Diacetyl Rest


By Jack Horzempa

What is Diacetyl

What is diacetyl?

Have you ever had buttered popcorn when going to the movie theater? You may not have known this but the butter flavoring of that popcorn is an artificial flavoring called diacetyl.
Diacetyl is a part of the class of compounds, vicinal diketone (VDK) and chemists refer to diacetyl as 2,3-butanedione. For those of you with an interest in chemistry the chemical formula is C4H6O2:
Diacetyl Chemical Makeup
Well, enough chemistry for now (until later).

What does the presence of diacetyl do to beer?

Perceptible levels of diacetyl will have flavor impacts for the majority of the beer drinking population. I attended a presentation at the 2015 National Homebrewers Conference by Pat Fehey, “Sensory and Flavor Testing”, where he stated that something like 20% of the population can’t perceive diacetyl in beer. In my opinion those are the lucky few! Pat even made mention that he personally can’t perceive the flavor of diacetyl but he has learned over the years that when he perceives a certain quality in a beer he knows that others will taste diacetyl – I believe he perceives a slickness in the mouthfeel as opposed to a flavor perception.

For the 80% of people who can perceive the flavor of diacetyl (like me) the descriptors often used to describe it are “buttery” or “butterscotch”. I am personally very sensitive to diacetyl and can perceive it even if the amount is low, and for my palate it is very buttery. And let me get it out of the way now – I am not a fan. To circle back to the presentation I discussed above they provided attendees with 5 small glasses of a blonde ale; one unadulterated beer and four with chemical compounds added from an off flavor kit. One of those beers was doctored with diacetyl. I could get that glass no closer than six inches from my nose and that is as close as that glass came to my mouth. I refused to take a sip even thought we were instructed to do so. I don’t remember if I actually did this but I would not be surprised if I said out loud: PU.

For those who would like to do something similar to what occurred at the presentation there are kits available:

Brewessence kits

According to Aroxa, a company that provides tools for professional taste panels and sensory analysis, “the flavor threshold of diacetyl in beer is 10 – 40 µg / l. The precise concentration varies with beer type”. I would not be surprised that I can pick up diacetyl at levels even less than 10 µg / l.

Diacetyl is acceptable in some beer styles

According to some beer judges it is “OK” for some beer styles to have a perceptible level of diacetyl. Two example beer classes are English Ales and Bohemian Pilsners (Czech Pale Lagers). I am not a certified beer judge but if I were invited to judge a homebrew contest I would request not to be placed in either of these two categories due to my strong dislike of butter flavors in beer; I would be biased here.

Below are some extracts from the BJCP (Beer Judge Certification Program) style guidelines on this topic:

English Ales

For the Bitter Ales category (Ordinary Bitter, Best Bitter, and Strong Bitter) there is the below verbiage with emphasis in bold by me:

Aroma: Generally no diacetyl, although very low levels are allowed.

Flavor: Generally no diacetyl, although very low levels are allowed.

Needless to say but “very low levels” is quite subjective and for a person like me who is very sensitive to the buttery flavor of diacetyl this would be problematic.

Bohemian Pilsner (Czech Pale Lager)

For the Czech Pale Lagers (Czech Pale Lager, Czech Premium Pale Lager) there is the below verbiage with emphasis in bold by me:

Aroma: Light (but never intrusive) diacetyl and light, fruity hop-derived esters are acceptable, but need not be present.

Flavor: “Diacetyl or fruity esters are acceptable at low levels, but need not be present and should never be overbearing.”

So, similar to what was detailed above for Bitter Ales the subjective verbiage “low levels” is utilized.

During my visit to Prague/Czech Republic in 2019 I often experienced levels of diacetyl which in my opinion were much higher than “low levels” and for some reason only when drinking draft mugs of Pilsner Urquell served at Tankovna (tank) Pubs. I had a discussion with the tour guide of the Pilsner Urquell brewery which I detailed in my article, “Pilsen Beer – The Family of Pilsners”:

“…the only beer I sometimes had diacetyl issues with was Pilsner Urquell and principally when at Tankovna Pubs which served Pilsner Urquell directly from tanks within the pub. When I visited the Pilsner Urquell brewery to take a tour I asked the tour guide about this topic. She recognized this situation but since she was not a brewer she had no answer to why there was variability here.”

Why is there diacetyl in the beer?

Bacterial contamination

Diacetyl can be produced by lactic acid producing bacteria, Pediococcus and Lactobacillus. But if the beer is suffering from a bacterial contamination it is likely there is more off about the beer than just a buttery flavor.

There is a straightforward solution to this potential issue: practice proper sanitation in the brewing of the beer.

For the case of serving beer via draft (bars, homebrewers that keg) the cause of perceptible diacetyl in beer can be caused by dirty lines (bacterial contamination in the beer lines).

On the off chance that the amount of contamination is low and there is still live yeast present in the beer there is a chance that the yeast can ‘clean up’ the diacetyl over time but it is best just to practice proper sanitation in brewing; this will nip the problem in the bud.

Yeast pathway to produce diacetyl

Diacetyl is produced during the primary fermentation of beer by the brewers yeast. During fermentation the yeast will produce amino acids, proteins and a number of other compounds. One of the amino acids produced by the yeast is valine via an intermediate compound, acetolactate. Not all of the acetolactate will be processed into valine and this excess acetolactate will exit the yeast cell. This acetolactate outside the yeast cell will be chemically converted to diacetyl in the beer. This chemical process is an oxidative reaction and higher fermentation temperatures will encourage this reaction. Beyond the aspect of fermentation temperature there are other factors which increase the production of diacetyl in the beer:

  • Poor wort nutrition (i.e., lack of valine) will encourage the yeast cell to increase valine production
  • Low levels of oxygen in the wort
  • Yeast strain

To further expound on the last factor listed above, yeast strain selection can have a notable impact here. For example the Ringwood yeast strain (e.g., Wyeast 1187) is renowned for producing a lot of diacetyl during primary fermentation.

The pathway towards diacetyl production is depicted in the figure below:

yeast cell producing diacetyl

Give it a rest – diacetyl rest

While the production of diacetyl occurs in every batch of beer there is an easy way to manage this situation: provide sufficient time post primary fermentation to permit the yeast to ‘clean up’ the diacetyl that is produced during fermentation. This maturation process is referred to as conducting a diacetyl rest.

During the diacetyl rest, diacetyl will cross the cell wall and enzymes convert it to acetoin and then subsequently to 2,3-butanediol. This process is depicted in the figure below:

Yeast cell proccessing Diacetyl

Warm temperatures accelerate the processing of diacetyl during the diacetyl rest (e.g., 65 – 70 °F). For ales this simply means permitting additional time (e.g., a few days) after primary fermentation is complete for the diacetyl levels to come down. For the case of lagers the suggested process is to raise the temperature of the primary from the lager fermentation temperature (e.g., 50 – 55 °F) when the primary is nearing completion to the mid-60’s °F. The rule of thumb is to conduct the increase in temperature when the beer is 2 to 5 specific gravity points away from the target terminal gravity. Just like for ales keep the lager at warm temperature for a few days and then commence the lagering phase.

Beyond just letting the beer sit for a few extra days for those folks who are industrious you can test the beer to ensure that diacetyl is no longer an issue. Take two small samples (just a few ounces) of the beer from the primary (remember to practice proper sanitation here). Place one sample in your refrigerator and heat the second sample to 140 – 150 °F and maintain that temperature for 20 minutes. Then place this sample into your refrigerator to chill down to the same temperature as the previous sample. Once both samples are at the same temperature take them out, give them a swirl and sniff and sip both samples. If they both smell and taste the same than the diacetyl rest has done its job. You can proceed to package you ale or proceed to the lager phase for your lager.

I mentioned earlier that the Ringwood yeast strain was noted for being a large producer of diacetyl. While having a conversation with the assistant brewer of a local brewpub it came up that they use the Ringwood strain as their house ale strain. He explained that they liked this yeast since it drops bright and therefore they have no need to filter their ales. During this conversation I exclaimed rather loudly “But I have never picked up diacetyl in any of your house ales”. He smiled and replied “That is because we give it enough time to clean up all of the diacetyl it produces”. He then stated rather sadly that he personally was ‘blind’ to diacetyl in beer and it was therefore the responsibility of the head brewer to take daily samples of the beer and decide when they are ready to be served.

Fighting Chemistry with Chemistry

There is a new product now available for commercial brewers:  ALDC enzyme which makes the concern of diacetyl a moot point.

Below is how this new product is detailed:

“ALDC - Alpha Acetolactate Decarboxylase (ALDC) prevents the formation of diacetyl by breaking down the precursor, alpha acetolactate, as it is formed during fermentation – converting it quickly and directly into acetoin. Since diacetyl can be formed from the precursor in the finished package, adequate removal helps ensure the product retains the intended quality.

ALDC can also help to reduce the rate limiting step in conditioning. Only 1 to 5 grams per barrel is required to be added to cold wort, prior to yeast pitching. As this enzyme is pH sensitive it is deactivated by the normal pH drop at the end of fermentation.”

From the chemistry detailed previously this enzyme negates the excess acetolactate that leaks out of the yeast cell during the production of valine.

For commercial brewers this product is a godsend since they will no longer have to conduct a diacetyl rest thereby significantly reducing their production timeline. Plus at only 1 to 5 grams per barrel not a lot of product is needed to achieve this goal.

I expect that at some point this product will be made available in quantities suitable for homebrewers.  Until then, a homebrewers best practices described above will keep diacetyl at bay.  In summary, practice good sanitation and provide a diacetyl rest and you will not have beer that tastes like buttered popcorn.

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