Doctoral Degrees (Chemistry and Polymer Science)

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Browsing Doctoral Degrees (Chemistry and Polymer Science) by browse.metadata.advisor "De Villiers, A."

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    Application of modern chromatographic technologies for the analysis of volatile compounds in South African wines
    (2009-12) Weldegergis, Berhane Tekle; Crouch, A. M.; De Villiers, A.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: The present study was initiated by the wine industry of South Africa to overcome the lack of available information on the flavor and aroma of South African wines. The aim was to develop new analytical methods and improve existing ones for the analysis of volatile compounds in the South African wines. Initially a new analytical method based on stir bar sorptive extraction (SBSE) in the headspace mode for the analysis of 37 pre-selected volatile compounds was developed and validated. Consequently, the method was improved by making important modifications and increasing the number of compounds analyzed to 39. This method was successfully applied to a large number of Pinotage wines of vintages 2005 and 2006. The quantitative data of these wines were subjected to chemometric analysis in order to investigate possible co- /variances. A clear distinction was observed between the two vintages, where the 2005 wines were more characterized by wood-related compounds and the 2006 wines by the fermentation compounds. The developed method was further applied to other cultivars of vintage 2005, including two white (Sauvignon Blanc and Chardonnay) and three red (Shiraz, Cabernet Sauvignon and Merlot) cultivars. In a similar fashion, the quantitative data of the six cultivars of vintage of 2005 were analysed by chemometric methods. Significant differences were observed between the two white cultivars and among the four red cultivars. It was shown that among these cultivars, the major role-players were the wood and fermentation related volatiles. A striking observation was the confirmation of the unique character of the Pinotage wines compared to the other red cultivars, mainly influenced by the high level of isoamyl acetate and low level of isoamyl alcohol, the former being categorized as a varietal compound for Pinotage expressed by a fruity (banana) odor. In addition, advanced chromatographic technology in the form of comprehensive twodimensional gas chromatography (GC × GC) coupled to time-of-flight mass spectrometry (TOFMS) was investigated for the detailed analysis of volatile compounds in young South African wines. This work focused primarily on Pinotage wines. In the first instance, solid phase micro extraction (SPME) in the headspace mode in combination with GC × GC-TOFMS was used. Due to the high resolution and large peak capacity of GC × GC, more than 200 compounds previously reported as wine components were identified. These compounds were dominated by the highly volatile and less polar compounds, mainly due to the characteristics of SPME. In an attempt to further extend these results, another selective extraction method, solid phase extraction (SPE) was used in combination with GC × GC-TOFMS analysis. Using this technique, more than 275 compounds, most of them unidentified using the previous method, were detected. These groups of compounds include volatile phenols, lactones as well as mostly aromatic esters and norisoprenoids, which can potentially influence the aroma and flavor of wine. The techniques developed as part of this study have extended our knowledge of the volatile composition of South African wines.