of about 800 mg L-1 was reached. Capillary gas chromatography on chiral and achiral phases showed a high enantiomeric purity of some of the chiral lactones (>98% ee). After the pioneering work using castor oil as a substrate for the fungal production of 4-decanolide (fruity-fatty odor) had ended up with yields of less than 5 g L"1 (77), the race for the best process was opened. During the past 10 years, every major flavor house started to develop its own lactone process. When, for example, species of the Mucor fungi were fed under aerated conditions with fatty acids or fatty acid esters, 4- and 5-alkanolides with the respective carbon numbers were produced in good yields (78). Similar effects of triacylglycerol substrates were reported for many other microorganisms.

Based on the universality of biochemistry, microbial models for studying flavor synthesis have widely replaced the inconvenient plant cell systems. One should, however, be aware that nature frequently provides more than just one pathway for a certain flavor molecule. This obviously restricts an unseen extrapolation of results. Genetic engineering should therefore be considered as a second step, after a biogenetic route has been fully elucidated and selected for gene transfer. Microbial versus plant formation of 4-dodecanolide (peachy-fatty odor; odor threshold in the fig L~1 range) may illustrate the situation: A chiral 10-hydroxystearic acid was obtained by bacterial conversion of oleic acid, and a baker's yeast concluded the sequence by /J-oxidation and lactonization (79). Data obtained with fruits of strawberry and peach, however, support a sequence starting with an epoxy fatty acid and proceeding through a /i-oxidative chain shortening of the co-3,4-dihydroxy fatty acid. Elimination of water from the hy-droxylactone followed by reduction of the enlactone concluded the biosynthesis (80). Both routes led to the same optically pure (R)-form of the lactone.

Only about 0.2% (equivalent to 20 to 30 tons) of the vanillin consumed worldwide is isolated from the botanical source, whereas the remainder is synthetic. This disproportion has stimulated research for a biotechnological substitution, despite some arguing about the details of its biosynthesis (20). Until now, neither plant cell cultures of Vanilla nor bacteria or fungi yielded more than trace amounts of the target compound. Again, as mentioned for the preceding bacterium (72), feeding of precursors (here lignin, eugenol, ferulic acid, curcumin, or benzoe siam resin) presented a solution. Different 13C-isotope ratios of the vanillins allowed to assign the flavor compound to a specific geographical or biochemical source (81).

Benzaldehyde, the second most important flavor molecule after vanillin, is obtained from amygdalin in peach or apricot kernels and is a character impact ingredient in stone fruit flavors, such as cherry coke. Equimolar amounts of hydrocyanic acid are formed concurrently and cause major safety problems. Among the biotechnological alternatives is the microbial degradation of natural l-phenylalanine. This amino acid has become easily available (from biotechnology), as it is an intermediate of the synthesis of the high intensity sweetener, aspartame (26). Strains of the basidiomycete Ischnoderma benzoinum afforded significant yields of benzaldehyde upon phenylalanine feeding (82). The metabolic pathways were elucidated using submerged cultures of this fungus and ring-labeled deutero-l-phenylalanine as a precursor (83). Depending on the phase of the cell cycle operating, phenylalanine was almost completely converted to benzaldehyde or to 3-phenylpropanol (flowery, roselike odor).

Generally, the supplementation of cell cultures with suitable (natural) precursor substrates, customary in numerous bioprocesses for pharmaceuticals, is an efficient tool to further increase the yields of volatiles. While high yields will continue to be key to the economy of a biopro-cess, a more critical evaluation of terms such as "optimization" or "best performance" has started: Optimization should not be regarded equivalent to the maximization of any one single parameter, but depends on the definition of a frame of reference to which the developmental activities are directed. A first step into this direction is the replacement of the term "yield" by time- and biomass-related productivity data.

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