What is a Diacetyl Rest?

by Dane Wilson | Last Updated: April 15, 2023

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Brewing beer is both a creative art and a precise science. You need to be patient, and precise, and keep a track of all the details that go into your homemade beer. If you are looking to increase your skills and get a cleaner tasting beer, the next thing to master is the diacetyl rest. What is it? How does it work? In this article, you are going to explore what a diacetyl rest is and how to execute one flawlessly.

Table of Contents

Popcorn colored by buttery yellow diacetyl rest, which may also be present in beer

What is Diacetyl?

First things first, let’s get to know diacetyl. Diacetyl is a compound that’s naturally produced during fermentation by some strains of yeast. It has a buttery or butterscotch aroma and flavor, and while it’s not harmful to humans, it can negatively affect the taste and aroma of beer. Diacetyl can make beer taste like movie theater popcorn butter, which is not what any beer brewer wants.

Diacetyl is often a problem with lagers, which need to be fermented at cooler temperatures than ales. If you rush into fermentation too quickly or do not cool the lager well enough, the yeast becomes stressed, causing an increased production of diacetyl.

For that reason, conducting a diacetyl rest when brewing up a lager can save your beer from tasting buttery.

Want to know more about diacetyl? Here is a refreshing perspective from Dr. Hans Brewery:

How Does Diacetyl Form in Beer?

Diacetyl may be naturally occuring, but there are some circumstances that can make it more prevalent in your beer. One way that diacetyl can form in beer is through the conversion of alpha-acetolactate, which is produced by yeast during fermentation. Alpha-acetolactate is a precursor to diacetyl, and it can be converted to diacetyl through a chemical reaction called diacetyl reduction. This reaction is catalyzed by an enzyme called diacetyl reductase, which is produced by yeast during fermentation.

Nutrient levels may also impact the formation of diacetyl. Let’s say, for instance, the amino acid known as valine is present in the beer during fermentation. The yeast will consume that amino acid and produce acetolactate, which may convert into diacetyl. Yeast requires certain nutrients, such as nitrogen and zinc, to properly metabolize and convert the precursors that lead to diacetyl formation. If the nutrient levels in the beer are insufficient, the yeast may not be able to efficiently metabolize the precursors, leading to a greater potential for diacetyl formation.

On the other hand, if the nutrient levels are too high, the yeast may metabolize the precursors too quickly, which can result in a lack of diacetyl formation during the early stages of fermentation. This can lead to the diacetyl not being reabsorbed by the yeast during the later stages of fermentation, and therefore remaining in the beer.

Another Case for Sanitizing Your Equipment

Sometimes you can do everything right when brewing your lager and still get diacetyl in your beer. What gives? Well, did you fully clean and sanitize all of your equipment? If not, the culprit may be lactic acid bacteria, or LAB. As part of the metabolic process of LAB, diacetyl is produced. If your beer is contaminated by LAB, you may end up tasting butter.

In short, do not forget to sanitize your brewing equipment every time you make a new batch.

What is a Diacetyl Rest?

Now that you know that diacetyl is your beer-brewing enemy, let’s learn about how to put a stop to its crimes. A diacetyl rest is defined as a period of time during the fermentation process where the temperature of the beer is raised just enough to get yeast to consume the diacetyl in the brew. Typically, a diacetyl rest lasts between 1-3 days and is done nearing the end of fermentation. When encouraged, yeast does an excellent job at consuming diacetyl and turning it into other flavorless compounds.

When Should You Perform a Diacetyl Rest?

Although a diacetyl rest is primarily performed at the end of fermentation, there are a couple of factors that determine exactly when it should be done:

  • Yeast strain. There are many kinds of yeast strains out there, and some of them naturally produce more diacetyl than others. Make sure you have researched how much diacetyl the yeast you are using will produce.
  • Temperature. Cooler fermentation temperatures often increase the amount of diacetyl in a beer. Anytime you are brewing a lager, you should conduct a diacetyl rest. Ales fermented at cooler temperatures would also benefit from a diacetyl rest.
  • Fermentation. Do the rest at the end of fermentation. Once the yeast has consumed the sugars in the wort, it will turn to diacetyl. That is why the end of fermentation is best, because it coaxes the remaining yeast to deal with the diacetyl compounds.

How to Conduct a Diacetyl Rest

Performing a diacetyl rest is a simple process and does not add too many additional steps to your typical brewing process. Here are the steps required to conducting a diacetyl rest:

Step One: Is a Rest Necessary?

Determine if a diacetyl rest is necessary for the beer style and yeast strain being used. Some yeast strains and beer styles naturally produce low levels of diacetyl and may not require a rest, while others may benefit from a rest to ensure the diacetyl is properly reduced.

Step Two: Raise The Temperature

Once the primary fermentation is complete and the beer has reached its target final gravity, raise the temperature of the fermenter to between 60°F and 70°F (15.5°C and 21°C). This can be done by adjusting the temperature control on the fermentation chamber or by moving the fermenter to a warmer location. The temperature should be raised gradually, by no more than 5-10 degrees Fahrenheit per day. This will prevent the yeast from getting too stressed, which can lead to off-flavors.

Step Three: Begin The Rest

Leave the beer at this elevated temperature for 1 to 3 days or longer, depending on the yeast strain and beer style. A longer rest may be necessary for some lager strains.

During the rest, the yeast will continue to metabolize and consume diacetyl, converting it to other compounds that are not detectable or have a more pleasant flavor. The rest also allows the yeast to clean up other fermentation byproducts and improve the beer’s overall flavor and aroma profile.

Step Four: Increase The Temperature

After the rest is complete, lower the temperature back down to the desired range for conditioning, carbonation, and packaging.

Monitor the beer during the rest and after the rest to ensure that the diacetyl has been properly reduced and that the beer is free of off-flavors or aromas. If diacetyl or other off-flavors are still present, additional steps may be necessary, such as a longer rest, repitching of yeast, or other corrective actions.

Need some help with the lagering process? Check out our lager temperature control tips.

How to Test if Your Beer Has Too Much Diacetyl

Let’s say you have done the diacetyl rest but aren’t entirely sure you have vanished those persistent buttery flavors for good. Is there a way to test how much diacetyl is in your beer? Or are you doomed to drinking butterscotch? Good news! There are a couple of ways to test for diacetyl at home.

Although you could simply sample your beer, you may not be able to detect the diacetyl right away.

The better way to go about detecting diacetyl is to take two samples. Use a 3-ounce Mason glass jar or something similar. Label each one. The first sample goes into the fridge. The second sample needs to be heated to 140ºF–150ºF (60ºC–66ºC). Maintain that heat for about 20-30 minutes. Once that time has passed, put the second sample in the fridge as well. Let those samples continue to chill so that they are the same temperature.

Swirl both jars and give each one a taste. Compare them. Should either sample taste buttery, it means you have some lingering diacetyl. You may have to continue on with the diacetyl rest. Should both samples taste delicious, congratulations! You are ready to bottle your beer.

Is There Ever a Time When You Want Diacetyl in Beer?

Generally, diacetyl gets a bad reputation for making beer taste odd, but let’s not demonize it entirely. There are some beer styles where a low to moderate level of diacetyl is desired as part of the flavor profile. For example, many English ales, Czech pilsners, and Munich Helles have a little diacetyl to add dimension. However, these styles of beer have other ingredients that balance the buttery quality of diacetyl as to not be too overpowering. In other types of beer, such as stouts or different types of IPAs, you do not want any detectable diacetyl.

Give Your Beer a Little Rest

The diacetyl rest is an important step in beer brewing that you shouldn’t overlook. Allowing your beer to rest at a slightly elevated temperature after fermentation works magic. You reduce the chance of any off-putting flavors and aromas, such as butterscotch or butter. Though the length and temperature of a diacetyl rest depends on the specific beer style that you are brewing and the yeast strain being used, it is one of the best methods for ensuring that your beer has a clean and delicious taste, free of foul flavors.

Have you done a diacetyl rest in the past? Let us know your thoughts on this process in the comment section!

Frequently Asked Questions

Is a diacetyl rest necessary?

While a diacetyl rest is not always necessary, it is generally recommended for many beer styles, particularly those that use lager yeast or certain ale strains that are prone to producing diacetyl. Without a diacetyl rest, the beer may have an unwanted buttery or butterscotch flavor and aroma, which can detract from its overall taste and drinkability.

How long should a diacetyl rest be?

The length of a diacetyl rest will depend on the specific beer style and yeast strain being used. Generally, a diacetyl rest lasts between 1 to 3 days, but some brewers prefer to do a longer rest of up to a week. Keep in mind that some styles of beer and strains of yeasts will require more or less time.

How do you do a diacetyl rest?

To perform a diacetyl rest, raise the temperature of the fermenter to between 60°F and 70°F (15.5°C and 21°C) for 1 to 3 days or longer, depending on the yeast strain and beer style. This takes place after primary fermentation. After the rest is complete, the beer can be cooled down to the desired temperature for conditioning, carbonation, and packaging.