Browsing by Author "Julies, Jerobiam Marvin"

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    Evaluating the vitamin requirements of wine-related yeasts and the resultant impact on population dynamics and fermentation kinetics
    (Stellenbosch : Stellenbosch University, 2019-04) Julies, Jerobiam Marvin; Bauer, Florian; Divol, Benoit; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: Within the vineyard environment, grape berries serve as a habitat to various microorganisms including bacteria, filamentous fungi and yeasts, of which some play distinct roles in winemaking. Studies on yeast species other than Saccharomyces cerevisiae, commonly referred to as non-Saccharomyces (NS) yeasts in oenology, have evaluated the ability of these yeast to modulate the sensory profile of wine. In the early stages of spontaneous fermentation when the ethanol concentrations are low, the NS yeast population increases, but is progressively replaced by S. cerevisiae, which is better adapted to the environmental conditions associated with fermenting grape juice. The overall sensory profile of wine is in part a result of the metabolite production of yeasts, and the extent of the contribution of each species will depend on the total metabolic activity of each species. Metabolic activity is directly related to the availability of nutrients such as carbon, nitrogen, vitamins and trace elements. These nutrients are indeed converted to biomass and other metabolites, many of which are aroma and flavour active by-products. Only limited information regarding the nutrient requirements of wine-related yeasts other than S. cerevisiae has been published. Several studies have explored the carbon and nitrogen requirements of some NS species, but the vitamin requirements of many biotechnologically relevant species remains to be determined. Vitamins are organic compounds, mostly of a complex chemical nature, and serve as cofactors in metabolic reactions. Vitamins occur in small quantities in grapes and grape juice, but some data suggest that they may in some cases be limiting for yeast growth in this environment, affecting metabolism and ultimately impact the final wine. This knowledge gap motivates the current study, which focuses on the growth and fermentation kinetics of different NS yeasts when presented with varying concentrations of the relevant vitamins: biotin, pantothenate, inositol, thiamine and pyridoxine. In a first section, a high-throughput microtiter plate assay was optimised to allow for the rapid screening of the vitamin requirements of NS yeasts. The results of this assay showed differences in the vitamin requirements amongst the different yeasts. The statistically most significant vitamin-dependent yeast phenotypes from the screen were selected for further investigation. These included the dependence of Viniflora® P. kluyveri Frootzen ™ on biotin and thiamine and of Viniflora ® L. thermotolerans Concerto ™ on inositol. The data obtained from this study provide a better understanding of the vitamin requirements of NS yeasts and how these requirements can potentially enhance the growth performance of NS yeasts. The data suggest that targeted nutrient additions may lead to a better modulation of the overall sensory profile of wine.