Browsing by Author "Mafata, Mpho"

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    A chemometric approach to investigating South African wine behaviour using chemical and sensory markers
    (Stellenbosch : Stellenbosch University, 2021-03) Mafata, Mpho; Buica, Astrid; Brand, Jeanne; Medvedovici, Andrei V.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: The aim of this dissertation was to demonstrate the value of comprehensive narratives and elucidate critical steps in data handling in Oenology, while highlighting some common misconceptions and misinterpretations related to the process. This compilation was a journey through different stages of dealing with oenological data, with increasing complexity in both the strategies and the techniques used (sensory, chemistry, and statistics). To achieve this aim, different strategies and multivariate tools were used under two prime objectives. Firstly, several multivariate descriptive approaches were used to investigate two oenological problems and lay out the contextual foundations for the statistics-focused work (Chapters 3 and 5). Secondly, in increasing levels of complexity, statistical strategies of constructing comprehensive data fusion as well as pattern recognition models were investigated (Chapters 4 and 6). A comprehensive literature review (Chapter 2) examined and addressed common misconceptions in the different stages of data handling Oenology. The first oenological problem, described in Chapter 3, investigated the evolution of the sensory perception of aroma, as well as the antioxidant-related parameters and volatile compound composition of Sauvignon Blanc and Chenin Blanc wines stored under different conditions and durations. The study applied an appropriate sensory method for this research question, namely, Pivot©Profiling. The study was able to show the evolution of Sauvignon Blanc from ‘fruity’ and ‘herbaceous’ and of Chenin Blanc from ‘fruity’ and ‘tropical’ both towards ‘toasted’, ‘oak’, and ‘honey’ attributes. Chemically, the volatile composition did not show any trends. However, wines stored at higher temperatures for longer periods had relatively higher UV-Vis absorbance, colour density as well as higher b* (yellow) values and lower clarity in terms of L* index, compared to the control. The second oenological problem, described in Chapter 5, investigated the typicality of South African old vine Chenin Blanc perceptually and conceptually using a typicality rating and a flexible sorting task. The sensory methodology followed published strategies for investigating typicality. This study did not find a unique sensory space of the old vine Chenin Blanc due to a lack of perceptual consensus among the industry professionals for the wines included in the study. However, it did find that the industry professionals had unified ideas about the attributes of an ideal old vine Chenin Blanc wine. The first of the statistics-focused studies, described in Chapter 4, explored data fusion at low and mid-level using principal component analysis - PCA (low and mid-level) and multiple factor analysis - MFA (mid-level). The study looked at data pre-processing and matrix compatibility, which are important data handling stages for data fusion. Like the contextual chapters (Chapter 3 and 5), and keeping with the aim of this compilation, this chapter gave a detailed descriptive narrative of the data handling. Through detailed examination of the process, the study found that MFA was the most appropriate data fusion strategy. The second statistics-focused study, described in Chapter 6, continued to exploit the multiple advantages of multiblock approach of MFA. Additionally, this chapter showed the reliability of fuzzy k-means clustering compared to agglomerative hierarchical clustering (AHC).
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    The effect of grape temperature on the phenolic extraction and sensory perception of Méthode Cap Classique wines
    (Stellenbosch : Stellenbosch University, 2017-03) Mafata, Mpho; Van Jaarsveld, Francois; Du Toit, Wessel J.; Buica, Astrid; Stellenbosch University. Faculty of AgriScience. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: The first sparkling wine in South Africa was released in 1971. The South African Cap Classique Producers Association (CCPA), formed for the appreciation of Méthode Cap Classique (MCC) traditional style sparkling wines (TSW), was established in 1992 and has since contributed to the growth of these wines on a competitive footing with the international market. Generally, studies on TSW have focused primarily on the foam capability, volatile composition and autolytic character of the wines and very little on phenolic content of the wines. Phenolic compounds are important quality indicators of wine. Their composition in wine is determined by various factors including grape variety, terroir, viticultural practice, and oenological practices. In this project, MCC wines were made by the traditional method using Chardonnay and Pinot noir grapes harvested from two regions (Robertson and Darling) and stored at 0, 10, 25 and 30ºC, over two vintages (2014 and 2015). The phenolic concentration of the wine samples throughout the winemaking process was analysed by spectrophotometer and the aroma and taste of the final 9 month old sparkling wines performed. The study was aimed at investigating the effect of the grape storage temperature on the phenolic content and the sensory properties of MCCs through a quantitative phenolic analysis. The study found that MCCs made from grapes stored at lower temperatures (0 and 10ºC) had lower total phenolic content, colour intensity and total hydroxycinnamates than wines made from grapes stored at higher temperatures (25 and 30ºC) showing that there was greater phenolic extraction from grapes stored at 25 and 30ºC. The total phenolics, as measured by spectrophotometer, was below the range cited in literature for Champagne made from the same cultivars. The sensory evaluation of the MCCs comprised a sorting analysis similar to that used for beers. Separating the aroma and taste sorting of the MCCs, the study showed a grouping of the MCCs according to temperature treatments for both vintages. There were, however, clear vintage differences in terms of the attributes cited and the frequency of citations. Based on frequency of citation, 2014 MCCs made from grapes stored at 0 and 10°C were described by judges as being fruity, fresh and crisp whilst those made from grapes stored at 25 and 30°C were described as having oxidised fruit, volatile acidity and solvent-like aromas. The judges perceived less oxidation and VA (in terms of the frequency of citation) in the aroma of 2015 MCCs, although higher temperature treatments were still associated with less desirable attributes compared to lower temperature treatments. Judges were better able to separate the Darling wines according to treatments compared to the Robertson wines. This study has shown that the grape storage temperature has an effect on the phenolic extraction and the sensory perception of MCCs aged 9-months with no changes in the phenolic content observed throughout winemaking.
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    Measuring thiols in single cultivar South African red wines using 4,4-dithiodipyridine (DTDP) derivatization and ultraperformance convergence chromatography-tandem mass spectrometry
    (MDPI, 2018) Mafata, Mpho; Stander, Maria A.; Thomachot, Baptiste; Buica, Astrid
    Wine varietal thiols are important contributors to wine aroma. The chemical nature of thiols makes them difficult to measure due to low concentrations, high sensitivity to oxidation, and low ionization. Methods for the measurement of thiols usually consist of multiple steps of sample preparation followed by instrumental measurement. Studies have collected large datasets of thiols in white wine but not in red wine, due to the lack of availability of suitable methods. In this study, for the first time, convergence chromatography was used to measure thiols in red wine at ultratrace levels with improved sensitivity compared to previous methods. Performance parameters (selectivity, linearity, limits of detection, precision, accuracy) were tested to demonstrate the suitability of the method for the proposed application. Red wine thiols were measured in South African Pinotage, Shiraz, and Cabernet Sauvignon wines (n = 16 each). Cultivar differentiation using the thiol profile was demonstrated.
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    Measuring thiols in single cultivar South African red wines using 4,4-dithiodipyridine (DTDP) derivatization and ultraperformance convergence chromatography-tandem mass spectrometry
    (MDPI, 2018-08-30) Mafata, Mpho; Stander, Maria A.; Thomachot, Baptiste; Buica, Astrid
    Wine varietal thiols are important contributors to wine aroma. The chemical nature of thiols makes them difficult to measure due to low concentrations, high sensitivity to oxidation, and low ionization. Methods for the measurement of thiols usually consist of multiple steps of sample preparation followed by instrumental measurement. Studies have collected large datasets of thiols in white wine but not in red wine, due to the lack of availability of suitable methods. In this study, for the first time, convergence chromatography was used to measure thiols in red wine at ultratrace levels with improved sensitivity compared to previous methods. Performance parameters (selectivity, linearity, limits of detection, precision, accuracy) were tested to demonstrate the suitability of the method for the proposed application. Red wine thiols were measured in South African Pinotage, Shiraz, and Cabernet Sauvignon wines (n = 16 each). Cultivar differentiation using the thiol profile was demonstrated.