Evaluating the effect of oxygen addition on yeast physiology, population dynamics and wine chemical signature in controlled mixed starter fermentations
Thesis (PhD)--Stellenbosch University, 2017.
ENGLISH SUMMARY: The use of commercial starter cultures of non-Saccharomyces yeast, usually together with Saccharomyces cerevisiae, has become a trend in the global wine industry in the past decade. Depending on the specific species of non-Saccharomyces yeast, the procedure may aim at enhancing aroma and flavour complexity of the wine, reduce acetic acid levels, and/or lower the ethanol yield. However, the contribution of non-Saccharomyces yeast strains depends on several factors, and in particular on the strains ability to establish significant biomass and to persist for a sufficient period of time in the fermentation ecosystem. For an effective use of these yeasts, it is therefore important to understand the environmental factors that modulate the population dynamics of such environments. In this study, we evaluated the effect of oxygen addition on yeast physiology, population dynamics and wine chemical signature in controlled mixed starter fermentations. The population dynamic in co-fermentations of S. cerevisiae and three non-Saccharomyces yeast species namely, Torulaspora delbrueckii, Lachancea thermotolerans, and Metschnikowia pulcherrima, revealed that oxygen availability strongly influences the population dynamics and chemical profile of wine. However, results showed clear species-dependent differences. Further, experiments were confirmed in Chardonnay Grape juice, inoculated with L. thermotolerans and S. cerevisiae with different oxygen regimes. The results showed a trend similar to those obtained in synthetic grape juice, with a positive effect of oxygen on the relative performance of L. thermotolerans. The results in this study also indicates that continuous stirring supports the growth of L. thermotolerans. We further analysed the transcriptomic signature of L. thermotolerans and S. cerevisiae in single and mixed species fermentations in aerobic and anaerobic conditions. The data suggest the nature of the metabolic interactions between the yeast species, and suggests that specific stress factors are more prominent in mixed fermentations. Both yeasts showed higher transcript levels of genes whose expression is likely linked to the competition for certain metabolites (copper, sulfur and thiamine), and for genes involved in cell wall integrity. Moreover, the transcriptomic data also aligned with exo-metabolomic data of mixed fermentation by showing higher transcripts for genes involved in the formation of aroma compounds found in increased concentration in the final wine. Furthermore, the comparative transcriptomics analysis of the response of the yeasts to oxygen provides some insights into differences of the physiology of L. thermotolerans and S. cerevisiae. A limited proteomic data set aligned well with the transcriptomic data and in particular confirmed a higher abundance of proteins involved in central carbon metabolism and stress conditions in mixed fermentation. Overall, the results highlight the role of oxygen in regulating the succession of yeasts during wine fermentations and its impact on yeasts physiology. The transcriptomics data clearly showed metabolic interaction between both yeasts in such ecosystem and provide novel insights into the adaptive responses of L. thermotolerans and S. cerevisiae to oxygen availability and to the presence of the other species.
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