Browsing by Author "Williams, Michell Teresa"

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    Characterisation of wine yeasts for varietal red wine production by using chemical, sensory and metabolomic tools
    (Stellenbosch : Stellenbosch University, 2018-03) Williams, Michell Teresa; Hart, Rodney; Khan, Wesaal; Stellenbosch University. Faculty of Science. Dept. of Microbiology.
    ENGLISH ABSTRACT: Modern day wine making includes direct inoculation of active dried yeast (ADY), primarily Saccharomyces cerevisiae, into relatively ‘neutral’ flavoured grape must. Subsequently, wine yeast strains influence wine quality through de novo synthesis or by converting odourless aroma precursors present in red grape must into aroma active compounds, which contribute to the varietal aromas and flavours ranging from ‘strawberry’, ‘raspberry’, ‘blackcurrant’, ‘plum’, ‘caramel’, ‘herbaceous and/or vegetative’, to ‘spicy’, and even ‘peppery’. Furthermore, yeast proteins produced and secreted during alcoholic fermentation were shown to have oenological importance, since they are critical during the release of some aroma compounds e.g. volatile thiols. Thus, it is important to select yeast starter cultures with the ability to enhance and complement varietal aromas and flavours. Therefore, this master’s study was undertaken with the aim of investigating the influence of a naturally isolated wine yeast strain i.e. ARC Nvbij 6 (S. cerevisiae) on typical red wine quality by utilising chemical, sensory, proteomic and metabolomics characterisation tools. Shiraz, Merlot and Cabernet Sauvignon winemaking trials were initiated during the 2016 and 2017 vintages with the inclusion of two commercial reference strains i.e WE372 (Anchor Oenologies, South Africa) and MERIT (Chr. Hansen, Denmark). The yeast strain ARC Nvbij 6 was shown to consistently produce Shiraz, Merlot, and Cabernet Sauvignon during the 2016 and 2017 vintages, equal and in some instances better than both commercial references. It is noteworthy that all wines produced with ARC Nvbij 6 also had a negative association with undesirable volatile acidity (VA) and acetic acid, which are known to impart unpleasant off-odours, thereby masking the sought-after varietal aromas and flavours. Furthermore, descriptive sensory evaluations showed that the ARC Nvbij 6 strain, for the most part, produced Shiraz, Merlot, and Cabernet Sauvignon wines with sought-after aromas and flavours. Gas chromatography (GC) also showed the ARC Nvbij 6 strain to be a better ‘3-mercaptohexan-1-ol (3MH) to 3-mercaptohexyl acetate (3MHA) converter’, as both commercial references also failed to convert 3MH to 3MHA during one vintage in two cultivars. In terms of aroma compounds i.e. esters (associated with fruity nuances), both commercial references mostly produced Shiraz, Merlot and Cabernet Sauvignon wines with higher ester concentrations than the ARC Nvbij 6 strain. Nonetheless, ARC Nvbij 6 consistently produced less of the undesirable compounds that are associated with wine off-odours, which can influence the wine sensory quality negatively. Furthermore, sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) showed that all yeast strains differentially expressed proteins within given molecular weights. It can be envisaged that peptide mass fingerprinting (PMF) in conjunction with matrix-assisted laser desorption ionization with time of flight mass spectrometry (MALDI -TOF MS) will be deployed to characterise specific yeast-derived proteins that were regulated and draw conclusions with regard to how they are associated with aroma compounds. Thus, proteomic tools may be used to select promising wine yeast strains with sought-after traits in terms of wine quality. The use of multiple omics approaches is also encouraged, as proteome does affect metabolome, which in turn determine wine chemical and sensory quality. Overall, the ARC Nvbij 6 strain proved that it has a commercial role to play in the production of varietal red wines, especially Shiraz, based on chemical and sensory attributes of all red wines included in this study.