In this multi-part mini-essay, I thought I’d write a little about diacetyl (or 2,3-butanedione) and why it is an important flavor compound in beer. Most of the text is based on my recently published review on diacetyl in brewery fermentations, so have a look at it as well and please cite the review rather than the text in this blog.

Diacetyl (2,3-butanedione) and 2,3-pentanedione are vicinal diketones (VDK) formed during beer fermentation as by-products of amino acid synthesis (valine and isoleucine, respectively) in Saccharomyces yeast. VDKs can have a significant effect on the flavour and aroma of beer, and lighter beers especially are more vulnerable. Diacetyl is known for its butter- or butterscotch-like flavour, and its flavour threshold is usually reported as around 0.1 – 0.2 ppm in lager and 0.1 – 0.4 ppm in ales (1, 2), although flavour thresholds as low as 17 ppb (3), 14 – 61 ppb (4), and 10 – 40 ppb (5) have been reported. This means that 100 µg (0.0001 g) of diacetyl is detectable in 1 litre of beer. 2,3-pentanedione has a similar flavour to diacetyl, though often described as more toffee-like, but it has a higher flavour threshold of around 0.9 – 1.0 ppm (1, 2). VDKs are most easily detectable in lighter beers, where the flavour is not masked by malt and hop flavours, and light lager beer can typically be troubled with diacetyl flavours. Presence of VDKs above their flavour threshold in beer is generally regarded as a defect, since their flavour is undesirable in many beer styles and it can also indicate microbial contamination, e.g. by Lactobacillus spp., Pediococcus spp., or Pantoea agglomerans (6-8). Nevertheless, diacetyl at detectable concentrations is acceptable in some beer styles, such as Bohemian Pilsner and some English ales (smell a freshly poured glass of Pilsner Urquell and you should be able to detect diacetyl).

Diacetyl concentrations in beer can be determined via a variety of analytical methods, including colorimetric assays (e.g. through complex formation with dimethylglyoxime or o-phenylenediamine), gas chromatography and liquid chromatography (10-12). During analysis, care must be taken in order to avoid interference by 2,3-pentanedione and α-acetolactate (a precursor to diacetyl, which we will come to later). During fermentation, the concentrations of free diacetyl in wort are usually low and α-acetolactate rather constitutes the majority of the ‘total diacetyl’ present (22-24). As a result, diacetyl concentrations are often expressed as ‘total diacetyl’ concentrations, i.e. the sum of the free diacetyl and α-acetolactate (‘potential diacetyl’), during analysis, in order to highlight potential diacetyl concentrations.

So how does yeast produce diacetyl? Well, yeast doesn’t actually produce diacetyl, rather it produces a precursor, which gets converted into diacetyl in the wort. The generally accepted pathways for diacetyl and 2,3-pentanedione formation and reduction in Saccharomyces spp. are presented in the figure (click to enlarge) above  (13-16). Diacetyl and 2,3-pentandione are formed indirectly as a result of valine and isoleucine anabolism, since they arise from the spontaneous non-enzymatic oxidative decarboxylation of α-acetohydroxy acids that are intermediates in the valine and isoleucine biosynthesis pathways. In yeast, valine and isoleucine synthesis is localized in the mitochondria (17). In the valine biosynthesis pathway, the reaction between α-acetolactate and 2,3-dihydro-isovalerate is rate-limiting, which means that during fermentation and yeast growth, some α-acetolactate is secreted out through the cell membrane into the wort (13,16-19). The reasons and mechanisms for α-acetolactate secretion by yeast are not fully understood, but may involve protecting the yeast from carbonyl stress (20). The α-acetolactate then spontaneously decarboxylates, either oxidatively or non-oxidatively, forming either diacetyl or acetoin respectively, and in both cases releasing carbon dioxide. The non-oxidative decarboxylation into acetoin can be encouraged by heating under anaerobic conditions and by maintaining a low redox potential in the wort (21). Diacetyl production thus increases with increasing valine biosynthesis, which in turn depends on the cell’s need for and access to valine and other amino acids. Hence, any fermentation conditions that favour rapid yeast growth can give rise to increased diacetyl production if wort free amino nitrogen content is insufficient, and more specifically if the yeast can’t access and uptake sufficient amounts of valine.

This is the end of the first part of the mini-essay. Upcoming parts will discuss what fermentation conditions favour diacetyl formation, what can be done to reduce diacetyl concentrations in the finished beer, and how yeast cells take up valine. The second part can be read here.

References:

• (1) Meilgaard, M., (1975) Flavor chemistry of beer: part II: flavour and threshold of 239 aroma volatiles. Tech. Q.  Master Brew. Assoc. Am. 12, 151-168.
• (2) Wainwright, T., (1973) Diacetyl – a review. J. Inst. Brew. 79, 451-470.
• (3) Saison, D., de Schutter, D., Uyttenhove, B., Delvaux, F., Delvaux, F.R., (2009) Contribution of staling compounds to the aged flavour of lager beer by studying their flavour thresholds. Food Chem. 114, 1206-1215.
• (4) Kluba, R., de Banchs, N., Fraga, A., Jansen, G., Langstaff, S., Meilgaard, M., Nonaka, R., Thompson, S., Verhagen, L., Word, K., Crumplen, R., (1993) Sensory threshold determination of added substances in beer. J. Am. Soc. Brew. Chem. 51, 181-183.
• (5) Aroxa (2013) Diacetyl beer flavour standard – 2,3-butanedione – butter, butterscotch. [Online] Available at: http://www.aroxa.com/beer/beer-flavour-standard/2-3-butanedione/
• (6) Boulton, C. and Quain, D., (2001) Brewing Yeast and Fermentation. Blackwell Science.
• (7) Priest, F., (2003) Gram-positive brewery bacteria, in Brewing Microbiology, (F. Priest & I. Campbell, eds.), pp. 181-217, New York: Kluwer Academic/Plenum Publishers.
• (8) van Vuuren, H., Cosser, K., Prior, B., (1980) The influence of Enterobacter agglomerans on beer flavour. J. Inst. Brew. 86, 31-33.
• (9) Martineau, B., Acree, T., Henick-Kling, T., (1994) A simple and accurate GC/MS method for quantitative analysis of diacetyl in beer and wine. Biotechnol. Tech. 8, 7-12.
• (10) European Brewery Convention. (2008)  Analytica–EBC. 7th ed. Section 9 Beer Method 9.24 Vicinal Diketones in Beer, Fachverlag Hans Carl: Nürnberg, Germany.
• (11) American Society of Brewing Chemists. (2011)  Methods of Analysis, 14th ed (online). Beer-25 Diacetyl. The Society: St. Paul, MN.
• (12) McCarthy, S., (1995) Analysis of diacetyl and 2,3-pentanedione in beer by HPLC with fluorometric detection. J. Am. Soc. Brew. Chem. 53, 178-181.
• (13) Chuang, L., Collins, E., (1968) Biosynthesis of diacetyl in bacteria and yeast. J. Bacteriol. 95, 2083-2089.
• (14) Radhakrishnan, A., Snell, E., (1960) Biosynthesis of valine and isoleucine. J. Biol. Chem. 235, 2316-2321.
• (15) Strassman, M., Shatton, J., Corsey, M., Weinhouse, S., (1958) Enzyme studies on the biosynthesis of valine in yeast. J. Am. Chem. Soc. 80, 1771-1772.
• (16) Suomalainen, H., Ronkainen, P., (1968) Mechanism of diacetyl formation in yeast fermentation. Nature 220, 792-793.
• (17) Ryan, E., Kohlhaw, G., (1974) Subcellular localization of isoleucine-valine biosynthetic enzymes in yeast. J. Bacteriol. 120, 631-637.
• (18) Dillemans, M., Goossens, E., Goffin, O., Masschelein, C., (1987) The amplification effect of the ILV5 gene on the production of vicinal diketones in Saccharomyces cerevisiae. J. Am. Soc. Brew. Chem. 45, 81-84.
• (19) Haukeli, A., Lie, S., (1971) The influence of 2-acetohydroxy acids on the determination of vicinal diketones in beer and during fermentation. J. Inst. Brew. 77, 538-543.
• (20) van Bergen, B., Strasser, R., Cyr, N., Sheppard, J., Jardim, A., (2006) α,β-dicarbonyl reduction by Saccharomyces d-arabinose dehydrogenase. BBA-Gen. Subjects 1760, 1636-1645.
• (21) Kobayashi, K., Kusaka, K., Takahashi, T., Sato, K., (2005) Method for the simultaneous assay of diacetyl and acetoin in the presence of α-acetolactate: application in determining the kinetic parameters for the decomposition of α-acetolactate. J. Biosci. Bioeng. 99, 502-507.
• (22) Haukeli, A., Lie, S., (1972) Production of diacetyl, 2-acetolactate and acetoin by yeasts during fermentation. J. Inst. Brew. 78, 229-232.
• (23) White, F., Wainwright, T., (1975) Occurrence of diketones and α-acetohydroxyacids in fermentations. J. Inst. Brew. 81, 46-52.
• (24) Landaud, S., Lieben, P., Picque, D., (1998) Quantitative analysis of diacetyl, pentanedione and their precursors during beer fermentation by an accurate GC/MS method. J. Inst. Brew. 104, 93-99.