Masters Degrees (Institute for Wine Biotechnology)

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Browsing Masters Degrees (Institute for Wine Biotechnology) by browse.metadata.advisor "Goussard, P. G."

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    Some aroma compounds of importance to the quality of Ferdinand de Lesseps and Kerner wines
    (Stellenbosch : Stellenbosch University, 2000-03) Rogers, Ilva Margaret; Van Wyk, C. J.; Goussard, P. G.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: Ferdinand de Lesseps grapes have a distinctive fruity varietal character and were often used judiciously by wine-makers to enhance the fruity bouquet of some white table wines. Ferdinand de Lesseps grape juice was investigated to identify the main contributing compounds responsible for its unique and intense aroma. The juice was recovered from grapes under anaerobic conditions and extracted using Freon 11. The concentrated extract was analysed using a combination of capillary gas chromatography-mass spectrometry and gas chromatography-sniffing techniques. Thirty-two compounds were reported. With the aid of GC-sniffing, it was concluded that the hybrid note of the Ferdinand de Lesseps grape was most likely attributed to the presence of 2,5-dimethyl-3(2H)-furanone and 0- aminoacetophenone. Esters, which featured prominently in the juice, consisted mainly of ethyl butanoate, ethyl- and methyl- 3-hydroxy butanoate, and to a lesser extent, ethyl 3- hydroxy hexanoate and ethyl-3-hydroxy propanoate. These esters are most likely responsible for the sweetish pineapple aroma. According to EEC regulations, the use of non- Vinifera grapes in the production of commercial wines is prohibited. The presence of 2,5-dimethyl- 3(2H)-furanone in wine could therefore be interpreted as a labrusca indicator should it be suspected that Ferdinand de Lesseps grapes were used in the blend. The quality of some South African Kerner table wines often is rated inferior owing to an unwanted odour described as "Elastoplast" or "medicinal". This odour is encountered occasionally in wines from other cultivars such as Gewurztraminer, Weisser Riesling, Muscat de Frontignan and Chenin blanc. The identification of the compounds responsible for the offodour was investigated, as well as possible relationships between its occurrence and viniviticultural practices. The compound responsible for the "medicinal" off- odour was identified as p-vinyl guaiacol. It is formed via decarboxylation of ferulic acid by yeast during alcoholic fermentation. lts concentration is directly related to the concentration of ferulic acid and the yeast strain. Other factors affecting p-vinyl guaiacol formation in Kerner wines are region, microclimate, exposure of grapes to sunlight and grape maturity. Wines made from grapes harvested from the warmer climatic regions and exposed to direct sunlight and increased maturity contained higher levels of p-vinyl guaiacol. Viticultural practices that can be applied to limit the formation of the p-vinyl guaiacol precursor in grapes are the use of a canopy manipulation to protect the grapes from direct sunlight and an earlier harvesting date. Various winemaking techniques can be used to lower p-vinyl guaiacol levels in wine. Careful consideration must be given to the choice of yeast, as different yeast strains differ considerably with their ability to form p-vinyl guaiacol. Fining oxidised grape juice with phenol adsorbing agents such as activated charcoal, polyvinylpolypirrollidone, casein, gelatin (particularly in combination with bentonite and "kieselsol") led to decreases in the p-vinyl guaiacol levels. Although fining with activated charcoal was the most effective must treatment for reducing the "medicinal" aroma of Kerner wines, it stripped the wines of colour and character. Excessive oxidation of must followed by fining involves extra costs and time for the winemaker and would not be used in making the reductive style of wines.