Diacetyl (dye-assa-TEEL, or dye-ASS-itle) is probably one of the most well-known flavors related to brewing. It’s buttery aroma is easily recognized at levels above threshold but, as much as is known and recognized about this compound, I’m constantly amazed and disappointed by how much “butter-beer” is still being produced. This post will briefly explore the various ways that diacetyl arises in beer.
Diacetyl (2,3-butanedione) is one of a class of compounds called “vicinal diketones” (VDK). In chemistry-speak, “vicinal” essentially means “adjacent”, and “diketone” means that there are two ketone functional groups (a ketone is an oxygen double-bonded to a carbon in the middle of a carbon chain). If you look at the molecular structure in the link above, you’ll see that it is a 4-carbon chain (hence the “butane” root in the name), and the two ketones are on the vicinal positions of the #2 and #3 carbons (ergo, 2,3-butanedione: two ketones on #2/3 carbons of a 4-carbon chain). The other main VDK in beer is 2,3-pentanedione (pentane – can you guess what this one looks like?), but it is usually found in beer at levels below that of diacetyl.
Diacetyl, as mentioned already, has a buttery or butterscotch-like aroma. Open a bag of microwave buttered popcorn and you’re hit in the face with diacetyl. At very high levels it can even start to affect the mouthfeel of the beer, causing a slick or oily mouthfeel. The detection threshold of diacetyl in beer is typically between 10 and 40ppb, although I have determined the personal thresholds for 11 of my panelists and they range from 7-190ppb, with an average of about 60ppb. Personally, my threshold is about 20ppb and beers with higher levels are so offensive to me that any beer with detectable diacetyl usually goes down the drain. Many times you may be served a beer which appears to have no hint of diacetyl, but as you work your way down to the bottom of the glass you begin to detect it. This is because the beer is warming up, which allows more diacetyl to volatilize into the headspace of the glass (and your nose). Too many times have I started in on a good and cold beer only to dump the second half because the “Big D” had begun to show its stinky face.
Common levels for diacetyl in beer range from 30ppb to over 1ppm . Until recently, I would have had a hard time believing that there was packaged beer out there with 1ppm diacetyl. That is, until my panel tasted a beer from a local micro-brewery which was so high in diacetyl that we just had to run it through our gas chromatograph to find out how much was actually in there: almost 900ppb. Nearly all of our beers that we make here at The Company are below 30ppb or so, with most falling easily below 20ppb. I would hazard a guess that the big American Lagers/Pilsners are about 10ppb or less, but I don’t know for sure.
There are a few different sources of diacetyl in beer, only two of which are widely discussed. The first is during fermentation, where it’s created by the brewer’s yeast. The other well-known source is from bacterial infection. The third source is probably the least known: beer aging. Below, we’ll examine each of these sources.