J. Marais

Terpenes in the Aroma of Grapes and Wines: A Review

This paper reviews some aspects of terpenes and terpene derivatives in grapes and wines, such as the identification of these compounds, their use in the identification of and differentiation between cultivars, the effect of grape maturity and wine-making techniques on terpene concentrations and wine quality, as well as transformations of terpenes in grapes and wine.

Oxygen in Must and Wine: A review

Oxygen can play an important role during the winemaking process. It can influence the composition and quality of the must and wine. Phenolic compounds are the main substrates for oxidation in must and wine. Oxygen addition leads to colour changes and the polymerisation of phenolic molecules in wine. Oxygen can, however, also influence the flavour and microbial composition of wine drastically, with certain off-flavours being formed and spoilage micro-organisms able to grow at too high oxygen additions to wine. A state-of-the-art, up-to-date review on the effects of oxygen in must and wine has, however, not been published recently. This review focuses on the effects of oxygen in must, during alcoholic fermentation, extended lees contact and during ageing of white and red wines. The effects it has on acetic acid bacteria and Brettanomyces are also discussed, as well as micro-oxygenation, a relative new technique used in wine production.

Effect of sunlight and shade on N orisoprenoid levels in maturing weisser riesling and chenin blanc grapes and weisser riesling wines

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.

Effect of region on free and bound monoterpene and C13-N orisoprenoid concentrations in Weisser Riesling wines

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 grape maturity and juice treatments on terpene concentrations and wine quality of Vitis vinifera L. cv. Weisser Riesling and Bukettraube

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 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

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.

Carotenoid levels in maturing grapes as affected by climatic regions, sunlight and shade

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.

Relationship Between Microclimatic Data, Aroma Component Concentrations and Wine Quality Parameters in the Prediction of Sauvignon blanc Wine Quality

Sauvignon blanc grape chemical and wine sensory data, as well as meteorological data (temperature and visible light radiation), collected in three climatically different wine regions in South Africa over three seasons and from two different canopy treatments were statistically analysed. A model for the prediction and/or definition of Sauvignon blanc wine quality was developed. The model utilises above- and within-canopy radiation and can explain 68.8% of the variation in the cultivar-typical vegetative/asparagus /green pepper intensity of Sauvignon blanc wine. Other significant correlations, e.g. between temperature and monoterpene concentrations, were also obtained. Further research is necessary to test and refine this model for application under different environmental conditions.

Effect of Grape Temperature and Yeast Strain on Sauvignon blanc Wine Aroma Composition and Quality

The study focused on the production of Sauvignon blanc wines from grapes grown in sub-optimal climatic conditions. Grape temperatures of l0°C and 25°C at harvest, and fermentation with different Saccharomyces cerevisiae yeast strains, i.e. VIN 13, VIN 7, NT 116 and NT 7, were investigated over three seasons (1998, 1999 and 2000). The highest quality wines were produced from the cooler 10°C grapes. Choice of yeast strain was complicated by effects of factors such as grape temperature, grape maturity and season. In general, the highest ester concentrations and lowest higher alcohol concentrations were produced by NT 116, and the highest wine quality by NT 7. Strain VIN 7 yielded the lowest ester levels, which in some cases were preferred, since the masking effect of esters was diminished with the result that Sauvignon blanc wines with more pronounced cultivar-typical green notes and consequently higher quality could be produced. Therefore when Sauvignon blanc grapes, known to produce neutral wines, are used, factors such as low temperature at harvest and yeast strains suitable for these conditions should be considered.

The Phenolic Composition of South African Pinotage, Shiraz and Cabernet Sauvignon Wines

Phenolic compounds are major constituents of red wines and impact on certain wine-quality parameters. This study has aimed to increase the knowledge base on the phenolic composition of South African red wines by HPLC quantification of 39 individual phenols and 2 polymeric, phenolic groups in 260 South African wines of the cultivars Pinotage, Shiraz and Cabernet Sauvignon. Statistical analysis of the collected data revealed significant cultivar differences in the levels of many of the analysed compounds, as well as in the ratios in which specific anthocyanins are found in the wines. These data may be helpful in the cultivar authentication of wines of these cultivars. Discriminant analysis of the data showed statistically significant separations of cultivars, based on their polyphenol composition, and also how the use of data from a specific wine-producing area or vintage improved the possibilities for successful authentication. The data were collected from both pure cultivar wines and commercial wines, which may contain up to 15 % of another cultivar wine. The collected data may therefore be further segmented into those obtained from pure cultivar wines and those obtained from blended wines in order to further enhance the accuracy of authentication of these respective groups of wines. The information obtained from this study opens several avenues for research on the impact of the noted cultivar differences on wine quality. Cultivar differences in the phenolic composition of young red wines also have important implications for the oenological management of oxidation reactions taking place during vinification and aging.