The threshold of detection for diacetyl is commonly reported to be between 100 and 200 parts per billion (ppb). That said, the level at which it can be detected is highly dependent on the beer style.
Lallemand Brewing, in partnership with the Siebel Institute of Technology, held a webinar putting laboratory analysis up against sensory analysis. Katie Strain provided lab analysis and Eymard Freire commented on the associated flavor perception.
While considered a technical flaw in most beers, threshold levels in Czech lagers and some English ales are acceptable. At those levels, common aroma and taste descriptors include buttery, butterscotch, and “movie theater popcorn.” At much higher levels, however, a slick mouthfeel and/or an unusual roundness can also be perceived.
Diacetyl is a normal product of fermentation. It is formed outside the yeast cell via a slow chemical reaction from an intermediate compound necessary for the synthesis of an essential amino acid. The extent to which the precursor of diacetyl is present depends primarily on yeast nutrients, but also on temperature, pH, and aeration level.
Towards the end of fermentation, in order to survive, yeast will simply reabsorb diacetyl and enzymatically reduce it to flavorless compounds. A very flocculant yeast could drop out before all the diacetyl has been processed.
Let’s not forget that the presence of diacetyl can also be due to microbial contamination such as Pediococci and Lactobacilli. Poor draft line sanitation is often the source of such contamination and the subsequent detection of buttery flavors.
Katie quantified the total vicinal diketones (VDK, the sum of 2,3-butanedione and 2,3-pentadione) by distillation and spectrophotometry and analyzed other highly volatile aroma compounds using GC-FID of the headspace in three beer styles namely: American Lager, Sweet Stout, and IPA.
For the sensory tasting, commercial samples of the various styles that were analyzed were spiked to 600 ppb. The perceived diacetyl character was qualitatively compared to the un-spiked control and also between each style.
As expected, the diacetyl characteristic note of butterscotch was more readily noticeable in the American lager and the IPA than in the Sweet Stout. For the latter, the complexity of the overall flavor profile does indeed mask some of the diacetyl even at a very high concentration.
Training your palate
Training always begins with assessing the ability to detect, identify, and use common terminology to describe a particular compound. Repeated and guided training sessions can improve one’s ability to recognize diacetyl and other flavor-active compounds. The Siebel Institute of Technology offers sensory test kits that include common off-flavors in beer like diacetyl, isovaleric acid, and hydrogen sulfide.
About the panelists
Eymard Freire is the recruitment and product manager at the Siebel Institute of Technology. He is a certified Diploma Bier Sommelier from Doemens Academy.
Katie Strain, M.S., is a laboratory services manager of Alcoholic Beverage Quality Assurance/ Quality Control and a lecturer in the School of Hospitality at the Metropolitan State University of Denver.