Diacetyl

Diacetyl (2,3-butanedione) is a vicinal diketone produced by yeast during fermentation that imparts a characteristic buttery or butterscotch aroma to beer and some wines. It is an off-flavour in most beer styles but is considered acceptable in certain ale styles and is nearly always an unwanted defect in dealcoholized beer.

Diacetyl is produced as a fermentation byproduct through the amino acid synthesis pathway — specifically as a spillover product of valine biosynthesis. Yeast excretes alpha-acetolactate during active fermentation, which oxidizes non-enzymatically to diacetyl outside the cell. Under normal fermentation conditions, yeast subsequently reabsorbs diacetyl and reduces it to the less flavorful acetoin and 2,3-butanediol — a cleanup process that requires adequate time and living yeast activity after primary fermentation (the 'diacetyl rest').

For NA beer production, diacetyl management is a significant quality consideration. Cold contact fermentation at low temperatures may not give yeast sufficient metabolic activity to complete diacetyl reduction before the fermentation is terminated. Arrested fermentation (stopped by filtration or pasteurization) also cuts short the diacetyl cleanup phase. Producers using these methods must ensure adequate post-fermentation conditioning time before dealcoholization or arrest to ensure diacetyl has been reduced below perception threshold (typically 0.1 mg/L in lager; higher thresholds in some ales).

The buttery diacetyl note is among the most commonly detected off-flavours in commercially produced NA beers, and consumer surveys consistently identify it as a key differentiator between mediocre and premium NA beer quality. Investment in proper diacetyl management — through appropriate fermentation temperatures, adequate yeast health, and conditioning time — is one of the most commercially significant quality improvements a NA beer producer can make.

A historical note: diacetyl at slightly higher concentrations is intentionally produced in some dairy fermentations (sour cream, butter, certain cheeses) to provide buttery flavor. The Lactococcus lactis subsp. cremoris strains used in these applications are specifically selected for high diacetyl production — the opposite of what brewers want, highlighting how the same compound plays opposite roles in different fermentation contexts.