Browsing by Author "Rapp, A."
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- ItemCarotenoid levels in maturing grapes as affected by climatic regions, sunlight and shade(South African Society for Enology and Viticulture, 1991) Marais, J.; Van Wyk, C. J.; Rapp, A.The effect of climatic region, degree of ripeness, sunlight and shade on carotenoid concentrations in Weisser Riesling and Chenin blanc grapes was investigated. Lutein and beta-carotene concentrations were higher in grapes from hot regions than grapes from cooler regions. In general, decreases in carotenoid concentrations were observed with an increase in ripeness. Also, grapes exposed to sunlight generally had lower carotenoid concentrations than shaded grapes. Both lutein and beta-carotene concentrations varied little between Weisser Riesling and Chenin blanc grapes at comparable ripening stages.
- ItemEffect of region on free and bound monoterpene and C13-N orisoprenoid concentrations in Weisser Riesling wines(South African Society for Enology and Viticulture, 1992) Marais, J.; Versini, G.; Van Wyk, C. J.; Rapp, A.Free and bound monoterpene and C 13-norisoprenoid concentrations of Weisser Riesling wines of different vintages and from different regions in South Africa, Germany and Northern Italy were compared. The concentrations of some of these compounds differed between climatic regions. For example, older Weisser Riesling wines from South Africa displayed significantly higher 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN), trans-vitispirane and trans- 1,8-terpin concentrations than those from the cooler European countries. Canonical and stepwise discriminant analysis of the wine data allowed the successful classification of the wines according to origin. A comparison of meteorological data of the countries revealed that in the cooler wine regions of Germany and Northern Italy the average daily temperatures were lower, the average monthly rainfall higher and the daily sunshine hours fewer than in the warmer South African wine regions.
- ItemEffect of storage time, temperature and region on the levels of 1, l ,6-trimethyl-1,2-dihydronaphthalene and other volatiles, and on quality of weisser riesling wines(South African Society for Enology and Viticulture, 1992) Marais, J.; Van Wyk, C. J.; Rapp, A.A survey of the concentration levels of 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN) in wines of different cultivars, vintages and regions was conducted. The TDN concentrations of South African Weisser Riesling wines were higher than those from Germany and Italy, and those of other cultivars. The effect of storage time and temperature on free and precursor TDN concentrations, on the concentrations of free trans-vitispirane, some monoterpenes, esters and higher alcohols, and on the quality of Weisser Riesling wines was investigated. Three Weisser Riesling wines from different regions were stored at 15°C and 30°C for four years and one year, respectively. These wines were analysed periodically by gas chromatography and evaluated sensorially. The concentrations of TDN, trans-vitispirane, 2,6-dimethyl-7-octen-2,6-diol and trans-1,8-terpin, and the intensity of the bottle-aged kerosene-like character increased significantly with ageing. During the same period, significant decreases occurred in the concentrations of diendiol-1, linalool, i-amyl acetate, ethyl caproate, hexyl acetate, 2-phenethyl acetate, hexanol, 2-phenyl ethanol, and in the intensity of young wine character. alpha-Terpineol showed significant increases followed by decreases under the same conditions. These changes in concentrations were more prominent at 30°C than at 15°C storage. The development of the kerosene character in Weisser Riesling wines was restricted to sensorially acceptable levels by storage at 15°C.
- ItemEffect of sunlight and shade on norisoprenoid levels in maturing weisser riesling and chenin blanc grapes and weisser riesling wines(South African Society for Enology and Viticulture, 1992) Marais, J.; Van Wyk, C. J.; Rapp, A.The effect of sunlight, shade and degree of ripeness on potentially volatile C13-norisoprenoid concentrations in Weisser Riesling grapes and wines and in Chenin blanc grapes, was investigated. Norisoprenoids were released from their bound forms by acid and enzymatic hydrolysis. With few exceptions, norisoprenoid concentrations were significantly higher in sun-exposed grapes than in shaded grapes. Significant increases in norisoprenoid concentrations were observed with an increase in ripeness. Microclimatic conditions during grape ripening for the production of Weisser Riesling wine with a potential to form lower concentrations of TDN during ageing are proposed.
- ItemFlavour components of whiskey. I. distribution and recovery of compounds by fractional vacuum distillation(South African Society for Enology and Viticulture, 2001) MacNamara, K.; Van Wyk, C. J.; Augustyn, O. P. H.; Rapp, A.A vacuum fractional distillation procedure is described for separating both the matrix components and flavour compounds of a whiskey into well-defined groups based on differences in azeotropic boiling points. The distillation was carried out at near ambient temperatures to accommodate both unaged and aged whiskies. Analytical and sensory data indicated good recovery of congeners. Individual fractions were reconstituted with ethanol and water to the original volume and strength dimensions of the whiskey. Undesirable thermal changes in the aged products were minimised by the low temperature fractionation and allowed changes in the flavour composition of whiskey due to maturation to be investigated for such unaged and aged reconstituted pairs.
- ItemFlavour components of whiskey. II. ageing changes in the high-volatility fraction(South African Society for Enology and Viticulture, 2001) MacNamara, K.; Van Wyk, C. J.; Augustyn, O. P. H.; Rapp, A.The volatile compounds isolated from whiskey by fractional vacuum distillation were identified by two-dimensional capillary gas chromatography/mass spectrometry. Changing levels with ageing were quantified for the most abundant compounds by direct split injection of whiskeys on a gas chromatograph equipped with a flame ionisation detector. The ageing decreases in volatile sulfides were similarly determined using a sulfur chemiluminescence detector. Large volume headspace injection sufficiently reproduced the distillation enrichment to allow direct twodimensional determination of similar ageing changes for other trace compounds. Seven compounds at μg/L and low mg/L levels were monitored and quantified.
- ItemFlavour components of whiskey. III. ageing changes in the low-volatility fraction(South African Society for Enology and Viticulture, 2001) MacNamara, K.; Van Wyk, C. J.; Brunerie, P.; Augustyn, O. P. H.; Rapp, A.The low-volatility wood-originating compounds isolated from whiskey by vacuum fractional distillation were analysed by high-resolution gas chromatography and mass spectrometry (GC-MS). Three phenolic esters previously unreported in whiskey were identified and confirmed by synthesis. Formation profiles for sixteen compounds were established in whiskeys aged for periods from 1.5 to 10 years in second-fill heavy-charred American Bourbon barrels. These profiles indicated significant increases for several compounds, especially in the older whiskeys. Ratios of aromatic phenolic aldehydes, and similar ratio changes during ageing, were different from reported data relating to other wood types and treatments. Further preparative separation by high-pressure liquid chromatography (HPLC) of the wood fraction followed by GC-MS allowed retention and mass spectral characterisation of additional compounds originating from wood. Sensory investigation indicated different and unique contributions from the HPLC cuts. Spiking of the three phenolic esters into a young whiskey gave a detectable increase in maturation intensity.
- ItemSome volatile aroma components of vitis vinifera L. cv. sauvignon blanc(South African Society for Enology and Viticulture, 1982) Augustyn, 0. P. H.; Rapp, A.; Van Wyk, C. J.Volatile aroma components of Vitis vinifera L. cv. Sauvignon blanc grapes were studied to identify the components responsible for the characteristic aroma of this grape cultivar. Thirty three volatile components were identified. Amongst these methional and trans-2, cis-6-nonadienal are reported present in Vitis for the first time. Evidence is also presented for the presence of three methoxypyrazines which are believed to be key substances responsible for the typical aroma present in grapes and wines of this cultivar.