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catalyzed the selective esterification of 2-pentanol with bu-tanoic acid in re-heptane. Proofed protocols for flavor ester synthesis can be obtained from some enzyme suppliers.

The step from hydrolytic/reverse hydrolytic reactions toward serial enzymatic reactions is bound to require cofactor-dependent enzymes (Table 4). With an ADH/ NAD+/CH3CHO-system for the production of geranial (lemon flavor constituent), 1500 cycles of regeneration of the cofactor were achieved by coimmobilization of NAD. The bienzymatic method for cofactor regeneration instead of the coupled substrate method was suggested to convert ethanol to acetaldehyde, cinnamic alcohol to cinnamalde-hyde, and leucine to 3-methylbutanal (malty odor) and 3-methylbutanol (fusel oil odor). In the latter case (50), Streptococcus enzymes were conencapsulated with substrate, NAD, and enzymes of Gluconobacteroxidans, which oxidize ethanol to acetic acid in a milk fat coat. Cheese that contained the enzyme capsules exhibited a stronger taste than controls. Such coimmobilizates of biocatalysts of different origin or of biocatalyst and substrate belong to a second generation of immobilized biocatalysts; they seem to be especially promising in the area of flavor production with its often poorly water soluble substrates and products.

An oxidoreductase from Candida parapsilopsis reduced the carbonyl function of keto esters; aliphatic, aromatic, and alicyclic ketones; aldehydes; and ketoacetals with high conversion rates (51). The products may be flavors or useful for flavor ester syntheses. To demonstrate its preparative value, methyl (S)-( + )-3-hydroxybutanoate, a versatile chiral building block, was synthesized with coupled coenzyme regeneration. Prenylation of small, polar compounds helps to increase their volatility and, thus, their odor activity. Prenylated odorants were found in roasted coffee and believed to contribute to the overall flavor (52). A transfer of the prenyl moiety onto benzoic acid derivatives was achieved using a membrane bound transferase from overproducing strains of Escherichia coli (53). This example documented particularly well the inherent advantages of enzyme catalysis: The substitution proceeded regio-selectively, and no isomerization of the substituent's double bonds occurred.

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