Over the course of primary fermentation, the compounds 2-acetolactic acid and 2-acetohydroxybutyric acid occur as by-products of yeast metabolism, and from these by-products, the substances diacetyl (2,3-butanedione) and 2,3-pentanedione are formed through oxidative decarboxylation. Diacetyl produces an off-flavor in beer in concentrations over 0.10 mg/l.
Determination of the content of 2,3-butanedione (diacetyl) and 2,3-pentanedione or the content of total VDK (free VDKs and precursors) after thermal conversion of the precursors
The method is suitable for beer brewed to any original gravity or to any alcohol content as well as for fermenting wort.
Diacetyl, 2,3-pentanedione and their precursors are determined by means of gas chromatography. A sample from the headspace – in equilibrium with the beer at 35 °C – is analyzed. Precursors are converted to the corresponding diketones through aeration and heating the beer to 60 °C. The evaluation of the results is performed by comparing the height of the peaks of diacetyl and against the peak for hexanedione as the internal standard.