Vicinal Diketones (VDKs) are a group of flavor components in beer, most notably 2,3-butanedione (generally referred to as diacetyl) and 2,3-pentanedione. Sweet butter, caramel, or butterscotch flavors and aromas are characteristics of diacetyl, while pentanedione contributes more honey-like notes to beer. See diacetyl. Both compounds are derived from two ketone groups on adjacent (vicinal) carbon atoms. See ketones. They are formed during beer fermentation in consistent proportions, and derivation in this ratio can indicate possible bacterial contamination. Excessive or unexpected VDK character can be a sign of improper fermentation or infection by bacteria or wild yeasts. In normal healthy fermentations, VDK production levels are strain dependent. Monitoring and controlling VDK levels plays an important role in flavor formation during beer maturation because VDKs are detectable by humans at fairly low levels. In light lagers, diacetyl is perceptible in the range of 20 to 60 parts per billion (ppb). Detection becomes more difficult in darker, more robust beers, in which diacetyl may be noticed at around 60 to 80 ppb. Research indicates that roughly 25% of all humans, however, are genetically blind to diacetyl detection, whereas many other beer drinkers taste it strongly and find the flavor and aroma repellent. Opinions about VDKs are divided among brewers as well. Although many brewers consider any notable trace of VDK character in beer a clear defect, others regard small amounts as desirable for rounding out a beer’s flavor and adding complexity.

Although VDKs are formed during fermentation, they are not produced directly by yeast. Instead, they are the result of a long-chain reaction. During synthesis of the amino acids valine and isoleucine, yeast cells excrete alpha-acetolactate and alpha- ketobutyrate, respectively. These are precursors to VDKs, and their levels peak roughly halfway through fermentation. Subsequently, they break down spontaneously into VDKs, whose levels peak near the end of fermentation. During beer maturation, however, VDK levels decrease again because yeast metabolizes them as an energy source. The result of this final process is the conversion of VDKs into less flavor-active molecules. If beer is removed from its yeast before this reabsorption is complete, VDKs may later appear in the finished beer.

As the fermentation progresses, brewers monitor VDK levels as an indication of beer maturity. Reduction of VDKs is one of the main goals of traditional lagering; resting the beer for many weeks after fermentation gives the yeast an opportunity to “mop up” VDKs.

On a practical level, brewers have several tools at their disposal for keeping diacetyl levels in check during fermentation. The most important of these is yeast strain selection. Some yeast strains produce only minimal amounts of VDKs and are capable of quickly metabolizing whatever levels they do generate, whereas others are not. A so-called diacetyl rest is another common way of managing VDK levels. It involves a temperature increase roughly halfway through the fermentation, usually by about 1°C to 2°C (2°F to 5°F). This change in temperature accelerates both the rate at which precursors are converted to VDKs and the rate at which yeast metabolizes and thus removes VDKs.

Kräusening is another time-honored method of diacetyl reduction. See kräusening. This involves adding actively fermenting beer to maturing beer. The fresh yeast that is introduced through kräusening consumes VDKs fairly quickly, in addition to contributing natural carbonation. Many industrial breweries have found other ways of reducing VDKs without adding maturation time. A Food and Drug Administration–approved enzyme isolated from bacteria (Acetobacter spp.) can be added to wort. It is capable of circumventing diacetyl formation entirely by converting alpha-acetolactate directly into flavorless acetoin. Another strategy is to use genetically modified yeast strains that produce this enzyme. A third method of VDK reduction is to pass warmed beer through an immobilized yeast bioreactor, a system that can drop VDKs to acceptable levels in only a few hours.See also immobilized yeast reactor.