Diacetyl [pronounced dahy-uh–seet-l] is a common off-flavor in homebrew that presents itself in the aroma and flavor as butter, butterscotch or liquid butter (think movie theater popcorn) or in the mouthfeel of the beer as slickness. The threshold for diacetyl is extremely low at around 0.5 parts per million in lighter beers such as american and international lagers. What is diacetyl, how is it produced, and how can we stop it from getting into our homebrew?The good news is that butanedione, butane-2,3-dione or the more common name diacetyl is relatively simple to prevent from getting into your beer. Diacetyl has the molecular formula C4H6O2 and is part of a family of vicinal diketones (VDK). Another common ketone in beer is 2,3-pentadione which tastes like honey but this article isn’t about this particular compound.
There are two primary mechanisms for getting diacetyl into your finished beer; normal yeast fermentation and microbial contamination. One thing to note is that diacetyl production is yeast strain dependent with some English strains producing more diacetyl than other yeast types.
I know what you’re thinking, “there’s no way that the flavor of artificial butter is acceptable in any style” and if the flavor ever reaches that intensity you are absolutely correct. However, there are some styles where a slight hint of diacetyl is acceptable.
- Czech Pilsners
- Czech Amber Lagers
- Czech Dark Lagers
- English Bitters
- English Porters
- All Scottish Styles
- Irish Red Ale
- Sweet (Milk) Stout
- Oatmeal Stout
- Tropical Stout
- Foreign Export Stout
- English Old Ale
- Wee Heavy Ale
- Flander’s Red Ale *
- Oud Bruin *
* The Flander’s Red Ale and Oud Bruin diacetyl is most likely from the microbial pathway whereas all other styles should be from the yeast pathway.
The Yeast Pathway:
During the yeast lag and growth phases, yeast cells are building up the components that they will need to convert fermentable sugars in the wort into alcohol and CO2. One of the items that the yeast cell will create is pyruvate, the simplest alpha-keto acid possible. This pyruvate is then converted by the yeast cell into acetolactate acid through enzymatic action. This acetolactate is then converted in the cell into valine, an amino acid essential for the fermentation metabolic pathway of yeast. Unfortunately, yeast will over-produce acetolactate and some of it will exit the yeast cell and end up in the beer. This acetolactate is then converted through a spontaneous oxidation reaction (not an enzymatic activity) into diacetyl. Two things have the potential to increase the generation of acetolactate; increased temperature and insufficient yeast nutrient. An increased fermentation temperature will increase the likelihood of acetolactate production, increasing the potential concentration of diacetyl in the beer. Additionally, insufficiency yeast nutrients, in particular the amino acid valine, will cause the yeast to increase production of acetolactate.
Once the diacetyl is in your beer it’s not the end of the world. During the yeasts stationary phase, yeast will re-adsorb diacetyl and through a reductive enzymatic reaction will create acetoin and finally 2,3-butanediol, both of which have a higher threshold for flavor in beer. By converting the diacetyl to these other components, the impact of diacetyl is reduced. This is commonly called the diacetyl rest phase in a fermentation profile where after the majority of fermentation is complete and you are within 2 to 3 points of your expected final gravity, the fermentation temperature is increased to approximately 65ºF to 68ºF to allow for the yeast to adsorb the diacetyl it produced. While this practice is common in lagers, it is possible that some ales which are fermented between 65ºF to 68ºF may required additional time sitting on the yeast to allow the proper cleaning up of the diacetyl. A common practice currently occurring in the homebrew world is that when the specific gravity hits the mark for the expected final gravity people will immediately cold crash the beer, add gelatin to clear the beer and put the beer on a keg. This may be causing diacetyl precursors like acetolactate to remain in the beer without yeast, eventually converting to diacetyl but not having the yeast available to adsorb and metabolize into less obtrusive forms.
The Bug Pathway:
Oh contamination, it seems to be a possible cause of so many of the off-flavors that we have in our homebrew and diacetyl is no exception. Two of the most common “beer spoilers” produce big quantities of diacetyl; lactobacillus and pediococcus. Diacetyl produced by these two bacteria are far from pleasant, presenting themselves more as a sour butter flavor. If a microbial contamination is thought to be the cause of the diacetyl in beer look towards your cold side equipment and serving lines. Diacetyl in restaurants is commonly caused by microbial contamination of draft equipment.
There are few things that can be done to reduce potential diacetyl in your beer.
- Pick a different yeast strain!
- If you are using a strain that kicks out a lot of diacetyl look into other strains that my not do that.
- Perform a diacetyl rest!
- Your lager is within 3 points of expected final gravity, bring that temperature up into the mid to high 60s for a couple of days then bring it back down to lagering temperatures.
- Your beer is finished (Yay for beer!), before you cold crash that beer let it sit for a couple of days to make sure that the yeast does it’s job and cleans up it’s mess.
- Help that yeast out!
- Build up a yeast starter instead of just pitching a package from the lab, healthy yeast produces less diacetyl.
- Ensure proper aeration, this goes back to making sure the yeast is healthy and not stressed out.
- Make sure you’ve got the nutrients in the wort, typically this isn’t an issue unless you have a very strange grain bill but it’s a possibility albeit a small one.
- Clean up that mess!
- Use the krausening technique to kick the cleanup into overdrive by adding a yeast that is at high krausen when you are reaching the end of your fermentation profile.