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.
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.