Diacetyl is a ketone, an organic compound produced when alcohol oxidizes during the primary fermentation of beer. Yeast, a remarkable organism, is responsible for producing bread, wine, distilled beverages, and beer. During the fermentation of beer, yeast produces over 500 different compounds that contribute to its unique flavor and aroma. Among these compounds is diacetyl, which is considered one of the less desirable ones. However, over time, yeast can convert some of the diacetyl into relatively odorless chemicals.
Diacetyl imparts a buttery butterscotch flavor to beer. In pale beers, diacetyl content above 0.1 parts per million (ppm) can be detected. Home-brewed beer typically has diacetyl levels ranging from 0.05 to 1 ppm.
Factors Affecting the Formation of Diacetyl
Diacetyl is produced in two steps. Firstly, when yeast undergoes metabolism, it produces a metabolic by-product called α-acetolactate. Then, outside the yeast cell, α-acetolactate is oxidized to diacetyl. The factors that influence diacetyl levels in beer include yeast strains, nutrient levels, contamination, and skipping the diacetyl rest. Diacetyl levels change during fermentation and maturation.
Yeast Strains
We have long used yeast to make bread and brew beer. The type of yeast used in brewing beer significantly affects diacetyl production. Different yeast strains have varying abilities to manage nutrients. For example, under certain conditions, some yeast strains produce excessive amounts of acetolactate, which is a precursor to diacetyl. Although all yeasts produce diacetyl, choosing yeast strains known for low diacetyl production can reduce the risk of off-flavors.
Nutrient Levels
In the beer fermentation process, the “food” available to yeast is as important as the yeast strain itself. If yeasts cannot find the necessary nutrients in their environment, they will find alternative ways to acquire them. However, this may lead to the production of unwanted compounds, including diacetyl. Adding nutrients along with the yeast can promote yeast growth, help reduce acetolactate production, and ultimately minimize diacetyl formation.
Contamination
Even professional winemakers dread contamination. Contamination can come from various sources, but the most common is improper disinfection of brewing equipment. If equipment is not adequately disinfected, it can harbor lactic acid bacteria (LAB), which are anaerobic organisms that thrive in high temperatures. Fermentation conditions, including temperature, provide an ideal environment for LAB growth. LAB not only produces a buttery flavor but also imparts acid wash, egg taste, and even metallic diacetyl. Therefore, LAB is considered a beer spoilage bacteria.
To prevent LAB contamination, ensure you use high-quality disinfectants to clean brewing equipment and fermentation tanks. If you are concerned about LAB contamination, you can leave some yeast behind when bottling beer. The remaining yeast will continue processing any diacetyl produced by LAB, reducing the formation of off-flavors.
Skip the Diacetyl Rest
Time is the culprit behind undesirable flavor compounds in beer. The beer brewing process requires time for maturation and development. Resist the temptation to shorten fermentation time. The remaining yeast in the beer will naturally address off-flavors after fermentation is complete. This is why proper beer conditioning is crucial, especially for Lager beer. Lager beer ferments at lower temperatures than ale, and the breakdown of diacetyl slows down at lower temperatures.
How to Perform a Proper Diacetyl Rest
Time and temperature are your allies in the battle against diacetyl. A longer fermentation time and higher fermentation temperature can improve yeast performance in breaking down diacetyl. Here’s how to conduct a diacetyl rest:
Start the diacetyl rest when the specific gravity of your wort is within 2 to 5 points of its final specific gravity or the final gravity of the finished beer.
As primary fermentation nears completion, plan for a two-day (or longer) diacetyl rest to allow the yeast to break down diacetyl.
During the last two days of fermentation, increase the wort temperature to between 65°F and 68°F (18°C and 20°C). This will enhance yeast activity and aid in removing any remaining diacetyl. Alternatively, you can relocate the fermenter to a warmer area to naturally raise the wort temperature. Additionally, using a heating jacket or heater can also increase the temperature.
After two days, test your wort. If it meets your desired specifications, proceed with bottling or place the beer on a shelf for refrigerated storage. If not, repeat the previously mentioned steps until satisfied.
How to Test Beer for Diacetyl Levels
After exercising patience and diligence during the diacetyl rest, it is time to test your beer and determine if diacetyl is truly absent. Testing beer for diacetyl is straightforward:
Collect two three-ounce samples of beer and seal them in containers.
Label the two containers as A and B or 1 and 2—use whichever labeling method allows for easy distinction.
Place the first sample in the refrigerator.
Heat the second sample to a temperature between 140°F and 150°F (60°C and 66°C) and maintain it for at least twenty minutes.
Remove the second sample from the heat source.
Place the second sample in the refrigerator to cool down.
Wait until both samples reach the same temperature.
Swirl and taste both samples.
