A Brewer's Guide to Diacetyl Off-Flavor | Discover Beer Taste Fixes!

Diacetyl is a kind of ketone, which is an organic compound produced when alcohol is oxidized during the primary fermentation of beer. Yeast is a magical organism that handles the production of bread, wine, distilled beverages, and beer. When yeast ferments beer, it produces more than 500 different compounds that give beer its unique flavor and aroma. Among these compounds, there is one of the least popular-diacetyl. Over time, the same yeast can convert part of the diacetyl into relatively odorless chemicals. Diacetyl gives the beer a buttery butterscotch flavor. In pale beers (such as Budweiser and Miller), the diacetyl content of more than 0.1 parts per million (ppm) can be tasted. The content of diacetyl in home-brewed beer will be above 0.05 to 1 ppm.

Contents hide

1 Yeast varieties

2 Nutritional level

3 Pollution

4 Skip the diacetyl rest

Factors affecting the formation of diacetyl

Diacetyl is produced in two steps. First of all, when yeast undergoes metabolism, it produces a metabolic by-product: α-acetolactate. Then, α-acetic acid lactic acid is oxidized to diacetyl outside the yeast cell. The factors that affect the level of diacetyl in beer are yeast varieties, nutritional levels, pollution, and skip diacetyl rest. Diacetyl levels will change during fermentation and maturation.

Yeast varieties

Throughout our long history, we have been using yeast to make bread and brew beer. The type of yeast you use when brewing beer can have a significant impact on the production of diacetyl. Different yeast strains have different abilities in managing nutrition. For example, under different conditions in other yeast strains, some yeast strains will produce excessive amounts of acetolactate (the chemical precursor of diacetyl). Although all yeasts produce diacetyl, you can choose yeast strains that are known to produce low levels of diacetyl to reduce the risk of diacetyl intrusion.

Nutritional level

In the fermentation process of beer, the “food” that feeds the yeast strain is as important as the type of yeast strain. If yeasts cannot find the “food” they need in the environment, they will find a way to make “food”. But, this will produce other unwanted compounds in your beer, which means there will be more potential diacetyl waiting to be formed. So, you must add nutrients when adding yeast to increase the production of yeast, and help cut the production of acetolactate, and ultimately reduce diacetyl.

Pollution

Even professional winemakers are afraid of the word pollution. Contamination can come from many sources, but the most common is that the brewery equipment is not properly disinfected. If the equipment is not properly disinfected, lactic acid bacteria (LAB) will be produced, which are anaerobic organisms that like high temperatures. This means that the fermentation conditions (including the fermentation temperature) are exactly the growth environment they need to thrive. To make matters worse, LAB not only produces a buttery taste, but also produces acid wash, egg taste, and even metallic diacetyl. This is why LAB is called beer spoilage bacteria. If you want to prevent the production of LAB, make sure to use high-quality disinfectants to disinfect the brewery equipment and fermentation tanks. If you are still worried that LAB will contaminate your beer, you can leave some yeast behind when bottling beer. The remaining yeast will continue to process any diacetyl that may be produced by the LAB, reducing the production of off-flavors.

Skip the diacetyl rest

The culprit in producing unwanted flavor compounds in your beer is time. The beer you brew takes time to mature and develop. Please resist the urge to reduce beer fermentation time. The yeast strains remaining in the beer will treat the off-flavor after the fermentation is complete. This is why the beer conditioning process should be carried out as much as possible. It takes time to condition beer, especially Lager beer. Because the fermentation temperature of Lager beer is lower than that of malt wine, and the decomposition process of diacetyl will slow down at lower temperatures.

Invite diacetyl?

Most brewers don’t like the presence of diacetyl in their beer, because the formation of diacetyl on time may cause fermentation or contamination issues. But some brewers want their beer to contain diacetyl in the final product. For example, Redhook ESB has a typical diacetyl taste. This is most likely caused by the yeast strain used or the fermentation profile used. Some yeast strains, especially flocculent British malt yeast strains, are heavily diacetyl producing this. The fermentation temperature can collapse after terminal gravity, which prevents the occurrence of diacetyl residues. The low concentration of diacetyl produced in this way may be pleasant. Many classic beer styles (such as Scottish ales and bitters) allow tasting low levels of diacetyl in beer.

How to take proper diacetyl breaks

In this battle with diacetyl, time and temperature are your allies. Longer fermentation time and higher fermentation temperature can improve the performance of yeast in decomposing diacetyl. The following is how to perform diacetyl rest:

Start when the specific gravity of your wort is within 2 to 5 points of its final specific gravity or the final specific gravity of the finished beer.
When your primary fermentation is nearing the end, plan to take a two-day (or longer) diacetyl break to let the yeast strain break down the diacetyl.
During the last two days of fermentation, increase the temperature of the wort to between 65°F and 68°F. This will increase the activity of the yeast strain and help remove any remaining diacetyl. You can also move the fermenter to a warmer place to let the wort heat up on its own. Of course, you may also increase the temperature by using a warm jacket or heater.
After two days, you can test your wort. If it is the location you want, then you can bottle the beer or put it on a shelf for refrigerated storage. If not, please continue the steps just mentioned until you are satisfied.

How to test the diacetyl level of beer

After using your patience and will, you have completed the diacetyl rest. Now is the time to test your beer to see if the diacetyl is really gone. The diacetyl test for beer is very simple:

Collect two three-ounce beer samples and seal them in cans.
Mark the two jars A and B, or 1 and 2. No matter what you like, as long as you can distinguish the two jars.
Put the first sample in the refrigerator.
Heat the second sample to 140°F-150°F (60°C-66°C) and hold for at least twenty minutes.
Remove the second sample from the heating source.
Put the second sample in the refrigerator and cool down.
Wait for the two samples to reach the same temperature.
Spin and taste two samples.

If you notice any signs of buttery taste in these two samples, you need to continue the diacetyl break. But, if these two samples have the exclusive style of beer, then the beer can be bottled or drunk.

Summary of Diacetyl Formation

Some yeast strains produce large amounts of diacetyl, while others produce less. Choose a yeast strain that produces less diacetyl, unless your brewing style allows for diacetyl.
Excessive fermentation temperature will promote the production of diacetyl.
Low aeration levels when adding yeast will produce less healthy yeast, and these yeasts tend to produce higher diacetyl.
Fermentation of malt wine produces more diacetyl because malt wine ferments hotter than lager beer, but the reduction of diacetyl occurs faster.
Storage fermentation requires “diacetyl rest” by raising the fermentation temperature before completing the fermentation.
A hydrometer should be used to measure the specific gravity to calculate when to start diacetyl rest. It starts when the beer reaches two to five points of its final gravity.
Fermentation should not be rushed, it is necessary to give the beer plenty of time to mature.
Thoroughly disinfect the equipment, especially when bottling, to limit the impact of bacterial contamination on diacetyl.

Brewers are extremely sensitive to the diacetyl flavor in beer because almost every aspect of brewing involves the formation or reduction of diacetyl. Opinions may always be different about the applicability of diacetyl flavor in various beer styles. But, as a professional winemaker, you should know how it is produced and how to control its level.

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