Doctoral Degrees (Institute for Wine Biotechnology)

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Browsing Doctoral Degrees (Institute for Wine Biotechnology) by Author "Fairbairn, Samantha"

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    Defining the chemical features of wine perception
    (Stellenbosch : Stellenbosch University, 2018-03) Fairbairn, Samantha; Bauer, Florian; Da Silva Ferreira, A. C.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: All wines evoke a product recognition, regardless of quality and cultivar, but what is the origin of this feature? The prevalence of this wine concept suggests that its formation occurs independent of the varietal, and ageing-related aromas, and is therefore potentially a function of yeast metabolism. Yeast utilise the nutrients present in grape must to produce biomass, and metabolites which ultimately signify the conversion of grape juice to wine. Consequently, the nutrient composition is highly influential on the aromatic outcomes of alcoholic fermentation. Synthetic grape must is widely used to evaluate all facets of the fermentation process but there remains much to learn. In this study, the impact of two nutrients, namely, amino acids and anaerobic factors, were evaluated with regard to their impact on yeast growth and aroma production under fermentative conditions. This work also examines the extent to which yeast de novo metabolism, both primary and secondary metabolism, contributes to the formation of the wine-like feature. In a single amino acid context, a linear relationship was apparent between the amino acid concentration and the production of their associated volatile products. This relationship was evaluated in more complex amino acid mixtures and as expected, this linear relationship was lost. Nonetheless, a significant degree of responsiveness between the amino acid and its catabolites remained. The impact of sterol (plant or yeast derived) or unsaturated fatty acid treatments, individually, as well as in combinations, were compared for their contributions to biomass formation and aroma production. Sterols had a greater impact on biomass development, as the fermentations treated with only unsaturated fatty acids displayed a poorer response. Moreover, they differently impacted aroma production. The unsaturated fatty acid lowered the production of acetate esters, medium chain fatty acids and their esters, whereas sterol supplementation generally bolstered the production of all compounds measured. This work highlights the importance of anaerobic factor management during winemaking. Although these nutrients certainly impact wine aroma, this study also sought to examine the degree to which these nutrients contribute to wine (product) recognition. Using a novel fermentation-based approach, Saccharomyces cerevisiae converted a synthetic grape must into a wine-like product. These synthetic products underwent sensory evaluations to rate the product’s resemblance to wine as well as to describe the aroma. This sensory data was used as a decision-making tool to decide upon treatments to be studied in subsequent fermentations. Ultimately, a wine-like character was created by altering the anaerobic factor composition of a synthetic grape must. The use of this synthetic grape must would allow for the more meaningful sensory characterisation of these synthetic products, in addition to providing a wine-like matrix used to evaluate the sensory implications of wine odorants.