What is a Diacetyl Rest?

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Diacetyl is a common off-flavor in beer. If you’ve ever tasted a buttery or butterscotch-like flavor in a beer, this is diacetyl. Luckily for brewers, it can easily be controlled and eliminated using a diacetyl rest.

What is a diacetyl rest?

A diacetyl rest is a fermentation step used by brewers to eliminate the common beer off-flavor, diacetyl. The diacetyl rest occurs at 68°F for a few days to ensure that yeast reabsorbs diacetyl, removing the butter-like flavor from the beer. This procedure is most common in lager brewing because yeast can only reabsorb diacetyl at warmer temperatures. Ales don’t typically need a diacetyl rest because they ferment in the temperature range that yeast can reabsorb diacetyl.

What is a diacetyl rest?

To perform a diacetyl rest:

  1. When your beer nears the end of fermentation, with about 5 gravity points left, warm the fermenter to 68°F.
  2. Hold the beer at 68°F for 2 or 3 days.
  3. Sample the beer to ensure final gravity has been reached and there are no traces of diacetyl.
  4. If diacetyl is present, give the beer another 2 days to continue to clean up.
  5. When happy with the flavor, package your beer as normal, or cold condition (lager) for up to 6 weeks.

To warm up the fermenter, simply increase the temperature in your fermentation chamber. If you don’t have a fermentation chamber, try using a heat wrap or you can move the fermenter to a warm area.

What is diacetyl?

Diacetyl – pronounced die-ass-se-til – is a common off-flavor in beer. The term comprises two similarly flavored molecules, Vicinal DiKetones (VDK) and Acetylpropionyl. In beer, these molecules are created as a fermentation byproduct.

As brewer’s yeast consumes fermentable sugar, alpha-acetolactate is excreted from the yeast cell. Once this is released into the beer, it reacts with oxygen to produce diacetyl.

When the yeast is finished consuming all of the fermentable sugars in the beer, it looks for other energy sources to consume. And luckily for beer drinkers, yeast likes to consume diacetyl.

In order for the yeast cells to reabsorb that buttery diacetyl, they need energy in the form of heat. Lager fermentation temperatures – 45 to 55°F – are not hot enough for adequate reabsorption. The reaction occurs most efficiently at typical ale fermentation temperatures, around 68°F.

Because of the need for warmer temperatures to reabsorb the diacetyl, the diacetyl rest stage is mostly used when brewing lagers.

What does diacetyl taste like?

Butterscotch, buttered popcorn, buttered corn on the cob… you get the picture.

Not only does diacetyl have a buttery flavor component, it also contributes to a slick or oily mouthfeel. The best way to describe the sensation is almost like drinking a fatty soup broth. In beer it’s usually to a much lesser degree, but it’s a similar sensory experience.

In certain beer styles, diacetyl is part of the flavor profile. The most notable example is English Bitter. Low levels of diacetyl have always been a hallmark of the style and appreciated by traditional ale drinkers.

Despite sometimes being to style, most modern craft brewers have grown to dislike the flavor of diacetyl. A presence of diacetyl in beer can be a sign of a bad brewer. For that reason, most British ales these days are relatively free from those butterscotch-like flavors.

Interestingly, diacetyl is not detectable by everyone. Due to differences in individual taste perception, some people are blind to diacetyl while others are very sensitive. Because of this, it’s vital for brewers to effectively eliminate diacetyl from their beers for a consistent flavor profile.

How is diacetyl measured?

Diacetyl can be measured using a spectrophotometer. Do you have one of those lying around?

Even most pro breweries don’t have the technology to test for diacetyl in house. Samples can always be sent to specialized labs to perform analysis if a brewery is having issues mitigating the off-flavor.

At the homebrew level, the best way to test if diacetyl is in your beer is through sensory analysis using a forced diacetyl test.

How do you perform a forced diacetyl test?

A diacetyl test, or “D” test, is a simple method to check if your yeast has finished reabsorbing the diacetyl.

There can still be precursors to diacetyl in your beer even as it nears the end of fermentation. You won’t be able to smell the butter-like aroma in a fermenter sample because it hasn’t formed yet, but the precursor may still be present.

It’s always a good idea to perform a forced diacetyl test. By warming a sample of beer, you’ll be able to know if there are still diacetyl forming precursors (alpha-acetolactate) in the beer.

To perform a diacetyl test:

  • Take a 100ml sample from your fermenter.
  • Warm it up to 140°F and hold it there for 10 to 15 minutes. You easily can warm up the sample in a hot water bath.
  • Let it cool to room temperature.
  • Take a fresh 100ml sample from the fermenter.
  • Carefully smell and taste each sample.
    • If neither smell or taste buttery: Ready to package beer.
    • If the warmed sample tastes like butter but the fermenter sample doesn’t: Give the beer more time to clean up, and re-do a test in a few days.

Unfortunately, if both of the samples are tasting like butter – you may have another problem on your hands. Contamination with unwanted microbes can also cause a diacetyl-like flavor in beer. If you have uncontrollable buttery flavor, make sure to pay closer attention to sanitation, yeast health, and proper pitch rate.

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Final Thoughts

Diacetyl is one of the easiest off-flavors to avoid when brewing beer. We’ve taken you through some basic yeast chemistry to help make sense of how diacetyl is formed and how it can be mitigated.

For ales, you’re normally in the clear due to the consistently warm fermentation temperature. When lager brewing, on the other hand, it’s always a good idea to perform a diacetyl rest. Simply warm up the fermenter to 68°F for a few days, test for diacetyl using a forced diacetyl test, and package when you’re happy with the flavor profile.

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