Pentanedione, properly called 2,3-pentanedione, is a vicinal diketone (VDK) normally produced by yeast during fermentation. It gives a honey-like flavor in beer and is considered an off-flavor for most beer styles. Pentanedione has a flavor threshold some 10-fold higher than the related buttery-tasting VDK diacetyl, but brewers usually seek to eliminate both from finished beer. Pentanedione is formed in fermenting beer by a spontaneous oxidative decarboxylation of alpha acetohydroxybutyrate, a precursor of the amino acid isoleucine. The pentanedione is reassimilated and removed by active yeast given prolonged contact.

To reduce the effect of 2,3-pentanedione on the overall flavor profile of the finished beer, many brewers employ either a “VDK rest” or kräusening when primary fermentation is complete, especially when fermenting lagers. The former method involves allowing the temperature in the fermenter to slightly rise over a specified period of time, usually from 1 to 3 days. The warmer temperatures raise the metabolic activity of the yeast, speeding the reabsorption of the offending compounds. The latter involves adding a small amount of freshly fermenting wort to beer that has finished its primary fermentation. The active yeast then performs the necessary VDK removal. Lagering, the storage of beer at cold temperatures for long periods, can eventually remove VDKs, but this can take months, and most brewers will use the methods above. 2,3-Pentanedione can be a useful marker for infections within a brewery. Brewery labs can perform gas chromatography tests to determine the ratio of diacetyl to pentanedione in beer. As many spoilage bacteria tend to produce diacetyl but not pentanedione, the presence of a vastly greater proportion of diacetyl than pentanedione is usually symptomatic of a contamination problem.