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A Technical Guide for Wine Producers
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Factors affecting Sauvignon blanc wine quality: A South African perspective
Johann Marais, ARC Infruitec-Nietvoorbij, Stellenbosch
Key words: Sauvignon blanc, methoxypyrazines, canopy microclimate, grape temperature, yeast strain
Local Sauvignon blanc wines have been the source of much discussion over the last few months. It is therefore appropriate to briefly review locally accumulated research data on matters affecting the quality of this desirable product.
Sauvignon blanc is one of the most important white wine varieties in the world. The typical aroma of this variety is described as grassy, green pepper, asparagus and gooseberry-like. These nuances are mainly caused by a specific group of chemical components, namely methoxypyrazines. Other components of equal importance are some sulphur-containing (mercapto) components, which are responsible for tropical nuances, like passion fruit. Sauvignon blanc is extremely sensitive to climatic, viticultural and production factors, partly due to the light-sensitivity of the above-mentioned methoxypyrazines.
The results of extensive vine and wine research over the last 10 years at Nietvoorbij presented some specific guidelines for Sauvignon blanc canopy management in different climatic regions, as well as optimum oenological conditions for the production of varietal-typical and high quality wines. Macro- and microclimatic differences between regions and seasons manifest in grape aroma composition, and consequently in wine quality. It was found that canopy manipulation contributes significantly to the quality of Sauvignon blanc wine. Canopy microclimate can therefore be optimised by viticultural practices to obtain grapes that will produce wine with the desired aroma composition, character, style and quality. Generally, grapes in cooler regions benefit from more sunlight exposure, while those in warmer regions benefit from more shade. Two dominant wine styles exist, namely the green pepper/asparagus "cool climate" style and the "warm climate" fruity/tropical style.
2-Methoxy-3-isobutylpyrazine (ibmp) concentrations, responsible for the typical green pepper-like aroma of Sauvignon blanc, are higher in shaded grapes than in sun-exposed grapes. On the other hand, the concentrations of certain varietal aroma components (monoterpenes and norisoprenoids), responsible for floral and fruity aromas, are higher in sun-exposed grapes than in shaded grapes. A close relationship exists between chemical, sensory and microclimatic data. During ripening, monoterpene and norisoprenoid concentrations increase, while that of ibmp decreases. Higher overall aroma concentrations are observed in cooler regions and seasons, which implies that Sauvignon blanc is better adapted to these conditions.
On the winemaking side, grape and juice temperature, oxygen contact of juice prior to fermentation, skin contact, the use of ascorbic acid and yeast strain are important parameters in the production of Sauvignon blanc wine. Harvesting of grapes at low temperatures yields more typical, delicate and higher quality wines. Since summer temperatures of 30°C and higher are common in South Africa, practices should be directed to harvesting of grapes at temperatures as low as possible. Furthermore, cooling of grapes to 10°C or lower directly after crushing is a necessity and, where practically possible, cooling prior to crushing should be conducted. Skin contact may enhance both desirable and undesirable component concentrations. This treatment is necessary to increase varietal-typical component levels, such as those of methoxypyrazines, but should also be conducted at temperatures of 10°C or lower.
Polyphenol concentrations, which may affect wine quality negatively, are lower in wines that are produced oxidatively. Although such wines are less sensitive to oxidation and pinking, oxidation of juices is not recommended. Reductive treatments are preferred in order to produce fresh and fruity Sauvignon blanc wines of high quality. The use of ascorbic acid also yields fresh/fruity wines, but it should be used judiciously, because it may cause oxidation during wine ageing. Must enrichment by reverse osmosis does not produce more typical wines from juices containing low levels of methoxypyrazines.
Different yeast strains are not all necessarily equally suited to the production of specific varietal wines under all conditions. The French yeast strain (VL3C) was specifically selected for use in Sauvignon blanc wine production. However, under South African experimental conditions, too high levels of sulphur-like off-odours were formed. Local yeast strains, such as NT 116 and VIN 13 yield relatively high ester concentrations and consequently fruity wines. They can be used with Sauvignon blanc wines with strong, typical varietal characters. However, in the case of neutral wines, the fruitiness may mask the sought-after green pepper/asparagus character. It is therefore recommended that yeast strains (e.g. VIN 7) which produce low ester concentrations, be used in such cases.
In conclusion, it is recommended that Sauvignon blanc should only be cultivated in selected cool localities. It is further recommended that canopy management be applied to obtain the correct light/shade balance that would secure optimum aroma balance, varietal typicity and wine quality. Furthermore, low temperatures during harvesting, crushing of grapes and skin contact and suitable yeast strains for specific wine styles should be applied in the production of typical and high quality varietal wines.
More information on aspects discussed in this article, as well as other aspects of importance can be obtained from the literature, cited below.
Conradie, W. J., Carey, V. A., Bonnardot, V., Saayman, D. & Van Schoor, L. H., 2002. Effect of different environmental factors on the performance of Sauvignon blanc grapevines in the Stellenbosch/Durbanville districts of South Africa. I. Geology, soil, climate, phenology and grape composition. S. Afr. Enol. Vitic. 23, 78-91.
Hunter, J. J., Volschenk, C. G., Marais, J. & Fouche, G. W., 2004. Composition of Sauvignon blanc grapes as affected by pre-v‚raison canopy manipulation and ripeness level. S. Afr. Enol. Vitic. 25, 13-18.
Marais, J., 1994. Sauvignon blanc cultivar aroma - A review. S. Afr. J. Enol. Vitic. 15, 41-45.
Marais, J., 1996. Pienkwordingsvatbaarheid van Suid-Afrikaanse Sauvignon blanc wyne. Wynboer Tegnies 79, 3.
Marais, J., 1996. Sauvignon blanc: Metoksipirasiene en mikroklimaat. Wynboer Tegnies 80, 7-8.
Marais, J., 1998. Effect of grape temperature, oxidation and skin contact on Sauvignon blanc juice and wine composition and wine quality. S. Afr. J. Enol. Vitic. 19, 10-16.
Marais, J., 2000. Die effek van mikroklimaat op Sauvignon blanc druif- en wynkwaliteit. Wynboer 128, 90-92.
Marais, J., 2001. Effect of grape temperature and yeast strain on Sauvignon blanc wine aroma composition and quality. S. Afr. J. Enol. Vitic. 22, 47-51.
Marais, J., 2001. The production of high quality Sauvignon blanc wines under different climatic conditions. VIII Latin American Viticulture and Enology Congress, 12-16 Nov. 2001, Montevideo, Uruguay (Proceedings on CD).
Marais, J., 2003. Effek van mosverryking op Sauvignon blanc samestelling en wynkwaliteit. Wynboer 165, 15-17.
Marais, J. & Swart, E., 1999. Research note: Sensory impact of 2-methoxy-3-isobutylpyrazine and 4-mercapto-4-methylpentan-2-one added to a neutral Sauvignon blanc wine. S. Afr. J. Enol. Vitic. 20, 77-79.
Marais, J., Calitz, F. & Haasbroek, P.D., 2001. Relationship between microclimatic data, aroma component concentrations and wine quality parameters in the prediction of Sauvignon blanc wine quality. S. Afr. J. Enol. Vitic. 22-26.
Marais, J., Hunter, J.J. & Haasbroek, P.D., 1999. Effect of canopy microclimate, season and region on Sauvignon blanc grape composition and wine quality. S. Afr. J. Enol. Vitic. 20, 19-30.
Marais, J., Minnaar, P. & October, F., 2004. 2-Metoksi-3-isobutielpirasien vlakke in 'n spektrum van Suid-Afrikaanse Sauvignon blanc wyne. Wynboer 175, 7-8.
Swart, E., Marais, J. & Britz, T.J., 2001. Effect of ascorbic acid and yeast strain on Sauvignon blanc wine quality. S. Afr. J. Enol. Vitic. 22, 41-46
Van Wyk, C.J. & Marais, J., 1997. Voorkoming van pienkwording van Sauvignon blanc-wyn. Wynboer Tegnies 100, 11-12.
For further information contact Dr. Johann Marais at (021) 809-3096 or maraisj@arc.agric.za.
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