Bioflavours and fragrances via fungi and their enzymes

Many fungi and yeasts have been found to produce de novo odorous compounds. Ceratocystis species and the yeasts Kluyveromyces lactis and Sporidiobolus salmonicolor produce a wide range of terpenes and lactones with fruity or floral flavours. The yeast Williopsis saturnus synthesizes de novo fruity ester flavours (i.e. volatile branched acetates); their yield can be improved by feeding fusel oil as a cheap source of precursor branched alcohols to the fermentation process. Geotrichum klebahnii also produces a broad spectrum of ethylesters of branched carboxylic acids, generating a pleasant fruity flavour. Also, precursor speciality fatty acids and PUFA’s can be converted by fungi (such as Penicillum sp. and Botryodiplodia sp.) into flavour compounds, that provide “green notes”, mushroom flavour, fruity lactones and cheese-flavoured methylketones. Similarly, a two step fungal process has been developed, whereby Aspergillus niger tranforms ferulic acid into vanillic acid, which basidiomycetes such as Pycnoporus cinnabarinus or Phanerochaete chrysosporium can further convert into vanillin. Furanone-flavours occur in many fruits, but have also been detected in microbial cultures. In this context, the soy sauce yeast Zygosaccharomyces rouxii forms the DMHF-furanone compound from glucose, when fed with fructose-1,6-biphosphate. Apart from precursored fermentation processes, enzymatic systems are also being developed to produce flavours i.e. yeast alcohol dehydrogenase can convert 1-phenyl-2-propanone into (S)-1-phenyl-2-propanol; in vitro co-enzyme regeneration often remains a bottleneck. Yeasts such as Torulopsis bombicola and Candida tropicalis can convert fatty acids or alkanes into musk-fragrance precursors. These examples indicate that interdisciplinary cooperation between microbiologists, biochemists, organic chemists and bioprocess engineers is needed to develop interesting laboratory findings into economic bioflavour production processes.