Acetoin/butanediol

Mass spectrometry (MS) has been applied mainly for the on-line detection and quantification of gases such as p02, pC02, pN2, pH2, pCH4 and even H2S [282] or volatiles (alcohols, acetoin, butanediol). The detection principle allows simultaneous monitoring and, consequently, control of important metabolites. 

Fig. 29. a Measured [77] and b simulated final product ratios of acetoin/butanediol as a function of specific power input during fermentation... 

Biacetyl is produced by the dehydrogenation of 2,3-butanediol with a copper catalyst (290,291). Prior to the availabiUty of 2,3-butanediol, biacetyl was prepared by the nitrosation of methyl ethyl ketone and the hydrolysis of the resultant oxime. Other commercial routes include passing vinylacetylene into a solution of mercuric sulfate in sulfuric acid and decomposing the insoluble product with dilute hydrochloric acid (292), by the reaction of acetal with formaldehyde (293), by the acid-cataly2ed condensation of 1-hydroxyacetone with formaldehyde (294), and by fermentation of lactic acid bacterium (295—297). Acetoin [513-86-0] (3-hydroxy-2-butanone) is also coproduced in lactic acid fermentation. 

Extracts of the summer chafer, Amphimallon solstitiale L. (Coleoptera Scarabaeidae), a common European scarab beetle were analyzed by GC-MS and GC-EAD. Both male and female extracts were shown to contain Acetoin — R) (5) > 9 1, as well as 2,3-butanediol — 2R, 3R) (25,35) meso =1 1 9. Although (25, 35)-butanediol did not show any activity, the other compounds elicited strong responses exclusively with male antennae. 

The presence of diacetyl at any stage of the process does not necessarily indicate an infection by pediococci, because diacetyl is normally formed during fermentation by oxidation of the precurser 2-acetolactate, which reaches a peak (1—1.2 ppm) at 24—36 h fermentation. The concentration of 2-acetolacetate is usually reduced to values of 0.01 ppm or less, and the diacetyl is reabsorbed by the yeast cells and enzymatically transformed through acetoin to butanediol. It is extremely important that 2-acetolactate as diacetyl is reduced below the threshold of 0.05—0.10 ppm (in terms of diacetyl). 

During the biological aging of sherry, the concentration of ethanol decreases because of its consumption by flor yeast. Its respiration via the tricarboxylic acid pathway (Suarez-Lepez and Inigo-Leal, 2004) provides the main source of carbon and energy. Acetaldehyde is the main organic byproduct of ethanol metabolism, but other volatile compounds, notably acetic acid, butanediol, diacetyl, and acetoin, can also be formed. In addition,... 

Merchuk et al. [276] investigated the dynamics of oxygen electrodes when analyzing mass transfer, and they reported whether and when an instantaneous response occurs. A semiempirical description of diffusion coefficients was provided by Ju and Ho [198]. Bacillus subtilis cultures change the product concentration ratio between acetoin and butanediol rapidly in the range of p02 =80-90 ppb [286]. This fact could be used for the characterization of the oxygen transport capabilities of bioreactors. 

Otsuka M, Mine T, Ohuchi K, Ohmori S (1996) A detoxication route for aldehyde metabolism od diacetyl, acetoin and 2,3-butanediol in liver homogenate and perfused liver of rats. J Biochem 119 246-251... 

Phytohormones and phytotoxins are two major classes of compounds produced and secreted by microorganisms that act on plants. These will be discussed in another chapter (4.10). Recendy, production of 2,3-butanediol (186) and acetoin (187) by two strains of plant growth-promoting rhizobacteria (PGPR) was reported. These compounds promote growth of A. thaliana and trigger induced systemic resistance (ISR) in the plant.108 109... 

Diacetyl, acetoin and 2,3-butanediol These compounds are produced by condensing of pyruvate with ethanal. This reaction produces acetolactate which is later decarboxylated. Diacetyl is produced if the decarboxylation is oxidative, whereas acetoin is produced if the decarboxylation is not oxidative. Acetoin can also be formed by directly reducing diacetyl. Finally, acetoin can be reduced to form 2,3-butanediol. This last reaction is reversible (Ribereau-Gayon et al. 2000c). Acetoin and especially diacetyl give off a buttery smell that may... 

Acetaldehyde is also involved in the formation of acetoin and 2,3-butanediol. Although some authors have stressed the significance of a chemical pathway, these compounds are more likely to originate from yeast metabolism (Romano and Suzzi 1996). 

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, and its reduction products, acetoin and 2,3-butanediol, are also derived from acetaldehyde (Fig 8D.7), providing additional NADH oxidation steps. Diacetyl, which is formed by the decarboxylation of a-acetolactate, is regulated by valine and threonine availability (Dufour 1989). When assimilable nitrogen is low, valine synthesis is activated. This leads to the formation of a-acetolactate, which can be then transformed into diacetyl via spontaneous oxidative decarboxylation. Because valine uptake is suppressed by threonine, sufficient nitrogen availability represses the formation of diacetyl. Moreover, the final concentration of diacetyl is determined by its possible stepwise reduction to acetoin and 2,3-butanediol, both steps being dependent on NADH availability. Branched-chain aldehydes are formed via the Ehrlich pathway (Fig 8D.7) from precursors formed by combination of acetaldehyde with pyruvic acid and a-ketobutyrate (Fig 8D.7). 

Dettwiller B, Dunn IJ, and Prenosil JE. Bioproduction of acetoin and butanediol Product recovery by pervaporation. In Bakish. R., ed., Proceedings of the 5th International Conference. Pervaporation Process in the Chemical Industry. Englewood, NJ Bakish Material Corporation, 1991 308-318. 

Bacterial species, particularly those belonging to Klebsiellae, Erwinia and E. colit are known for their ability to metabolize hexoses and pentoses to produce either neutral compounds (butanediol, acetoin and ethanol) or mixed acids and ethanol under specific cultural conditions. Research on the production of ethanol from pentoses by bacteria has revolved around the improvement of such bacteria through genetic recombination. 

In the course of transformation of 2,3-butanediol the products of dehydrogenation of one and two alcohol groups were formed. Thus the reaction results not only in the formation of butadione but in formation of acetoin as well. The amounts of both products formed change markedly with the butanediol conversion degree. 

Dehydrogenation of butanediol on Zn-Cr-oxide catalyst in a wide temperature range allowed to obtain data on the content of acetoin and diacetyl at different conversions of 2,3-butanediol. Besides, the transformation of acetoin into butadione was studied (Figure 1.). One can see that at low temperature (310-340°C) when the conversion was less than 50% mainly acetoin was presented in the reaction products (diacetyl/acetoin molar ratio - 0,5). At 375°C the curve of acetoin content reaches... 

Figure 1. Dehydrogenation of acetoin (to the left, pale dots) and of 2,3-butanediol (to the right) on zinc- chromium oxide catalyst, LHSV=1.6 h" 2, acetoin (as initial material) 3, butadione 1, butanediol (as initial material) 2, acetoin (formed as intermediate from butanediol) 3,butadione.

The results plotted at Figure 1 seem to be useful for elucidation of the reaction pathway. In the course of butadione formation two hydrogen molecules of 2,3-butanediol must be eliminated. The elimination can proceed either simultaneously or step by step via consecutive elimination of hydrogen molecules and intermediate formation of acetoin. One can see that conversion of the latter into diacetyl proceeds faster and at lower temperature as compared to the conversion of butanediol. By increasing the temperature and conversion of butanediol the curve of acetoin formation passes maximum and the curve of diacetyl formation has an induction period. Thus, one can believe that the conversion proceeds mainly via consecutive elimination of hydrogen molecules and intermediate formation of acetoin ... 

As in the dehydrogenation on Zn-Cr oxide catalyst, at low butanediol conversion acetoin is formed preferably. At 180°C diacetyl acetoin molar ratio was equal 0.66. Maximum amount of acetoin in the reaction products was observed in the range of 180-220°C. With further increasing temperature the acetoin content declined in favor of diacetyl. This dependence of acetoin content on butanediol conversion allows it to extend the above conclusion of the intermediate formation of acetoin to oxidative dehydrogenation of butanediol on V205/Mg0 catalysts. 

The optimum reaction conditions in butadione synthesis providing yields of 60-62% and high selectivity have been found. Kinetic data and data on the reaction mechanism were obtained. The conversion of butanediol is believed to proceed via consecutive elimination of hydrogen molecules and intermediate formation of acetoin. 

Oxidation of meso 2,3-butanediol gave (S)-acetoin at 61% yield and its optical purity was 72%. This was determined by comparison of optical rotation with a reference (5). This is the antipode of the product into which meso 2,3-butanediol was transformed by lactic acid bacteria. Oxidation of meso and a racemic mixture of 2,4-pentanediol afforded 4-(R)-hydroxy-2-pentanone at 63% yield and its optical purity was 93%. This was determined by comparison of optical rotation with a reference (9). These results proved the feasibility of the stereoselective oxidation of alcohols by the yeasts. 

The acetoin is reduced with NADH to 2,3-butanediol, while a third molecule of pyruvate is converted to ethanol, hydrogen, and CO2 (Eq. 17-26). The reaction provides the basis for industrial production of butane-diol, which can be dehydrated nonenzymatically to butadiene. 

The most significant role of 2,3-butanediol is in maintaining an oxidation-reduction balance with acetoin (or acetylmethyl carbinol) and diacetyl (Figure 2.7). This compound (2,3-butanediol) is formed following the reduction of acetoin, produced by the condensation of two ethanal molecules. 

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] ... 

---