Doctoral Degrees (Viticulture and Oenology)

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Browsing Doctoral Degrees (Viticulture and Oenology) by Author "Bagheri, Bahareh"

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    Evaluating the effect of environmental parameters on the dynamics of the wine yeast consortium
    (Stellenbosch : Stellenbosch University, 2018-03) Bagheri, Bahareh; Setati, Mathabatha Evodia; Bauer, Florian; Stellenbosch University. Faculty of AgriSciences. Dept of Viticulture and Oenology.
    ENGLISH ABSTRACT: Wine fermentation is a complex biochemical process which is characterized by the sequential development of various weakly and strongly fermentative yeast species. Thus, in such a multispecies consortium, individual species interact with one another and with their environment. Consequently, final chemical composition of wine will be affected significantly by the contribution of individual species as well as yeast-yeast interactions present in a wine consortium. The contribution of individual strain in the wine consortium is affected by several biotic (ecological interactions, killer factors, and grape variety) as well as abiotic parameters (temperature, sulphur dioxide, oxygen and nutrient availability). However, each strain will be affected differently by the combination of these parameters. Previous studies in wine fermentations have mainly focused on mixed culture fermentations composed of two species. Thus, fundamental rules underlying the effect of these parameters in a multi-species ecosystem are not fully understood. To decipher the principles that govern the complex wine ecosystem, a simplified model consortium comprising eight species commonly encountered in South African grape microbiota was established. An Automated ribosomal intergenic spacer analysis (ARISA) method was also developed in order to monitor population dynamics of the yeast consortium. The influence of presence of Saccharomyces cerevisiae as a biotic stress was investigated on the dynamics of yeast consortium in synthetic must using plating method and ARISA. Furthermore, the yeast consortium was used as an inoculum in Chenin blanc grape must where the population dynamics were monitored by plating method. The results confirmed that a selective pressure applied by the keystone species, S. cerevisiae modified the pattern of population dynamics. Wine ecosystem was characterized by supportive and inhibitory interactions. Furthermore, in spite of the differences between the two grape matrices, a similar pattern of population dynamics was observed in both fermentations. This observation suggested broad applicability of the model consortium to study the ecological interactions in the wine fermentation. In the next step, the variation in initial cell densities of each member of the consortium was used as a tool to untangle the contribution of individual strain in the population dynamics and wine aroma. The data suggest that S. cerevisiae applied a selective pressure to suppress the growth of main competitor in the wine ecosystem. Moreover, the presence of individual non-Saccharomyces species at a higher cell density, favoured the growth of some non-Saccharomyces species while suppressing the growth of others in the yeast consortium. Lastly, the effect of temperature and sulphur addition on the dynamics of the yeast consortium was evaluated in the synthetic must and real grape must fermentations. The results demonstrated that ecological interactions are largely independent of the matrix, confirming that the constructed multispecies consortium is a robust model that can be used as a tool to predict microbial behavior from a simple ecosystem to the complex natural environment. Furthermore, the effect of temperature and sulphur dioxide on the growth of non-Saccharomyces species was species/strain dependent. The results suggest that environmental parameters modify the pattern of population dynamics. However, ecological interactions seem to drive the wine ecosystem. The current study for the first time revealed the potential of a multi-species yeast consortium in understanding the ecological interactions in wine fermentation.