Diacetyl is the butterscotch or buttery flavor that can ruin your home brewed beer. This week, as part of my ongoing series on beer flavors and off flavors, we’ll discuss diacetyl in your beer and how you can control it. Earlier articles in the series include DMS in home brewed beer, Esters in Home Brewed Beer and Judging Beer.
If you want to learn more about diacetyl I also recommend listening to my podcast/video interview with Charlie Bamforth – he does a great job of explaining it.
What is Diacetyl?
Diacetyl is a natural by-product of fermentation. It is one of two major Vicinal Diketones (VDKs) produced during fermentation, the other being Pentainedione. Diacetyl tastes like butter or butterscotch and in fact is used in the production of artificial butter flavors. Pentainedione provides a honey like flavor. Both can be detremental to the finished beer, and are considered off flavors if too much is present.
Diacetyl is most noticible in very light flavored beers, such as light lager, due to its relatively low flavor threshold. It is far less of an issue in full bodied english ales or dark beers because ales are fermented at higher temperature which helps break down VDKs after fermentation. Also other flavors in ales often mask the flavor of remaining diacetyl.
Diacetyl can also be produced by bacterial infection, and in fact bacteria produces more diacetyl than pentainedione. It is most often produced by contaminated keg lines (particularly in bars), but can occur even in home brewed beers that are infected.
Diacetyl is a natural byproduct of fermentation, so it cannot be completely eliminated. However healthy yeast can also “mop up” or break down both diacetyl and pentainedione into other substances that have a much higher flavor threshold. To do this, the yeast needs to remain in contact with the beer, and also it needs to remain healthy.
This brings us to the first important method for controlling diacetyl, which is simply to pitch a proper quantity of healthy yeast with a good yeast starter at the start of the fermentation. Properly pitching the right amount of yeast will result in a strong healthy yeast concentration after fermentation and allow the yeast to reabsorb the VDKs (diacetyl and pentainedione). Underpitching can lead to diacetyl.
A second method, called “krausening”, involves pitching fresh active yeast after the beer has completed fermentation. This active yeast will break down the diacetyl and pentainedione and significantly reduce the levels of both in the finished beer.
A third method, often used with lagers, is to add a diacetyl rest. A diacetyl rest is allowing the fermentation temperature to rise slightly to 57F or 14 C for lagers (roughly a few degrees higher than your fermentation temperature) near the end of the fermentation to help activate the yeast so it can absorb some of the VDKs. Diacetyl rests are rarely needed with ales, as ales are already fermented at a high temperature that promotes VDK reduction.
Diacetyl can also come from bacterial contamination, so it is important to sanitize everything that might touch your beer. Dirty keg lines also contain bacteria that produces diacetyl, so keeping your keg lines clean is very important.
Finally, some commercial brewers use a product called Maturex that is a chemical that helps reduce VDKs. In some cases, microbrewers and home brewers can obtain this as well, but it must be used with care in the appropriate quantity.
- Diacetyl (butterscotch flavor) and pentainedione (honey flavor) are a natural biproduct of fermentation, but both can be broken down by healthy yeast when fermentation completes.
- The primary method for controlling diacetyl (and pentainedione) is to pitch the appropriate quantity of healthy yeast from a yeast starter and ensure that the yeast remains healthy.
- A diacetyl rest for lagers (raising it a few degrees during fermentation) can also help reduce diacetyl in the finished beer by helping the yeast break down VDKs.
- For some really light lagers where eliminating all of the diacetyl is important, a method called krausening can also be used. In krausening, fresh active yeast is added after fermentation has completed to break down remaining VDKs. This method is relatively quick – taking less than a day in most cases.
- If you are kegging, be sure to keep your keg lines and taps clean as bacteria often creates diacetyl within the dirty lines and taps.
- Poor sanitation can lead to bacterial infection which will also produce large quantities of diacetyl. Keep your equipment clean.
- Finally, some commercial brewers use chemical additives (Maturex) to chemically break down diacetyl and pentainedione. This may be available for microbreweries and home brewers if you can find a good source of it.
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