SUNScholar - Browsing by Author "Van Wyk, C. J."

Browsing by Author "Van Wyk, C. J."

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Carotenoid 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.
Characterisation of the aroma of the hybrid Ferdinand de Lesseps (Vitis vinifera x Vitis labrusca)
(South African Society for Enology and Viticulture, 2000) Rogers, I. M.; Van Wyk, C. J.
Freon 11-extracted volatiles of Ferdinand de Lesseps grape juice were studied by capillary gas chromatography, capillary gas chromatography-mass spectrometry and gas chromatography-sniffing. The berries were crushed under an inert atmosphere and the juice extracted with Freon 11 for 20 hours. Thirty-one compounds were reported. The juice was quantitatively characterised mostly by esters, particularly hydroxy esters. These esters are most probably responsible for the sweetish pineapple aroma of Ferdinand de Lesseps grapes whereas o-aminoacetophenone and 2,5-dimethyl-4-hydroxy-3(2H)-furanone could contribute to its hybrid note.
Chenin blanc wine volatiles and the intensity of a guava-like flavour
(South African Society for Enology and Viticulture, 1982) Van Rooyen, P. C.; De Wet, P.; Van Wyk, C. J.; Tromp, A.
A guava-like flavour occurring in some South African Chenin blanc and Colombard dry white wines was investigated. Headspace volatiles of some of these wines, as well as those of fresh guava fruits, were analysed by means of a gas chromatograph equipped with a "sniffer-detector". In contrast to the headspace concentrate of guava fruits, no single fraction with a typical guava flavour was found in the wines analysed. The pattern recognition system "ARTHUR" was therefore used to investigate relationships between the more important wine volatiles and the intensity of a guava-like flavour of a set of wines originating from the same Chenin blanc must. Sensory scores for this flavour were used in both category and continuous property analysis in each case. Category and correlation to property plots were used to elucidate the results. Several promising variables and ratios between variables were singled out for further investigation, notably ethyl butyrate and the ratios ethyl butyrate/ethyl decanoate and ethyl butyrate/ethyl octanoate.
Effect of grape maturity and juice treatments on terpene concentrations and wine quality of Vitis vinifera L. cv. Weisser Riesling and Bukettraube
(South African Society for Enology and Viticulture, 1986) Marais, J.; Van Wyk, C. J.
The effect of grape maturity aud different juice treatments, namely free-run, skin-contact, pressing and heat treatment on individual terpene concentrations in Weisser Riesling and Bukettraube juices and wines, as well as on different wine quality parameters, was investigated. Linalool, hotrienol, alpha-terpineol, nerol, geraniol, citronellol, 3,7-dimethylocta-1,5-dien-3,7-diol and the furan and pyran Iinalool oxides were analysed gas chromatographically. Wine quality parameters, such as terpene-like character, cultivar authenticity and overall wine quality of these wines were sensorially evaluated. Increases in grape maturity and the application of juice treatments, like skin-contact and heat treatment, caused significant increases in the concentrations of the majority of terpenes analysed. These changes were, to a limited extent, reflected in the intensities and qualities of some wine characteristics. Heat treatment emerged as the single factor, causing the most prominent increases in terpene concentrations and improvement in overall wine quality.
Effect 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.
Effect 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.
Effect 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.
Flavour 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.
Flavour 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.
Flavour 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.
Histamine and tyramine content of South African wine
(South African Society for Enology and Viticulture, 1985) Cilliers, J. D.; Van Wyk, C. J.
The histamine content of 184 wines and tyramine content of 156 wines, produced in South Africa was measured. The histamine and tyramine content of the wine was found to be similar to those of wines produced in other countries. The average histamine content of South African red wines that had undergone malo-lactic fermentation was more than double that of red wines that had not undergone malo-lactic fermentation. All the red wines containing relatively large amounts of histamine had pH's above 3, 7. Six selected strains of malo-lactic bacteria were tested for their ability to form histamine and tyramine in white and red wine. No histamine or tyramine was formed.
A "phenolic" off-odour in white table wines : causes and methods to diminish its occurrence
(South African Society for Enology and Viticulture, 2000) Van Wyk, C. J.; Rogers, I. M.
The quality of some Kerner table wines often is rated inferior owing to the presence of an objectionable odour designated as "medicinal", "elastoplast" (band-aid) or "phenolic". Occasionally this odour is also encountered in wines from other cultivars such as Gewiirztraminer, Weisser Riesling, Muscat de Frontignan and Chenin blanc. The objectives of this study were to identify the compounds responsible for the off-odour and to establish possible relationships between their occurrence and vini-viticultural procedures. The component predominantly responsible for the "medicinal" odour was identified as p-vinyl guaiacol. This compound is known to be formed during alcoholic fermentation via decarboxylation of ferulic acid. Yeast strains, however, differed appreciably with respect to their ability to produce p-vinyl guaiacol. Most, if not all, of this component is apparently formed during alcoholic fermentation of grape juice. Only in one exceptional case was the presence of this odour detected in grapes from a crossing of Cruchen blanc x Servan blanc, which also contained relatively high levels of p-vinyl guaiacol. Wines made from grapes harvested at an advanced degree of maturity and in particular those exposed to sunlight, contained higher levels of p-vinyl guaiacol than did those from shaded grapes. Oxidation and subsequent treatment of grape juice with phenol-adsorbing fining agents such as activated charcoal, polyvinyl polypirrolidone, casein and gelatine (in combination with "kieselsol" and bentonite) led to decreases in the p-vinyl guaiacol levels and the concomitant "medicinal" odour intensity.
A rapid and quantitative HPLC method for determination of diethylene glycol
(South African Society for Enology and Viticulture, 1986) Van Rooyen, T. J.; Van Wyk, C. J.
A method was developed for determining diethylene glycol (DEG) by HPLC. Good separation was achieved by analysis of standard mixtures containing normal wine components and DEG. Addition of DEG to wine samples proved that DEG did not elute with any of the known constituents. Recovery studies proved the method to have a good repeatability. A selection of South African and imported wines were analysed and it was found that none contained any DEG.
Some 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.

Browsing by Author "Van Wyk, C. J."

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