Molecules with a double aldehyde group produced during fermentation by nitrogen metabolism. The common VDK is diacetyl produced from acetolactate, an intermediate of valine metabolism. Acetolactate is released into the wort and decarboxylates to diacetyl which has a very strong butterscotch flavour at low concentrations. Levels rise towards the end of fermentation but diacetyl is then reduced by yeast to produce 2,3 butane diol, a compound with a much higher flavour threshold. These reactions occur during normal processing but if yeast is removed too early high levels of diacetyl may result in beer.

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