A new generation of yeasts for the differentiation and improvement of wine quality

The optimized use of the grape environment indigenous microbiota has been proposed as an innovative tool to mimic the complexity of the spontaneous fermentation process, without taking the risks of having poor-quality wines due to the uncontrolled growth of spoilage organisms or having a “standardized” product lacking originality caused by the inoculation of a single starter culture of Saccharomyces cerevisiae exhaustively employed worldwide. With the aim of exploring new alternatives for the improvement of wine quality, it was proposed the screening of innovative yeasts to be used on mixed fermentations together with S. cerevisiae. After the mining of more than 400 yeasts isolates in samples coming from multiple Italian wine-producing regions, a collection of non-Saccharomyces yeasts was thoroughly described morphologically and identified. About one quarter of them belonging to three more interesting and prevalent oenological species, namely Starmerella bacillaris, Lachancea thermotolerans and Metschnikowia spp., were picked for an in-depth molecular and physiological characterization. After the genotyping, stress tolerance assays, enzymatic activity trials and single inoculum fermentations, important differences were acknowledged between the strains and species, allowing for the selection of a reduced number of isolates showing potentially positive oenological traits. Furthermore, the isolates of S. bacillaris and L. thermotolerans were also submitted to a safety assessment of virulence factor related to human pathogenicity and were challenged to inhibit the growth of the phytopathogen Botrytis cinerea, responsible for massive quantitative and qualitative losses on the wine industry, across in vitro and in vivo assays, in order to become candidates for an integrated vitivinicultural strategy of biocontrol and mixed fermentation. The chosen isolates from the preliminary selection steps were then applied in sequential inoculations with a commercial S. cerevisiae in natural grape must. The physicochemical analysis and the aromatic profile confirmed the great potential of all three species to a successful winemaking routine, mainly focused on the glycerol increase and reduction of acetaldehyde concentration for S. bacillaris, limitation of acetic acid and boosted aromatic complexity for Metschnikowia spp., and diminution on ethanol content and enhanced natural acidity by lactic acid production for L. thermotolerans. This last feature was intriguingly highly variable among the strains and thus received a more detailed attention, revealing that sequence mutations and transcriptional regulatory mechanisms could be involved in the strong variance of a very relevant metabolic pathway. Indeed, many of the distinctive features of alternative yeasts are species- or strain-dependent and the efficient selection of a new generation of starter cultures could provide satisfactory answers to multiple challenges of modern winemaking.