Off-Flavors: Diacetyl aka Butter

Diacetyl, pronounced (die-uh-sea-tul), is an organic compound known for its buttery or “movie popcorn” flavor.  Diacetyl is naturally found in beer as it is a byproduct of the fermentation process.  It is produced in the early stages of fermentation through Kraeusen but as the yeast completes the fermentation process they begin to break down or metabolize the diacetyl through enzymatic action into acetoin and 2,3-butanediol which are flavorless compounds.

 

What causes Diacetyl or the “Buttery” flavor in beer?

Well there are many ways that a beer can taste buttery and sometimes its not the brewery.  A common way for diacetyl to be present in beer is by pulling beer off the yeast cake to early in fermentation.  In order for yeast to metabolize the diacetyl, it needs to be present.  Another way diacetyl becomes present is through infections. Lactobacillus and Pediococcus are bacteria that are known for producing lactic acid (sourness) but they can also produce large amounts of diacetyl.  Mutated yeast can also lead to diacetyl.  One of the most common mutations of yeast is the one that causes the yeast to loose the ability to metabolize diacetyl.  Introducing oxygen late in the fermentation process will cause yeasts to release more alpha acetolactate which will lead to an increase in Diacetyl.  These are all ways that the “buttery” flavor can be introduced by the brewery.  Diacetyl can also be introduced into beer from the tap! Dirty lines are filled with bacteria that will impart the buttery flavor into your beer.  So before you blame the beer, make sure the lines are clean.

 

How to prevent the “Buttery” flavors in beer?

Some ways to prevent the flavors from the above mentioned causes. Make sure you do not pull the fermenting beer off the yeast cake to early.  Take gravity measurements and wait to cold crash or filter your beer until after primary fermentation is complete and your final gravity is met.  Clean your equipment! Clean equipment means clean beer.  If you intend on using Lactobacillus and Pediococcus in your beer to sour do some research on the different strains. Some just sour and some also ferment. Pediococcus is the larger producer of diacetyl in sour beers. One way is to use an ale yeast to ferment or brettanomyces to consume the diacetyl.  Lastly, make sure the lines are clean.  Pour houses and breweries should be cleaning their lines about every 2-4 weeks.