Diacetyl and 2,3-pentanedione are two aromatic compounds produced by normal yeast metabolism that are perceived as butter, buttered popcorn, or butterscotch, and are typically seen as a flaw in any beer style. Diacetyl has a lower aromatic threshold than 2,3-pentanedione, though specific thresholds vary based on the individual and style of beer. Chemically, they are classified as vicinal diketones and are commonly referred to as VDKs.
Yeast produces VDKs as a byproduct of the synthesis of the amino acids valine and isoleucine. In a healthy fermentation, yeast will subsequently reduce these VDKs to compounds with significantly higher aromatic thresholds. Yeast health, fermentation pH, free amino nitrogen (FAN) content, and temperature are all factors that directly affect VDK production and efficient reduction. If these variables are properly addressed, adequate VDK reduction should not be an issue.
What are Vicinal Diketones?
In the brewing world, Vicinal Diketones (VDKs) is a general term used to refer to two molecules in particular: 2,3-butanedione, commonly called diacetyl, and 2,3-pentanedione. Both of these molecules are byproducts of normal yeast metabolism during fermentation. They each have aromas of butter, buttered popcorn, and butterscotch, which can be acceptable in some styles and are clear off-flavors in others.
The aromatic threshold for these compounds varies based on individual sensitivity and what medium they are present in. For example, the same concentration of diacetyl may be perceptible in a light lager but not in an IPA, and may be perceived differently in a white wine than in a Pilsner. A general aromatic threshold range for diacetyl is 50-150ppb, though higher and lower threshold values have been reported. An accepted aromatic threshold for 2,3-pentanedione is 900ppb (Boulton and Box, 2003; Mielgaard, 1975).
The Rise and Fall of VDKs
Yeast produces both diacetyl and 2,3-pentanedione as a byproduct of making the amino acids valine and isoleucine, which are essential during cell growth for the synthesis of structural and enzymatic proteins. The production of valine and isoleucine occur via individual but very similar pathways. For the purposes of this document, the valine synthesis pathway and subsequent production and reduction of diacetyl will be the focus due to the lower aromatic threshold of diacetyl. The isoleucine synthesis pathway can be understood to functionally mirror the outlined processes.
A rate limiting step in the synthesis of valine creates a bottleneck that leads to a buildup of a-acetolactate, which is then excreted out of the cell into the fermenting beer. Once in the beer, a-acetolactate spontaneously decarboxylates to become diacetyl. Warmer temperatures can lead to a faster conversion rate, though in all practical beer fermentation situations a-acetolactate will eventually convert to diacetyl.
Yeast will naturally uptake the diacetyl in the beer and reduce it to 2,3-butanediol, a compound with a significantly higher aroma threshold of 4500ppm (Krogerus and Gibson, 2013). This uptake and reduction of diacetyl is a rapid process in healthy yeast cells. As the time in the fermentation tank increases and the physiological condition of the yeast deteriorates, the diacetyl reduction rate can slow proportionally (Boulton and Box, 2003).
Variables Affecting VDK Formation
The Role of the Brewer
Keep your yeast happy.
Pitch at rates that take into account the starting gravity of your fermentation. Aim for 0.75 million cells/ml/ºP (mcmp) for Ale fermentations with starting gravities under 17ºP; target 1.0 mcmp for Ale fermentations over 17ºP. For Lager fermentations, pitch at 1.0 mcmp for starting gravities under 17ºP, and 1.5 mcmp for gravities above 17ºP. Starting gravities of 23ºP and higher may benefit from higher pitch rates.
Consider your recipe and adjust accordingly.
Brews with high levels of adjuncts may result in low wort FAN, and may benefit from the addition of a nitrogen-containing yeast nutrient.
Oxygenate appropriately.
Yeast needs oxygen to maintain its cell membrane integrity and to make new cells. An adequate number of healthy cells in the fermentation is critical for achieving a timely reduction of diacetyl. A wort dissolved oxygen (DO) concentration of 10-12ppm at the time of knockout or inoculation is adequate for most Ale and Lager brewing strains, though some strains like A38 Juice or other London Ale variants may benefit from higher levels of oxygen. If you have questions about how to quantify your wort DO or what level of oxygen a specific strain needs, please contact us.
Plan for a Diacetyl Rest.
Allow the fermentation temperature to rise some in the final 20% of gravity drop and hold there until diacetyl has been reduced to below threshold levels. An increase to 72ºF for Ales and 60ºF for Lager fermentations will be adequate to encourage the rapid decarboxylation of a-acetolactate to diacetyl, thus shortening the overall time to diacetyl reduction.
Monitor your fermentation pH.
The overall process of reducing diacetyl to below threshold concentrations occurs slower when the pH is near 4.5 or higher. If this range of pH is observed and diacetyl reduction issues are also occurring, adjustments may need to be made on the brewhouse or to fermentation parameters.
Allow more time.
If certain factors have affected the health of the yeast population or the favorability of fermentation conditions, the beer may just need more time to reduce the diacetyl concentration. Time won’t fix everything though. If little improvement in diacetyl concentration is occurring over several days, krausening with actively fermenting beer may be the best option.
Testing for Diacetyl
There are a number of ways to test for and quantify the diacetyl / VDK concentration in a fermentation, many of which require expensive equipment like a spectrophotometer, gas chromatograph, or colorimeter. If you have access to those instruments and have the know how to use them, the American Society of Brewing Chemists has some great protocols for testing for diacetyl.
Check Out: Beer-25 in the ASBC Methods of Analysis.
If you are like most brewers and don’t have access to those instruments, there is an easy, low-tech method for running a VDK sensory analysis called a Forced Diacetyl Test. Through the action of warming and then cooling a sample of beer, unconverted a-acetolactate will decarboxylate to diacetyl. This allows for sensory detection of would-be diacetyl that is not aromatically perceptible prior to heating, reducing the possibility of cooling a beer prior to adequate diacetyl reduction.