Browsing by Author "Du Preez, Brannigan Jovern"

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    The evaluation of tannin activity in South African red wines
    (Stellenbosch : Stellenbosch University, 2022-04) Du Preez, Brannigan Jovern; Aleixandre-Tudo, Jose Luis; Du Toit, Wessel J.; De Villiers, Andre; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: Proanthocyanidins are complex and important phenolic compounds, which play a significant role in defining overall red wine quality. In fact, some of the beneficial bioactive components associated with moderate red wine consumption can be attributed to these non-volatile, polar compounds. Additionally, they are involved in multiple chemical reactions with other compounds from the onset of fermentation throughout periods of long-term ageing, which ultimately influence the organoleptic properties of red wine. Therefore, it is beneficial to quantify proanthocyanidin content and composition in red wine. Several chemical methods exist for the analytical quantification of tannin content and composition in red wines and/or their grape skin-, seed- or tannin extracts. These methods are based primarily on spectrophotometry and chromatography. Many methods have been used in chemosensory studies to understand factors affecting red wine mouthfeel. In recent years, the use of tannin activity has gained traction as a useful chemical measurement to understand red wine mouthfeel, with astringency in particular focus. However, from a South African context, such studies are lacking. To address this, an explorative study was performed on experimental red wine extracts, separated into various mass fractions by Sephadex LH-20 by flash column-chromatography. Following chemical characterisation of these fractions based on tannin concentration and composition, tannin activity values were shown to be strongly influenced with proanthocyanidin molecular mass. Moreover, tannin activity-concentration relationships were identified using HPLC-tannin estimation methods, while MCP tannin concentrations were poorly correlated with activity values. Although tannin estimation methods were well correlated with one another, the study showed that other factors were also involved, setting the basis for the second part of the study. The effect of wine vintage on tannin composition and activity values was investigated across multiple vintages. Sixteen Pinotage wine samples (2003-2018) isolated by an optimised FCC method rendered four fractions (F1-F4) differing in degree of polymerisation and subunit composition, increasing in size from F1 to F3, with F4 pooled from these three samples. Similar findings were reported as in the explorative study. The extent to which MCP and RPLC- based tannin estimation methods were correlated with one another, was strongly dependent on wine age and tannin size. Activity values increased with tannin size but decreased in older wines, as a function of DP. This was indicative of reactions related to age, such as oxidation and precipitation. Finally, chemosensory analysis illustrated that low positive correlations were observed between fractions of high molecular weight, and bitterness and astringency, suggesting that oligomeric and polymeric proanthocyanidins play a greater role in modifying tannin activity values. More research is needed to understand how purified tannin fractions of various sizes affect tannin activity and mouthfeel attributes, by a chemosensory approach. Additionally, this may also be integrated with other experiments to evaluate how activity values are modified by tannin origin or maturity, and other processes such as tannin oxidation or precipitation.