beer is crucial for many styles, as excessive levels can lead to off-flavors that detract from the overall drinking experience. Brewers take several measures to minimize diacetyl production and ensure that the finished beer is free from detectable levels of this compound. Here are some key strategies for avoiding diacetyl in beer:


Healthy Yeast Management: Start with healthy yeast cultures and ensure proper yeast pitching rates. Healthy yeast cells are better equipped to metabolize precursor compounds and minimize diacetyl production during fermentation.

Fermentation Temperature Control: Maintain consistent fermentation temperatures within the optimal range for the yeast strain being used. Sudden fluctuations in temperature can stress the yeast and promote diacetyl production. Additionally, some yeast strains produce more diacetyl at certain temperature ranges, so controlling fermentation temperature is crucial.

Diacetyl Rest: For lager beers and lighter beers, such as Kolsch, conduct a diacetyl rest towards the end of fermentation. This involves raising the temperature slightly (usually to around 60-65°F or 15-18°C) for a short period to encourage the yeast to reabsorb and reduce diacetyl levels naturally. After the diacetyl rest, the beer is typically cooled for conditioning.

Yeast Strain Selection: Choose yeast strains known for their low diacetyl production characteristics, especially for styles where diacetyl levels should be minimal. Many yeast strains have been specifically bred or selected for their ability to produce clean-tasting beers with minimal off-flavors.

Proper Wort Aeration: Ensure adequate aeration of the wort before fermentation begins. Proper oxygenation of the wort provides yeast cells with the necessary oxygen to carry out healthy fermentation and minimize off-flavors like diacetyl.

Monitoring and Testing: Regularly monitor the progress of fermentation and test samples for diacetyl levels throughout the brewing process. This allows brewers to catch any potential issues early and take corrective actions if necessary.

Conditioning and Maturation: Allow the beer to undergo a proper conditioning and maturation period after fermentation. This allows any remaining diacetyl to be naturally reduced through yeast activity and chemical reactions. Extended conditioning times can help ensure that the beer is free from diacetyl before packaging.


By implementing these strategies and closely monitoring the brewing process, brewers can minimize the risk of diacetyl formation and produce clean-tasting beers that meet quality standards.



Diacetyl formation and reabsorbtion.

Detecting Diacetyl



Buttery Notes: Diacetyl can impart a buttery aroma to beer, reminiscent of buttered popcorn or butterscotch candies. This aroma is often considered undesirable when present in excessive amounts.

Vinegary or Sour Smell: In some cases, diacetyl can produce a vinegary or sour smell, which can detract from the overall aroma profile of the beer.

Fruity Esters: While not directly related to diacetyl, the presence of fruity esters can sometimes mask or interact with diacetyl aromas, making detection more challenging.



Buttery Flavor: The most noticeable flavor associated with diacetyl is a buttery taste, similar to melted butter or artificial butter flavoring.

Slick Mouthfeel: Diacetyl can also contribute to a slick or oily mouthfeel in beer, which can detract from the overall drinking experience.

Off-putting Aftertaste: Beers containing high levels of diacetyl may leave an unpleasant aftertaste, described as cloying or overly sweet, lingering on the palate.


The presence and intensity of these aromas and flavors may vary, influenced by factors such as yeast strain, fermentation conditions, and aging process. When diacetyl is carefully balanced and woven into the beer's flavor profile, it can enhance the complexity for some styles. However, excessive levels of diacetyl, resulting in overpowering or undesirable buttery flavors, are generally unwelcome in beer.


Sensory Training:

Taste Different Beers: Sample a variety of beers across different styles, paying attention to their flavor profiles. Look specifically for beers known to exhibit diacetyl characteristics, such as English ales, Bocks, and Marzens. Try to detect the buttery flavor in these beers.

Structured Tasting Notes: Keep structured tasting notes when sampling beers, noting any buttery or butterscotch-like aromas and flavors you detect. Describe the intensity of the diacetyl and how it affects the overall beer experience.

Experiment With Food Pairings: Pairing beers containing diacetyl with certain foods can enrich the sensory experience by highlighting flavor interactions. This exposure helps individuals better discern nuances in the beer's flavor profile, including the presence of diacetyl. Through contrast and comparison, it aids in identifying specific flavor components and training the palate to detect subtle differences in taste. Additionally, it serves as an educational experience, deepening understanding of flavor pairing principles and enhancing overall beer appreciation. Here are some foods that can be recommended for pairing with beers containing diacetyl:

     ° Creamy pasta dishes like fettuccine alfredo or carbonara.

     ° Buttery croissants or brioche bread.

     ° Grilled chicken or pork chops with a buttery sauce.

     ° Creamy and buttery cheeses like Brie or Camembert.

     ° Nutty salads with toasted nuts like almonds or walnuts.

     ° Caramel desserts like flan or sticky toffee cake.


By consistently practicing these methods and actively engaging in sensory evaluation, you can develop a more discerning palate and improve your ability to detect diacetyl in beer accurately.

Recommended Beers For This Lesson

While diacetyl is generally considered an off-flavor in beer, there are some beer styles where it is either acceptable or even expected as part of the flavor profile. These styles typically include:

It's important to note that while diacetyl may be acceptable or even desired in these beer styles, the levels should still be balanced and not overpowering. Brewers often strive to achieve a subtle presence of diacetyl that enhances the beer's complexity without overwhelming other flavors. Additionally, personal preferences and regional variations may influence the acceptance of diacetyl in specific beer styles.