Generation of odorous acyloins by yeast pyruvate decarboxylases and their occurrence in sherry and soy sauce

Volatile acyloins (alpha-hydroxy ketones) were 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. Sherry and soy sauce flavors were analyzed: 2-hydroxy-3-pentanone and 3-hydroxy-2-pentanone were identified in soy sauce for the first time; these and 2-hydroxy-5-methyl-3-hexanone and 3-hydroxy-1-phenyl-2-butanone were isolated from sherry 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 conversions with purified pyruvate decarboxylase demonstrated the suitability of crude enzyme extracts as cost-effective biocatalysts in acyloin formation. Conversion rates of >50% showed that the potential of this type of enzyme to catalyze the formation of aliphatic acyloins has been underestimated before.