BIODIVERSITY OF METSCHNIKOWIA PULCHERRIMA AS A RESOURCE FOR INNOVATION IN FERMENTED BEVERAGES

The present PhD thesis was developed with the general aim of exploring the diversity and the biotechnology potential of non­conventional yeasts ascribed to the genus Metschnikowia, and in particular to the species M. pulcherrima, for application in winemaking and fermented beverages. Indeed, they could be considered promising bioresources, useful to contribute to innovation and sustainability in the agrifood sector in the framework of the Sustainable Development Goals, specifically SGDs 9 and 12, pertaining to “industry, innovation and infrastructure” and “responsible consumption and production”, respectively. The first part of this thesis was focused on a single strain M. pulcherrima DBT012, previously selected for winemaking, on its taxonomic placement and possible industrialization. Its genome was sequenced, and comparative genomics allowed to support its identification at the species level as well as the very recent reclassification of the entire clade, which was previously formed by 11 species. As for strain industrialization, the protocol for dry biomass production was successful and, as the strain retained high viability during rehydration and showed suitable activity when inoculated in grape must. In the second part of the thesis, the genotypic and phenotypic biodiversity of 193 isolates, gathered from different isolations sources (plants, flowers, fruits, and musts), different countries (Italy and Poland), and in different years (from 2016 to 2020) was explored. The de­replication process allowed the recognition of 129 strains displaying a high genotypic variability as well as significant phenotypic diversity in terms of β­glucosidase and esterase activities and H2S production. A subset of the collection, representative of the phenotypic diversity, was chosen for in-depth technological characterization, aimed at selecting 3 strains for lab­scale fermentation of commercial apple juice. The fermentations were followed by microbiological, chemical, and sensorial analyses of the fermented juices. Remarkably, despite the different enzymatic activities observed in vitro during the screening step, the results of the volatile compounds in the fermented juices did not reveal significant variations of important aroma­active molecules, especially esters, among the three strains. Those discrepancies suggested that in vitro screening procedures were not predictive of strain abilities in a real fermentation matrix; therefore, alternative enzymatic screening procedures should be devised and biocatalytic mechanisms should be further investigated as well. The collection realized in this study will be deposited in the Verona University Culture Collection (VUCC­DBT) and, to date, it represents the biggest repository of Metschnikowia pulcherrima yeasts that will be publicly available. Finally, during the PhD, a model system based on yeast resting cells was performed to test for biocatalytic properties of microbial strains. The study was published, and it is reported in the appendix of the thesis.