Written by PAUL HUGHES, PH.D.
DEALING WITH DIACETYL Is it Good or Bad? Management is the Key.
E
very brewer and winemaker is familiar with diacetyl. An intensely flavored compound, it can be a positive or negative flavor component depending on its beverage context. Reminiscent of butterscotch or cultured buttermilk, diacetyl works well in red wines and more flavor-intense — usually darker — beers, whilst it is eschewed in products such as lighter beers. Indeed, the famous brewing scientist Dr. Takashi Inoue, an emeritus of the Kirin Research Laboratory in Japan, dedicated a whole book to diacetyl in foods (Inoue, 2008). So what is diacetyl? It’s a relatively simple
chemical, more rigorously known as 2,3-butanedione (1, Fig. 1). As a pure chemical compound, in the absence of a food or beverage context, it has a pleasant aroma with a wide range of sensory associations reminiscent of many foods and drinks. The range of these sensory descriptors betrays the broad range of fermentative sources of diacetyl and its understudy, 2,3-pentanedione (2, Fig. 1). Together these compounds are often referred to as vicinal diketones (VDKs), the term vicinal referring to the fact that the carbonyl moieties are on adjacent carbon atoms. While a rather old-fashioned term, it persists in the
Fig. 1. Summary of the pathways to the principal flavor-active vicinal diketones
(VDKs) formed during yeast fermentations. They are a minor side-product of the biosynthesis of the amino acids isoleucine and valine and, less directly, leucine.
110
various sectors of the alcohol industry. Both compounds are ultimately derived from the biosynthesis of the amino acids valine, isoleucine, and less directly leucine during fermentation, either by conventional yeast strains or contaminating microorganisms such as lactic acid bacteria (Fig. 1). A leak in the amino acid biosynthetic pathways leads to the formation of (1) and (2) by oxidative decarboxylation. Critically, both VDKs are highly flavor-active, particularly (1). A detailed sensory evaluation of (1) in a neutral grain matrix by Lee et al., (2000) determined that the detection and recognition thresholds of diacetyl were 2 and 6 μg/l respectively in grain spirit. Although the thresholds of 2,3-pentanedione were not determined in this study, in other systems such as beer it is generally considered to have thresholds around ten times higher ( i.e., less flavor-active) than diacetyl. To provide context, the diacetyl thresholds were the lowest reported by Lee et al., in their studies, which included the potent sulfur compound dimethyl trisulfide (onion-like, savory, meaty). The sensory impact in detectable levels of VDKs can in principle be positive or negative, but they are unlikely to make a positive contribution in spirits that are required to create neutral grain spirit-based categories such as gin or vodka, therefore VDKs must be controlled or eliminated. Given the boiling points of (1) and (2, Table 1) it is unsurprising that diacetyl in particular is challenging to remove from
W W W . ARTISANSPIRITMAG . C O M