Doctoral Degrees (Viticulture and Oenology)

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Browsing Doctoral Degrees (Viticulture and Oenology) by Author "Garrido Banuelos, Gonzalo"

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    Factors influencing the colour and phenolic composition of Shiraz wine during winemaking
    (Stellenbosch : Stellenbosch University, 2018-03) Garrido Banuelos, Gonzalo; Du Toit, Wessel J.; Buica, Astrid; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: Phenolic compounds are considered important quality contributors of red wines. However, lot of unknowns exist in terms of their role in wine due to their complexity and continuous evolution which starts in fresh grapes, during alcoholic fermentation and wine ageing. In this study, we have evaluated different parameters which could affect the colour and phenolic composition of Shiraz wines. All the work has been carried out within the same vineyard during three harvest seasons. This study has interconnected topics in order to reach a broader understanding on how certain factors influencing the phenolic composition in fresh grapes can affect phenolic extractability and subsequent wine ageing. In the first part of the study, the colour and phenolic composition of wines was changed by harvesting the grapes at different ripening levels. Differences observed in the phenolic composition in young red wines were also followed during prolonged wine ageing. The initial phenolic composition of young wines influenced the wines’ colour and phenolic evolution during bottle storage. The sensory profiles of the wines were also influenced by the grape ripening and ageing. This work also evaluated the impact of vintage, grape ripeness and alcoholic fermentation on the transformation of the grape berry cell wall. All these factors have been shown to affect the final colour and phenolic composition of young red wines. Grape ripeness was relevant for the study, but the vintage effect was shown to have the major impact on the phenolic content and the cell wall composition in fresh grapes. Nonetheless, compositional changes in the grape berry cell wall, partly influenced by the de-pectination during grape ripening and fermentation, also affected the release of grape phenolics into the wines. In addition, the extraction of grape proanthocyanidins with a longer polymer length was also partly influenced by the cell wall breakdown during fermentation, enhanced in riper berries. The last part of the study has shown the influence of different anthocyanin/tannin ratios and oxidation on the evolution of colour, phenolics and the precipitate formed over time in a wine-like system (WL). The use of advanced chromatographic and untargeted techniques has allowed us to measure the impact of different seed additions on the phenolic composition and development of the WL. A larger amount of tannins, extracted from higher additions of grape seeds, influenced the initial phenolic concentration and also the polymerisation reactions over time, which were enhanced in the presence of oxygen. Differences in the precipitate formed over time were also influenced by the initial amount of seeds used. This work highlights the importance of a better understanding of the grape cell wall composition and its evolution. This can lead to better control of the extraction of grape phenolic compounds and their ratios during the winemaking process, which can also influence the ageing of the wines.