Diacetyl

What it tastes and smells like

• butter
• butterscotch
• buttered popcorn
• slick, coating mouthfeel

Responsible compound: 2,3-butanedione.

Styles this tends to affect

Not where it belongs — where it is most likely to appear when something has gone wrong.

• Altbier — Düsseldorf’s copper-coloured hybrid: ale-fermented and cold-conditioned, with firm bitterness over nutty, toasty malt.
• Baltic porter — A strong dark lager descended from exported English porter: deep roast and dark fruit, remarkably smooth for its strength.
• Bock — A strong, dark, malt-dominated lager with deep bread-crust and toasty character and only enough hop to keep it from being sweet.
• California Common — Lager yeast fermented warm in the Californian manner, producing a clean amber beer with woody, minty hop character.
• Czech amber lager — A Bohemian amber lager balancing caramel malt against Saaz hop character, sitting between the pale and dark Czech beers.
• Czech dark lager — A dark Bohemian lager richer and sweeter than its German counterparts, with caramel and light roast over a soft body.
• Czech pale lager — The Czech everyday lager: lower in strength than the premium version but keeping the soft bready malt and generous Saaz hopping.
• Czech premium pale lager — The original pale lager: rich bready malt, generous Saaz hopping and a rounded bitterness that soft Bohemian water makes possible.
• Doppelbock — Intensely malty strong lager, dense with bread crust, caramel and dark fruit, and remarkably clean for its strength.
• Dortmunder Export — A pale lager sitting between Helles and Pils: fuller and firmer than the first, softer and less bitter than the second.
• Dunkelweizen — A dark Bavarian wheat beer where banana and clove meet bread crust and light caramel from Munich malt.
• English brown ale — A malt-led brown ale with hazelnut and caramel character, moderate strength and a dry, clean finish.
• Festbier — The golden, stronger lager actually served at modern Oktoberfest — malt-forward and smooth, but pale rather than amber.
• German Pils — Pale, dry and firmly bitter, with a crisp finish and a spicy-floral noble hop character — Germany’s answer to the Bohemian original.
• Helles Bock — A pale, strong lager with the body of a bock but a brighter, more hop-balanced character — traditionally a spring beer.
• Irish red ale — An easy-drinking malt-led ale with caramel sweetness and a distinctive dry, faintly roasty finish.
• Kellerbier — Unfiltered, gently carbonated lager served young from the cellar, with a soft yeasty fullness filtration removes.
• Märzen — A rich amber lager brewed in March for autumn drinking, with deep toasty malt and a clean, firm finish.
• Munich Dunkel — Deep brown Bavarian lager led by rich Munich malt — bready and gently chocolatey, without real roast bitterness.
• Munich Helles — Bavaria’s everyday pale lager: soft, bready and malt-forward, with just enough hop to balance and nothing to distract.
• Ordinary bitter — The everyday British cask ale: low in strength, gently bitter, and built to be drunk in pints over a whole evening.
• Schwarzbier — Nearly black but remarkably light on the palate: smooth roast, clean lager finish and none of the heaviness the colour suggests.
• Scottish ale — Malt-forward Scottish ales with caramel depth and very restrained hopping, traditionally graded by shilling price.
• Vienna lager — An amber lager built on gently kilned Vienna malt: toasty and lightly sweet, but finishing clean and dry.
• Wee heavy — A strong, intensely malty Scottish ale with deep caramel and dried fruit and almost no hop presence.
• Weissbier — Bavarian wheat beer defined by its yeast: pronounced banana and clove over a soft, cloudy, highly carbonated body.

If you are drinking it

A strong butter or butterscotch note usually means the beer was rushed, or that the pub’s dispense lines need cleaning. If one beer in a venue tastes buttery and others do not, the lines are the likely cause and it is worth mentioning. It is harmless.

Not a safety issue. Beer’s alcohol, hop compounds and low pH make it inhospitable to organisms that cause illness.

What causes it

If you are brewing it

• Brewer: Hold the beer at a few degrees above fermentation temperature for 24–48 hours near terminal gravity, and confirm by forced diacetyl test before cooling. (prevents)
• Brewer: Identify and eliminate the contamination source. Beer already affected by bacterial diacetyl is not recoverable. (cannot be reversed once present)
• Venue: Clean dispense lines on a strict schedule, typically every seven days, and replace worn line and keg couplers. (prevents)

Where in the process this comes from

Steps that can produce this fault when they go wrong. Each links to what the step is for and how it is controlled.

• Bottom fermentation — Ferment cool with Saccharomyces pastorianus, suppressing yeast-derived fruit and spice so malt and hop character stand exposed.
• Conditioning — Mature beer after primary fermentation so flavours settle, harsh young compounds are cleared and carbonation reaches equilibrium.
• Diacetyl rest — Raise a fermentation’s temperature near its end so the yeast reabsorbs and reduces the diacetyl it produced earlier.
• Dry hopping — Add hops to beer after fermentation, without heat, to extract aroma compounds while adding essentially no bitterness.
• Hop creep — Explain the renewed fermentation that dry hopping can start in beer that had already finished, and the over-carbonation and dryness that follow it.
• Lagering — Store beer cold for an extended period after fermentation to clarify it and produce the clean, rounded character lagers are named for.
• Pitching — Add yeast to cooled, oxygenated wort at a rate matched to the volume, gravity and fermentation temperature.
• Primary fermentation — Convert wort sugars into ethanol, carbon dioxide and the esters, phenols and higher alcohols that give beer much of its character.
• Sensory panel — Assess beer against its own specification using trained tasters, screened and calibrated against known spiked standards.

Often confused with

• Unintended sweetness — cloying sweetness, syrupy body

Questions