Quality Control: Dealing with Diacetyl

All brewers encounter diacetyl, and it’s likely that all our customers experience it, too—whether they realize it or not.

Diacetyl is one of the more common off-flavors that can occur in beer, and yet it’s easily preventable. There are natural ways to reduce diacetyl, and there are tests you can perform to monitor its levels during fermentation. Whether the source is bacteria or yeast, you can still avoid diacetyl in your finished product.
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What Is Diacetyl, Really?

Diacetyl is a chemical compound, and in high enough concentrations, it gives off the distinct aroma of butterscotch or popcorn butter. Also known as butane-2,3-dione (or butanedione), it’s one of two significant vicinal diketones (VDKs.) The other is pentane-2,3-dione, also known as acetylpropionyl.

VDKs are reported in parts per billion (ppb), and the averag human threshold is about 60 ppb. However, some people are completely unable to detect diacetyl, or else they can detect it only at very high levels, so the actual threshold varies from person to person.

Diacetyl is easier to detect in styles with relatively lighter flavors. In light lagers, for example, the perceptible range of diacetyl is about 20 to 60 ppb, while darker beers have a higher range of about 60 to 80 ppb. This is why many brewers view diacetyl as especially unwelcome in pale lagers—although the Czechs are known for a more tolerant view of it at low levels, in balance with other flavors. Likewise, many brewers consider it acceptable at low levels in stouts, porters, and some British ale styles.

Yeast Are Here to Help

When diacetyl is the result of a bacterial infection, the source is likely Lactobacillus or Pediococcus. These commonly show up in tap lines that aren’t being sufficiently cleaned or flushed, resulting in butterscotch-like pours of beers that had no off-flavors when kegged. In that case, pouring off more beer or switching to a new line can solve the issue. And of course, thoroughly cleaning your lines every two or three weeks can prevent the issue from arising.

However, if a fermentor or barrel has growth of Lacto or Pedio, there’s nothing you can do to reverse it. Consistent testing on microbiological media specifically designed to detect these bacterial strains is the best way to prevent infection-related scenarios of diacetyl.

However, most occurrences of diacetyl come from yeast metabolism, which is part of every standard fermentation. Good news: The yeast will clear this up on their own if you give them enough time to do so. Butane-2,3-dione is a by-product of the amino-acid metabolism of valine. During the production of valine, alpha acetolactate is also released. When this compound escapes from the cell, it becomes diacetyl. Pentane-2,3-dione is produced by the same mechanism but does not have a significant sensory impact. Given time, yeast cells will reabsorb butane-2,3-dione and convert it into a similar compound with minimal flavor. Failing to give the yeast enough time to reabsorb and convert diacetyl results in higher levels of it in the finished product.

Give It a Rest

Allowing the yeast to clear up diacetyl takes time and patience, two things often in short supply at a craft brewery with limited tank capacity. The surest way to avoid being stuck with diacetyl in your cans or kegs is to plan for a diacetyl rest period—typically a few days, often combined with a slight increase in temperature.

Because the metabolic activity of yeast increases at higher temperatures, this bump helps the yeast to absorb diacetyl at a faster rate than if the vessel were left at the standard fermentation temperature, particularly in the case of cooler-fermenting lagers. Once the beer is at or near final gravity, a rise of 3–4°F (roughly 2°C) is sufficient to assist with diacetyl reabsorption; this usually takes two to four days.

It’s important not to cold-crash the fermentor before the yeast are done clearing up the diacetyl. Once you crash the tank and the yeast begin to slow their activity and settle out, it takes significantly longer for reabsorption to occur. So, patience is a key element.

A new way that brewers can work to avoid this step is through the use of alpha acetolactate decarboxylase, also known as ALDC. This is an enzyme that consumes the alpha acetolactate as it is being produced. This quick conversion can help avoid the production of diacetyl. There are now yeast strains that have the ALDC gene encoded directly into the yeast genome, but there is also the option to use ALDC as an enzyme addition.

However, there is a phenomenon that requires more time and patience than the standard diacetyl rest. This is known as hop creep, and it occurs in conjunction with high levels of dry hopping during fermentation. (For more about hop creep, see The Causes and Effects of Hop Creep—and How to Prevent It.) Essentially, enzymes in the hops begin to break down previously complex and unfermentable sugars in the beer, leading to additional fermentation and, thus, more diacetyl production.

You need to be aware of this phenomenon to avoid getting diacetyl into your packaged product. Not only is the off-flavor unwelcome in hop-forward styles—ruining all that hard work and all those wonderful aromas—but it also leads to higher (and out-of-spec) ABV, greater attenuation, and the potential for exploding cans. (For more on how brewers and researchers are working on better ways to avoid hop creep, see Revenge of the Hop Creep.)

How to Test for Diacetyl

You can be patient, cross your fingers, and hope for the best, but the most reliable way to avoid diacetyl in packaged beer is to monitor its reduction through regular testing.

Even if you’re a super-sniffer, it can be difficult to detect diacetyl by sensory means without any additional modifications to the sample. This is especially the case once the yeast have cleared up most but not all of it. Most of you are familiar with the forced diacetyl test, a simple method to get an idea of how much time is needed until the diacetyl rest is considered complete.

As the name suggests, this method “forces” diacetyl out of the sample by heating it up for a short period of time. The sample is then cooled and compared to an unaltered control sample to see whether diacetyl is still present in the heated sample. Notably, this is a qualitative method and, thus, it is highly dependent on the individual who is smelling the sample. The best way to perform this analysis is with a few different people, so you can account for different sensitivities. It’s also good practice to regularly train your team using brands spiked with different amounts of diacetyl, so that they can get familiar with how it shows up in different beers.

If you’re concerned about the subjective nature of the forced diacetyl test, or if you want to compile some data, VDK analysis via distillation is your best bet. This is a quantitative method that takes a bit more investment in equipment and regular regeneration of chemical solutions. However, this method provides a reading in parts per billion, so that you can tell exactly how much diacetyl is in your beer. This is especially useful data if you can identify the threshold requirements for a particular brand.

Besides distillation equipment, this method also involves a pricey piece of equipment known as a spectrophotometer. This will measure the absorbance of the distillate after it undergoes a color-changing reaction. If your brewery can justify the investment of perhaps $1,000 to $3,000 for a spectrophotometer, the good news is that it can also be used to analyze IBUs and color (SRM).

Especially as brewing professionals, it’s always disappointing to produce or buy a beer and find it to be tarnished by diacetyl. Whether in the tap lines or in the tanks, this is an issue that can be identified and prevented. Knowing how it’s formed, how to test for it, and how to reduce it is critical to packaging and serving the best beer we can—and to raising the level of quality throughout the industry.