Biotechnological Production and Sensory Evaluation of ω1-Unsaturated Aldehydes.

Lipid extracts of the fungus Flammulina velutipes were found to contain various scarce fatty acids including dodec-11-enoic acid and di- and tri-unsaturated C16 isomers. A biotechnological approach using a heterologously expressed carboxylic acid reductase was developed to transform the fatty acids into the respective aldehydes, yielding inter alia dodec-11-enal. Supplementation studies gave insights into the fungal biosynthesis of this rarely occurring acid and suggested a terminal desaturation of lauric acid being responsible for its formation. A systematic structure-odor relationship assessment of terminally unsaturated aldehydes (C7-C13) revealed odor thresholds in the range of 0.24-22 µg/L in aqueous solution and 0.039-29 ng/L in air. In both cases, non-8-enal was identified as the most potent compound. All aldehydes exhibited green odor qualities. Short-chained substances were additionally associated with grassy, melon-, and cucumber-like notes, while longer-chained homologs smelled soapy and coriander leaf-like with partly herbaceous nuances. Dodec-11-enal turned out to be of highly pleasant scent without off-notes.

Biotechnological Production of Odor-Active Methyl-Branched Aldehydes by a Novel α-Dioxygenase from Crocosphaera subtropica.

As a result of their pleasant odor qualities and low odor thresholds, iso- and anteiso-fatty aldehydes represent promising candidates for applications in flavoring preparations. A novel cyanobacterial α-dioxygenase from Crocosphaera subtropica was heterologously expressed in Escherichia coli and applied for the biotechnological production of C12-C15 branched-chain fatty aldehydes. The enzyme has a sequence identity of less than 40% to well-investigated α-dioxygenase from rice. Contrary to the latter, it efficiently transformed short-chained fatty acids. The kinetic parameters of α-dioxygenase toward unbranched and iso-branched-chain substrates were studied by means of an oxygen-depletion assay. The transformation products (C12-C15 iso- and anteiso-aldehydes) were extensively characterized, including their sensory properties. The aldehydes exhibited green-soapy, sweety odors with partial citrus-like, metallic, peppery, and savory-tallowy nuances. Moreover, the two C14 isomers showed particularly low odor threshold values of 0.2 and 0.3 ng/L in air as determined by means of gas chromatography-olfactometry.