Doctoral Degrees (Viticulture and Oenology)

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Browsing Doctoral Degrees (Viticulture and Oenology) by browse.metadata.advisor "Buica, Astrid"

Now showing 1 - 3 of 3
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    Characterising sensory interactions between volatile phenols and other taint-causing compounds in South African red wines
    (Stellenbosch : Stellenbosch University, 2019-04) McKay, Marianne; Buica, Astrid; Bauer, Florian; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: South African red wine competes for limited market space, and must maintain consistent, excellent quality. One of the important modalities in assessing the quality and typicality of wine is odour perception (Hopfer et al., 2015). Most investigations quantify aroma compounds by chemical/analytical means and compare levels to odour detection thresholds (ODTs) provided by scientific literature. If malodourous compounds are present at concentrations above their ODTs, they may be considered a threat to wine quality as they exhibit odour activity values >1 (Louw et al., 2010; Prida & Chatonnet, 2010). A review of the literature reveals that studies in wine can use inappropriate ODTs for work carried out in a new wine matrix, and matrix effects on the activity and perception of a compound are often ignored. To properly scientifically evaluate the effect of any compound to wine aroma, formal sensory evaluation in the study matrix is essential (Villamor & Ross, 2013; Perry & Hayes, 2016). Certain volatile phenols (VPs) can contribute to a continuum of smoke-taint related off-flavours including ‘burnt’, ‘bretty’, ‘smoky’, and ‘ashy’ attributes in wine (Jiranek, 2011; Kennison et al., 2011) at higher levels but are generally accepted as being benign to wine aroma at subthreshold levels (Boidron et al., 1988; Prida & Chatonnet, 2010). Useful sensory studies on VPs in wine have been carried out (Simpson et al., 1986; Boidron et al., 1988; Chatonnet et al., 1992; Kennison et al., 2008; Petrozziello et al., 2014), but at supra-threshold levels, and the effect of combinations of subthreshold levels of VPs is not taken into account. The main aims of this research were to characterise the sensory contribution of specific VPs at low levels to red wine odour, and to assess their effect on the perception of attributes through interactions with themselves and other compounds known to be involved in specific wine offflavours. Chapter 2 addresses key knowledge and gaps within the literature around origin, perception and odour detection threshold of VPs associated with specific off-flavours (including ‘smoke’ and ‘ashiness’), and previous studies concerning interaction of aroma compounds which may have relevance to the current study. Chapter 3 investigated trends within the chemical (GC-MS) and sensory (Descriptive Analysis) results for twelve commercial samples of smoke-affected wine. Associations between negative attributes and bushfire events prior to harvest were found. Results also showed that certain sensory effects could have resulted from combinations of subthreshold levels of VPs. The need arose to formally test perceptual interactions at subthreshold levels to see if various effects could be explained. It was thus decided to investigate effects of three VPs: guaiacol, ortho-cresol, 4-ethylphenol, and two compounds associated with certain off-flavours in wine, viz. 3- isobutyl-2- methoxypyrazine (IBMP) and 2,4,6-trichloroanisole (TCA). The work was carried out in partially de-aromatised Shiraz, and it was necessary to establish if ODTs provided in the literature were appropriate for this matrix. Formal sensory work is known to be complex, time-consuming and expensive, particularly in establishing ODTs, and thus a pragmatic sensory approach to the work is outlined in Chapter 4. Chapters 5 and 6 address the perceptual effects of combinations of two, three, four and five offflavour compounds on red wine aroma, which has not been conducted on this scale before. The results of this DA sensory study showed olfactory opposition between clean controls, wines spiked with single compounds (generally fruity and sweet-associated), and wines spiked with complex combinations of VPs and IBMP (linked to negative attributes). Chapter 7 demonstrates another pragmatic approach, using projective mapping (PM) with a large sample size (n=18). Comparable results to the DA interaction study for four compounds in red wine were shown. Chapter 8 investigates effects of combinations of two VPs on four cultivars in order to establish whether there were perceptual olfactory effects that were common to all cultivars, or whether the matrices responded differently from an aroma perception perspective, and shows that samples spiked with combinations of VPs and IBMP show consistently negative olfactory attributes that are independent of cultivar. This research contributes to the sensorial and chemical characterization of selected VPs in red wines, and shows that subthreshold levels of VPs in combination with very low levels of IBMP and TCA can lead to olfactory interactions that cause various olfactory effects, some of them negative. This may help inform winemaking decisions, particularly when dealing with smokeaffected grapes, and/or cultivars that naturally have higher levels of methoxypyrazines, like Merlot and Cabernet Sauvignon. This study also emphasises the importance of understanding effects of VPs on wine aroma, and escalating awareness and sensitivity to these issues in the wine industry.
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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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    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.