What Is Diacetyl Rest and Why is it Essential?

by Karl Updated on March 21, 2021

What Is Diacetyl Rest?

Once the beer has finished the fermentation process to near-final gravity, you raise the temperature of the lager to roughly 3 to 4 degrees Fahrenheit above the original fermentation temperature.

Then, let it sit for two to four days.

The purpose is to allow the yeast to reabsorb this chemical, which is a natural byproduct of the fermentation process.

And while this can be done at a colder temperature, too, the absorption occurs at much shorter times at a warmer temperature, when the yeast’s metabolic activity is sped up.

In the end, the warmer temperatures’ purpose is to help lower the necessary overall aging time and make a cleaner tasting beer. Diacetyl Rest is an essential step in making the best lagers, ales, or beers. But why?

Below, we will tell you what it is about. There is also a section that will tell you how to perform it correctly and check your ale.

If you want to become an expert homebrewer, you should know all the details about the Diacetyl Rest.

What is Diacetyl?

This compound is famous for creating a rich, buttery flavor. It is one of the more than 500 chemicals that yeast produces when it ferments your beer at the right temperature.

Diacetyl is a ketone [R], a type of organic compound created when alcohol is oxidized during primary fermentation. The same yeasts can eat and convert it into relatively flavorless chemicals with time.

No real brewer really knows how to pronounce this word, so we just say “D-rest.”

What Causes the Off Flavor in Beer?

Humans can taste or smell a particular note if it reaches the absolute threshold. [R] The lower the value, the easier it is to detect.

Diacetyl’s absolute threshold is pretty low, at just 0.0001 parts per billion (ppb), which means it does not take much to taste or smell it.

Many homebrews can have levels of up to 1 part per million (ppm), so dealing with Diacetyl is an essential part of brewing, regardless of your style or any tricks you may use.

Yeast Strain

The type of yeast starter you use for your lager’s fermentation will significantly affect Diacetyl levels in your final flavors.

Different strains of yeast will have different capabilities in managing nutrients.

For example, a certain starter will produce extra amounts of acetolactate [R], the chemical processor of dulcetly, in the same conditions that another type of starter will not.

While all yeast has a part in Diacetyl production, you can reduce its risk by choosing yeast strains known to produce low levels of it, like this one [R].

Yeast Nutrients

Yeast will find a way to make food if they cannot find what they need in their environment. However, this can also produce unwanted compounds in your beer.

For example, yeast can synthesize valine, an amino acid. However, this will also produce acetolactate, which increases the chances of making Diacetyl.

So you must add nutrients [R] when you pitch your yeast to boost it and help keep acetolactate production to a minimum and ultimately reduce Diacetyl.

Contamination

Contamination can come from many sources and is sure to strike fear in the hearts of all brewers.

The most common cause is improperly sanitized equipment.

This can produce lactic acid bacteria (LAB), which are anaerobic critters who love heat. This means the conditions of fermentation, including the fermentation temperature, are precisely what they need to thrive.

Worse, LAB produces Diacetyl that is sour, eggy, or even metallic. This is why it is known as beer-spoilage bacteria.

To prevent LAB, make sure you are using a high-quality sanitizer for brewing [R]. Moreover, leave some yeast behind when bottling beer, so it will continue processing any Diacetyl the LAB may produce.

Skipping the Diacetyl Rest

Resist the urge to give your lager the shortest possible fermentation time. Your beer needs time to mature and develop, and that is why the Diacetyl rest is critical to your success.

The yeast in your fermenting wort goes through three phases:

  1. The Lag Phase – it settles in and adapts to your wort and temperatures
  2. The Log Phase – growth explodes logarithmically, and your yeast starts processing the sugars in the wort into alcohol and producing the various compounds that go along with it
  3. The Stationary Phase – everything slows down, and the yeast begins to break down the compounds made in the log phase into other compounds

This final phase is critical to remove the distinctive taste of Diacetyl.

However, it takes time, especially for lagers, because they have lower fermentation temperatures than ales, which naturally slows down Diacetyl’s breakdown.

Diacetyl Rest: How to Do It Right

Time and temperature are the critical factors you need. The longer the time and the higher the temperature, the better your yeast will perform as they break down Diacetyl.

Here are the tips, tricks, and steps to performing the Proper Diacetyl rest:

  1. Start when your wort’s specific gravity is 2 to 5 points of its terminal gravity or the final gravity of your finished lager.
  2. As your primary fermentation finishes, plan on a 2-day Diacetyl rest to allow the yeast to break it down.
  3. For the last two days of fermentation, raise the wort’s temperature to between 65°and 68°F to increase yeast activity and remove any remaining Diacetyl.
  4. After 2 or 3 days, you can test your wort, then bottle and rack it for lagering in cold storage if you are satisfied. The period will also depend on different beer styles.

There are various ways to raise temperatures. You can do this by moving it to a warmer location, using a warm-water bath, or a carboy-warming jacket [R] or carboy heater [R].

Test Your Beers’ Diacetyl/Yeast Level

Here are some tips and the steps on testing your beer, lager, or ale:

  1. Collect 2 3-ounce samples and seal them in a jar.
  2. Mark them so you can tell them apart.
  3. Put the first sample in the fridge.
  4. Heat the second to a temperature of 140ºF–150ºF (60ºC–66ºC) and keep it there for at least 20 minutes. 
  5. Remove the second sample from heat, then put it into the fridge to chill.
  6. Wait for the two samples to reach the same temperature.
  7. Swirl and taste both.

If you taste hints of butter, you will need to continue the Diacetyl rest. But if it has the taste and feel you want, it is time to bottle your lager.

Conclusion

Now that you know how to elevate your brew, give it a try at home and let us know how you do!

Karl S: Lead marketer, brewer, dad, and husband. Pretty much an all-round awesome guy.