Browsing by Author "Wilson, Christine Leigh"

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    Chemical evaluation and sensory relevance of thiols in South African Chenin Blanc wines
    (Stellenbosch : Stellenbosch University, 2017-03) Wilson, Christine Leigh; Buica, Astrid; Brand, Jeanne; Du Toit, Wessel J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: South African Chenin Blanc is gaining recognition for its high quality both domestically and abroad. As the most widely-planted cultivar in the country, there is interest in research which can provide additional knowledge to producers and further increase Chenin Blanc wine quality. One of the sensory modalities contributing to wine quality is wine aroma, which is studied through sensory analysis and the chemical quantification of volatile compounds. Commercially-available South African Chenin Blanc wines had been characterized previously for a variety of chemical compounds, but not for thiols. Thiols, including 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexyl acetate (3MHA), are volatile sulphur compounds which are important to the ‘tropical’ and ‘green’ aromas of many wines, especially Sauvignon Blanc. The main aims of this research were to chemically characterize 3MH and 3MHA levels in a variety of commercially-available dry South African Chenin Blanc wines and explore the sensory contribution of these compounds to Chenin Blanc wine aroma. Chapter 3 reported the chemical analysis results of 3MH and 3MHA in South African Chenin Blanc Wines and explored trends within the chemical results. Chapters 4, 5, and 6 addressed the sensory relevance of thiols to South African Chenin Blanc wines. In Chapter 3, both 3MH and 3MHA were quantified in South African Chenin Blanc wines at levels above their odour thresholds. The average levels found were 893 ng/L for 3MH and 23 ng/L for 3MHA, with ranges of 380-2929 ng/L for 3MH and 0-305 ng/L for 3MHA. Significant differences were found for 3MHA levels by wine age, vine age, wood contact, price, and lees contact were found, while 3MH only differed significantly for wine origin. In Chapters 4 and 5, the sensory contribution of thiols was analysed through interaction studies. In Chapter 4, interactions of a thiol (3MH), an ester (ethyl hexanoate), and a terpene (linalool) in partially-dearomatized Chenin Blanc wine were analysed by descriptive analysis. Interaction effects were identified, such as the antagonism between the ‘tropical’ attributes of 3MH and the ‘floral’ character of linalool. The second interaction experiment, reported in Chapter 5, analysed combinations of 3MH and 3MHA in different matrices by projective mapping (PM) with intensity. This study showed that the perception of thiols was affected by the volatile and non-volatile wine matrix. The addition of an intensity measure to the ultra flash profiling step of the method provided more detailed data, which made the rapid sensory method better suited to interaction studies. In all sensory studies, wines with high thiols, especially high 3MHA, were described with ‘tropical’ and ‘green’ terms In Chapter 6, polarized projective mapping (PPM) was used to characterize commercial South African Chenin Blanc wine aroma, and sensory results were compared with extensive volatile chemical analyses. Results showed a sensorial and chemical opposition between wooded and unwooded wines. The levels of 3MHA in the wines correlated with the unwooded wines and thiol-related descriptors. PPM was applied for the first time to wine, validating a method which increases the maximum sample size of wines in rapid sensory analysis. The results of this research made contributions to the sensorial and chemical characterization of South African Chenin Blanc wines, as well as the validation of PPM and PM with intensity in wine. The knowledge that thiols are present in Chenin Blanc wines, together with existing research on practices affecting thiols can help inform viticultural and oenological decisions in the future of Chenin Blanc winemaking.