2,3-butanedione fermentation

Hydroxy-2-butanone (acetoin) is a characteristic constituent of butter flavour used for flavouring margarine and can be obtained as a by-product of molasses-based and lactic acid fermentations [49, 71]. The closely related 2,3-butanedione (diacetyl) has a much lower organoleptic threshold than acetoin and is an important strongly butter-like flavour compound in butter and other dairy products [72] in buttermilk, for instance, the diacetyl concentration is only about 2-4 mg [73]. a-Acetolactate (a-AL) is an intermediate of lactic acid bacteria mainly produced from pyruvate by a-acetolactate synthase. In most lactic acid bacteria, a-AL is decarboxylated to the metabolic end product acetoin by a-AL decarboxylase (ALDB) [71] (Scheme 23.5). 

Fumaric Acid or Butanedionic Acid, HOOC.CH CH.COOH mw 116.07, colorless prisms, mp 286—87°(in sealed tube), bp 290° si sol in w in eth sol in ale. Can be prepd by fermentation of molasses isomerization of maleic acid or catalytic oxidation of benz. 

The most significant ketone produced by yeast is diacetyl (2,3-butanedione), a vicinal diketone, although malolactic fermentation is a more important source, when it is used in wine production. Having a sensory threshold of 0.2-2.9 mg/L, according to the type of wine, it is characterised by a nutty , toasty or buttery aroma depending on concentration (Martineau et al. 1995). Dry white wines tend to contain lower concentrations (0.1-2.3 mg/L) than red wines (0-7.5 mg/L) (Bartowsky et al. 2002 Martineau et al. 1995). Acetoin, which produces a buttery flavour, is formed by partial reduction of diacetyl, and is itself reduced to 2,3-butanediol. Acetoin is usually present at concentrations (<80 mg/L) much lower than its sensory threshold of 150 mg/L (Romano and Suzzi 1996). 

Diacetyl Diacetyl (butanedione, CH3COCOCH3) can be industrially obtained by oxidation of 2-butanone using a copper catalyst at 300 °C, by dehydrogenation of 2,3-butanediol over a copper or silver catalyst in the presence of air, or with 3-hydroxy-2-butanone (acetoin) as a by-product [144]. On the other hand, this compound is naturally produced by LAB conferring a strong buttery aroma to many fermented dairy products (butter, buttermilk, and cheese). Diacetyl is synthesized by oxidative decarboxylation of the intermediate product a-acetolactate [55]. The most important diacetyl-producing LAB species have been shown to be Lc. lactis, Lactobacillus spp.. Strep, thermophilus, and Leuc. mesenteroides [65] ... 

Some species of the LAB group such as Leuconostoc mesenteroides subsp. cremoris, Leuconostoc mesenteroides subsp. dextranicum, and Lactococcus lactis subsp. lactis biovar diacetylactis, are known for their capability to produce diacetyl (2,3-butanedione) from citrate, and this metabolism appears especially relevant in the field of dairy products (Figure 13.4). Actually, selected strains belonging to the above species are currently added as starter cultures to those products, e.g., butter, in which diacetyl imparts the distinctive and peculiar aroma. Nevertheless, in particular conditions where there is a pyruvate surplus in the medium (e.g., in the presence of an alternative source of pyruvate than the fermented carbohydrate, such as citrate in milk or in the presence of an alternative electron acceptor available for NAD+ regeneration) (Axelsson, 2(X)9, pp. 1-72), even other LAB such as lactobacilli and pediococci can produce diacetyl by the scanted pyruvate (Figure 13.5). Thus, in addition to butter and dairy products, diacetyl can be present in other fermented foods and feeds, such as wine and ensilage (Jay, 1982). 

Citrate is present in milk, fruit, and vegetables. It can be co-metabolized with sugars by citrateutilizing LAB. Citrate utilization results in an excess of pyruvate, which is thus converted to diacetyl (2,3-butanedione), acetoin (2-hydroxy-3-butanone), and 2,3-butanediol to equilibrate the redox balance of cellular metabolism (Collins 1972 Bartowsky and Henschke 2004). Some LAB can also synthesize 2,3-pentanedione from pyruvate and threonine (Ott et al. 2000). Diacetyl and 2,3-pentanedione are associated with a buttery aroma, which positively contributes to the flavor of a range of fermented dairy products such as butter (MalUa et al. 2008), yogurt (Routray and Mishra 2011), and cheese (Curioni and Bosset 2002). Diacetyl also contributes to wine style, while it is responsible for flavor defects in beer. Diacetyl is widely produced by LAB, including species of the Lactococcus, Streptococcus, Leuconostoc, Lactobacillus, Pediococcus, and Oenococcus genera. 

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