In the beer world, there's no greater profanity than saying the "D" word. And even worse is putting that "D" in your mouth. (Get your mind out of the gutter! ...That's not what I meant!)
I am referring to Diacetyl.
Pronunciation aside (die-ASS-a-teel, DIE-a-see-till, or die-assa-TEEL), this compound causes a movie popcorn, almost rancid butter-like flavor and aroma in beer. Although Diacetyl is produced by all yeast strains during normal metabolism, its quantities are greatly affected by many factors. The main factor is the amino acid Valine and how much of it is present in the wort. Regardless of this, Diacetyl is always produced at some rate during the first few days of fermentation.
*Warning! Chemistry up-ahead!*
|2, 3-Butanedione (AKA: Diacetyl)|
During the initial stages of fermentation, the yeast cell creates Valine as part of its metabolism cycle. Along this pathway to create the amino acid, the compound α(Alpha)-Acetolactate is also produced. A non-enzymatic oxidative decarboxylation of α-Acetolactate (I know, right?) occurs outside the yeast cell and is converted to 2,3-Butanedione, the compound we know as "Diacetyl."
The presence of Valine is directly related to the amount of Diacetyl produced because the enzyme that produces α-Acetolactate is deactivated when the wort contains high quantities of Valine. We know Valine is an amino acid and amino acids are the building-blocks of proteins. In turn, we can assume the high concentration of proteins in wort should significantly reduce the amount of Diacetyl produced. High protein levels could cause some other problems in the finished beer, so large protein quantities is not a viable solution to control Diacetyl.
For you chemist out there, these two compounds are Vicinal Diketones. "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)." (Beer Sensory Science)
As fermentation wraps up, most of the previously produced 2,3-Butanedione and 2,3-Pentanedione is reabsorbed by the yeast into its cell. Through more enzymatic activity, they are converted into 2,3-Butanediol and 2,3-Pentanediol, which have a much higher threshold for us humans. In other words, Diacetyl is turned into something we can't taste.
For you brewers out there, this is typically referred to as a "Diacetyl Rest." Brewers usually aid the reabsorption of these compounds by raising the temperature during the last day or two of fermentation. This rise in temperature excites the yeast and can more easily and vigorously convert Diacetyl into the non-flavored compound.
If Diacetyl is detected in your beer, it is usually an indicator of rushed production or improper fermentation practices.
Another common contributor to Diacetyl is a bacterial infection. The most likely culprits are the little bugs called Pediococcus and/or Lactobacillus. Both of these lovely bacteria create lactic acid and can produce Diacetyl as part of their fermentation products. While they may be a welcome addition to some wild ales and artisanal sour beers, it is quite unpleasant when found in dirty draft lines that haven't been maintained or cleaned sufficiently. Infections can also occur in many stages of the fermentation process, but dirty draft lines seem to be the biggest contributor for Diacetyl via bugs.
One more discovery is Diacetyl production during beer aging. Although the levels of Diacetyl in aged beer may be undetectable because of low levels or hidden behind the veil of oxidation compounds, Diacetyl could be nonetheless present.
In very small quantities, Diacetyl can be a contributing factor to the overall flavor and aroma profile of certain styles. Most English and Irish beers tend to have some detectable levels of Diacetyl adding a pleasant buttery undertone. An interesting fact is Diacetyl is one of the differentiating characteristics between a German Pilsner and a Czech Pilsner (Czech Pilsners are expected/accepted to have low levels of Diacetyl). However, in high quantities this compound can impart a butterscotch, movie popcorn, or a rancid butter flavor and aroma.
Another interesting fact to point out is while a great number of beer drinkers might be able to detect Diacetyl with ease, there are an equal amount of us that are completely blind to it. On that same note, some are very sensitive to Diacetyl while others will only find it detectable in extremely high quantities. Should you fall under any of these parameters, it is perfectly normal since it is simply genetics.
I think that's enough foul language for one lesson. I need to go wash my mouth out with some beer... Er, soap. And put a coin in the tip jar... Er, swear jar.