Vicinal diketones (VDK) comprise the compounds diacetyl and 2, 3-pentanedione, which are spontaneous breakdown products of the yeast metabolites α-acetolactate and α- acetohydroxybutyrate respectively.
Since they are virtually indistinguishable by typical chemical tests, they are grouped together. Both are natural by products of fermentation, formed from minor metabolic products produced during the initial stages of yeast growth and fermentation, which leak out of the yeast cells into the beer. The highest concentrations are found in the initial stages of fermentation, during the reabsorbed by yeast in final phases of fermentation and are metabolized to relatively flavorless diol compounds.
High-temperature fermentation both produces higher levels of VDK, but does an even better job of reducing them as long as the yeast remains active until the end of fermentation. Bacterial infections, notably Pediococcus and Lactobacillus, can produce VDK in high concentrations, usually in conjunction with numerous other off-flavors and aromas as well. This is a common problem in infected (dirty) draught beer lines.
The beer flavor is adversely affected by the presence of high levels of diacetyl. The taste threshold of diacetyl is in the low ppb (μg/l) range for most trained tasters. The maximum removal of VDK’s (specifical diacetyl) occurs in the fermenter. Yeast activity must not be halted since the yeast reabsorbs the diacetyl. Normally chill back is delayed until diacetyl levels ≤40 μg/l are obtained in order to ensure levels in the beer of ≤15 μg/l.
For analyzing VDK, samples are taken from FV ( fermenting vessel), SV ( Storage Vessel), BBT ( Bright Beer Tank) and packaged beer samples. FV concentrations are monitored after a specific number of days in fermentation.
The vicinal diketones are distilled from the beer. The distillate is mixed with a solution of o-phenylenediamine to form derivatives of quinoxaline. After acidification, the amount of reaction product is measured spectrophotometrically. The concentration of vicinal diketones is calculated using a calibration factor.
The vicinal diketone (VDK) determination is a measure of the amount of free diacetyl, 2,3 pentanedione and their precursors present in a sample i.e. the total VDK concentration in a sample.
Produced during fermentation as a byproduct of valine synthesis when yeast produces α-acetolactate, which escapes the cell and is spontaneously decarboxylated into
diacetyl. The yeast then absorbs the diacetyl and reduces the ketone groups to form acetoin and then 2,3-butanediol. Healthy yeast has about 10 times the ability to absorb diacetyl as to produce it. Diacetyl is typically detectable at 0.5 to 0.15 mg/l, although
the ability to taste diacetyl is genetic. Some people can taste diacetyl down to 0.2 mg/l, while others are insensitive to it! It is described as tasting like artificial butter, butter, butterscotch, toffee or vanilla.
2, 3-pentanedione is produced during fermentation as a byproduct of isoleucine synthesis when yeast produces α-ketobutyrate, which escapes the cell and is spontaneously decarboxylated into 2, 3-pentanedione. The yeast then absorbs the 2, 3-pentanedione and reduces the ketone groups to form relatively flavorless compounds. Compared to diacetyl, pentanedione is much less important, since the perception threshold is 10 times higher than that of diacetyl and most yeast strains produce far less pentanedione than diacetyl. It is detectable at 0.90 mg/l. It is detectable in aroma and flavor as honey or honey-like perfume.
- Yeast strain selection – some produce more VDKs, especially those which flocculate well and those which produce respiratory-deficient mutants.
- Improper yeast management. Insufficient or excessive yeast growth. Yeast mutation. Reusing yeast collected too early in previous fermentation cycle. Poor yeast health (e.g., Mineral deficiency, overpitching, low dissolved oxygen, high gravity wort, >9% ethanol concentration, delayed yeast collection, insufficient cooling of fermentor cone
- Improper Sanitation. Wild yeast infection (bacterial contamination can produce high levels of 2, 3- pentanedione).Pediococcus Damnosus infection produces large amounts of diacetyl, which isn’t reduced with time. Lactobacillus infection during storage.
- Configuration & size of fermenting vessels can affect VDK production – but only for large commercial tanks where there is high pressure at the bottom of the tank.
- VDK precursors are increased by yeast strain, higher oxygen levels, and higher yeast pitching levels, but the latter two factors also help yeast reduce VDK in the final stages of fermentation.
- Conversion of VDK precursors to VDK is increased by
drop in pH (optimal at 4.2-4.4 pH), high levels of oxygen and
temperature increases, especially during the fermentation stage
of primary fermentation. These factors also help yeast reduce
- Recent studies show that Dry Hopping can also lead to an increase in diacetyl level in the final beer. The enzymes in your dry hops have acted upon the starches and dextrins in your beer, breaking them down into fermentable sugars. The yeast that you have left in your beer has refermented those sugars. Yes, it’s secondary fermentation. While additional alcohol and CO2 have been produced, a natural by-product of fermentation called a-acetolactate has been produced by the yeast and has gone through its normal oxidative decarboxylase reaction to form diacetyl. I discuss how this happens in a recent Youtube video below. Because your packaged beer is probably refrigerated, the yeast cannot adequately clean up the diacetyl as per normal during a diacetyl rest thus leaving it in your beer after its formation.
- CO2 buildup in the fermentor.
- Keepadjuncts low (<40%) or add yeast nutrient.
- Good yeast management. Aerate wort well after pitching yeast.
- Use optimum fermentation temperature for yeast strain (not too cold).
- Use good quality moderately flocculating yeast, which is not susceptible to mutation or contaminated by wild yeast.
- Allow fermentation/diacetyl reduction to finish before racking off, lowering temperature or adding finings. Sufficiently age beer on yeast (rouse yeast if necessary, while avoiding aeration). Minimize aeration during transfer.
- Use good sanitization practices to avoid bacterial infection.
- Yeast will naturally reduce VDK as fermentation progresses, so healthy, vigorous yeast activity during primary fermentation (down to about 90% of terminal gravity) will reduce most diacetyl.
- The yeast’s ability to remove diacetyl drops during secondary fermentation (i.e., lagering).
- Higher yeast concentrations and increased contact between yeast and fermenting beer (i.e., adding new yeast or rousing existing yeast into suspension) also increase yeast’s natural ability to reduce VDK.
- An enzyme called Maturex (alphaacetolactate decarboxylase) significantly reduces the production of diacetyl and thereby makes it possible to reduce the beer maturation time. The enzyme breaks down the alphaacetolactate directly to acetoin and is added to the cold hopped wort at the start of fermentation.
When Are VDKs Appropriate? Low levels of diacetyl are acceptable in Bohemian Pilsner, English Pale Ales, Scottish Ales, English Brown Ales, Brown Porters, Robust Porters, Sweet Stouts, Oatmeal Stouts, Foreign/Extra Stouts, English IPA and Strong Ales. They are a fault in other styles of beer, especially most lagers.
Low (sub-threshold) levels of diacetyl can give the illusion of richness or body in any beer style, although this is undesirable in thin-bodied beers.