Alcohol Formation

Lactic acid bacteria metabolize pyruvate anaerobically, yielding varying amounts of carbon dioxide, acetoin, diacetyl, 2,3-butanediol, acetic acid, ethanol, and lactic acid (81). Volatile acyloins (alpha-hydroxy ketones) are obtained by condensing either aldehydes with pyruvate or 2-keto acids with acetaldehyde in a reaction catalyzed by yeast pyruvate decarboxylases (EC 4.1.1.1). Odor qualities and threshold values of 34 acyloins were evaluated, and 23 of them possessed distinct flavor properties. Soy sauce flavors were analyzed: 2-hydroxy-3-pentanone and 3-hydroxy-2-pentanone were identified in soy sauce for the first time. The biocatalytic efficiencies of crude pyruvate decarboxylase preparations from Zygosaccharomyces bisporus, Saccharomyces cerevisiae, Kluyveromyces lactis, and Kluyveromyces marxianus were compared. Product yields comparable to those of conver sions with purified pyruvate decarboxylase demonstrated the suitability of crude enzyme extracts as cost-effective biocatalysts in acyloin formation (82).

Leucine degradation seems to involve chain reactions that commence by transamination, producing a ketoisocaproic acid, followed by nonoxidative decarboxylation into 3-methylbutanal, which is then oxidized or reduced to acid or alcohols (83). The formation of the higher alcohols n-propanol, isobutyl alcohol, active amyl alcohol, and isoamyl alcohol has been studied intensively (84). These higher alcohols are derived either from the deamination or transamination of extracellular amino acids (Ehrlich pathway) or directly from amino acid biosynthetic pathways. In the Ehrlich pathway, the uptake and transamination of isoleucine, valine, and leucine results in the formation of a-keto-h-methyl-valerate, a-keto-isovalerate, and a-keto-isocaproate, respectively, which are converted into active amyl alcohol, isobutyl alcohol, and isoamyl alcohol, respectively. Recently, it was suggested that the Ehrlich pathway is not the only pathway involved in the catabolism of the branched-chain amino acids (isoleucine, valine, and leucine) (85). 2-Phenyl ethanol is considered to be the common product produced by yeast fermentation. Formation of 2-phenyl ethanol follows a similar pathway. Similar to that of higher alcohol, Strecker degradation product of methionine leads to the formation of methional, and its reduced product methionol are key aroma compounds of soy sauce flavor (86). Both methional and methionol have very low threshold values (2.0 x 10_1 and 3.0 x 10_1 ppb in water, respectively. (See Table 7.)

Table 7 Aroma Compounds Generated During Manufacture of Fermented Soy Sauce

Classes

Compounds

Precursors

Alcohol

Acids Furanones

Pyrazines Esters

Isobutanol n-Butanol

Isoamy alcohol

Acetol

Acetoin

2,3-Butanediol

Furfuryl alcohol

Methionol

2-Phenyl ethanol

Maltol

4-Ethyl guaiacol Acetic acid Lactic acid Succinic acid HDMF

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