Browsing by Author "De Kock, Marli Christel"

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    Investigating osmotic stress in mixed yeast cultures and its effects on wine composition
    (Stellenbosch : Stellenbosch University, 2015-04) De Kock, Marli Christel; Divol, Benoit; Bauer, Florian; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: Grape must gives rise to various stress conditions for the yeast inoculated for alcoholic fermentation. These include hyperosmotic stress due to the high initial sugar concentration and redox imbalances due to the fast depletion of oxygen. Under these stress conditions, Saccharomyces cerevisiae tends to produce glycerol as an osmoprotectant and to regenerate reducing equivalents. However, the production of glycerol often leads to increased acetic acid production. According to literature, it seems that many non-Saccharomyces yeasts have a different metabolic response to the above-mentioned stress conditions, especially since it has been found that they produce low levels of acetic acid. Only recently non-Saccharomyces yeasts were researched to be used as starter cultures in wine fermentations. It is found that they can confer beneficial characteristics to the resulting wine. However, most of the non-Saccharomyces yeasts lead to stuck fermentations as confirmed by this study. Therefore, if the positive characteristics of these yeasts were to be exploited in wine making they need to be inoculated together with S. cerevisiae. When two yeasts are inoculated together, they affect each other and consequently the wine. In this context, the aim of this study was to investigate the metabolic response to hyperosmotic stress during wine fermentation of the following wine-related non-Saccharomyces yeasts: Lachancea thermotolerans, Torulaspora delbrueckii and Starmerella bacillaris. Fermentations were performed in a synthetic grape must medium with pure cultures of the mentioned strains as well as mixed cultures of each non-Saccharomyces yeast with S. cerevisiae. The fermentation behaviour was monitored and concentrations of various wine-related metabolites were determined. Concerning polyol concentrations, S. cerevisiae produced only glycerol while the non-Saccharomyces yeasts also produced other polyols. The low production of acetic acid in the non-Saccharomyces fermentations was confirmed especially in the case of L. thermotolerans. Moreover, this yeast produced high levels of the higher alcohols butanol and propanol. St. bacillaris produced significant levels of acetoin and isobutyric acid and T. delbrueckii produced an increased concentration of succinic acid. All these metabolites might play a role in maintaining intracellular redox balance. However, a more extensive systematic study is needed to investigate the extent of their involvement. The mixed cultures completed fermentation and had higher final glycerol levels than the control and lower acetic acid concentrations and therefore can contribute positively to the wine aroma. Furthermore, the mixed culture fermentations showed the potential of lowering the ethanol concentrations of wine. Furthermore it has been shown in literature that the yeasts present in the mixed culture can affect each other on gene expression level as well. However, there is little genetic information available on non-Saccharomyces yeasts. In this study, we sequenced the genes involved in glycerol and acetic acid biosynthesis of L. thermotolerans and T. delbrueckii. The gene sequences are fairly homologous with only a few differences. These gene sequences can be used to study gene expression of GPD1 and ALD6 from fermentation samples in order to determine to what extent the yeasts in a mixed culture influence the gene expression of one another.