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See also Sauvignon Blanc - the dust hasn't settled
2-Methoxy-3-Isobutylpyrazine Levels in a Spectrum of South African Sauvignon Blanc Wines
ARC Infruitec-Nietvoorbij, Stellenbosch
Key words: Sauvignon blanc, iso-butylmethoxypyrazine
Introduction
Sauvignon blanc is one of the most important white wine cultivars in South Africa. The cultivar's total plantings increased by 146% from 1986 (2618 ha) to 2002 (6450 ha) (Martin, 2003). Typical Sauvignon blanc wines have a characteristic cultivar character. This complex character consists of various nuances that are described, inter alia, as green, grassy, green pepper-like, asparagus-like, herbaceous, fig, gooseberry, litchi and tropical fruit. It is well known that certain methoxypyrazines are responsible for the typical green pepper/herbaceous nuances in Sauvignon blanc, Sémillon and Cabernet Sauvignon grapes and wines. 2-Methoxy-3-isobutyl pyrazine (iBMP) is by far the most important because it occurs in much higher concentrations than the other methoxypyrazines. The particularly low threshold value of iBMP, namely 2 ng/L in water (Buttery et al.,1969) and 1 ng/L in white wine (Allen et al., 1988), indicates that this component can have a huge impact on wine quality.
The above nuances and the complexity of the aroma differ depending on the climatic conditions under which the grapes are cultivated. In local investigations the highest iBMP levels occurred, together with the accompanying highest wine quality, in wine from cool areas such as Elgin, in cool seasons such as 1997, and in shady conditions (Marais et al. 1999). The temperature and light sensitivity of methoxypyrazines often result in South African Sauvignon blanc wines having hardly any to no cultivar character whatsoever as far as the typical grassy/greenpepper aroma is concerned. This obviously poses a problem, seeing that South Africa must compete on overseas markets with countries that have cooler climate conditions, such as New Zealand. Typical iBMP concentration levels, as determined in a number of overseas Sauvignon blanc wines, ranged from 5 to 40 ng/L in France, from 10 to 35 ng/L in New Zealand and from approximately 2 to 15 ng/L in Australia (Lacey et al., 1991). Seeing that the Australian viticultural areas are similar to the South African areas from a climatological point of view, the same order of iBMP levels might be expected locally.
Methoxypyrazines are obviously not the only flavour components that play a role in Sauvignon blanc aroma. Certain mercapto compounds that are formed during fermentation are responsible for the characteristic tropical flavours of Sauvignon blanc, while numerous other chemical components also play a lesser or more important role. Given the complexity of wine aroma, high levels of iBMP should therefore not necessarily be associated with high wine quality. Nevertheless iBMP is considered an important measure of quality. The purpose of this investigation was therefore to compile a blueprint or profile of typical iBMP levels in South African Sauvignon blanc wines.
Trial procedure
A spectrum of Sauvignon blanc wines, their origin and number being as far as possible representative of the plantings in the South African wine industry, were obtained from the entries in the 2002 and 2003 Veritas Wine Shows (Table 1). In the case of the 2003 season six wines were also obtained from the Young Wine Show to supplement the number of wines in two regions. If there were still no sufficient wines from a region, wines were obtained from adjacent regions with compatible climates. Altogether 88 wines were obtained from the 2002 vintage and 115 from the 2003 vintage. iBMP was extracted from the wines and analysed using gas chromatography/mass spectrometry according to the technique of Kotseridis et al. (1998).
Table 1. 2-Methoxy-3-isobutylpyrazine (iBMP) concentrations in a
representative spectrum of Sauvignon blanc wines from different
regions and seasons.
Region % of total plantings 2002
Season 2003
Season Number of wines iBMP (ng/L)
Number of wines iBMP (ng/L)
Cool
regions1 II2 <1 11 2.183 (<1.0
- 3.96)4 11 3.70 (<1.0
- 14.13) Durbanville III <1 5 5.50 (<1.0
- 11.59) 6 0.95 (<1.0
- 1.19) Stellenbosch III 35 30 3.67 (1.48
- 9.08) 35 2.08 (<1.0
- 9.41) Paarl IV 18 12 2.86 (<1.0
- 8.10) 21 2.87 (<1.0
- 7.71) Robertson IV 15
13 1.75 (<1.0
- 4.16) 17 1.47 (<1.0
- 3.02) Worcester IV 11 11 1.54 (<1.0
- 3.78) 11 1.84 (<1.0
- 5.82) Swartland5 IV 15
3 3.26 (1.18
- 6.46) 10 1.47 (<1.0
- 3.21) Olifants
River V
5 3 2.84 (<1.0
- 4.27) 4 1.96 (<1.0
- 1.17) 1
= Includes Constantia, Elgin and Hermanus. 2
= Classification according to Le Roux, 1974. 3
= Average concentrations. 4
= Lowest and highest concentrations. 5
= Includes Darling, Perdeberg and Porterville.
2002 Season
The iBMP concentrations in 88 Sauvignon blanc wines from the 2002 season are presented in Table 1. Based on the average iBMP concentrations it was only the Durbanville region, considered to be a cool area, that showed relatively high values. However, a few wines with iBMP concentrations that are considerably higher than the threshold value also occurred in the Stellenbosch, Paarl and Swartland regions. The three wines from the Swartland region were all from the Darling area which is also considered to be fairly cool. There is no explanation for the relatively low average value in the other "cool regions" (Constantia, Elgin and Hermanus). On the other hand the 2002 season was a particularly warm season which could definitely have played a role with regard to the overall iBMP levels.
2003 Season
The iBMP concentrations in 115 Sauvignon blanc wines of the 2003 season are also shown in Table 1. Indications are that the 2003 season was cooler than the 2002 season. The percentage of gold medals earned by Sauvignon blanc at the 2003 Veritas wine show also exceeded that of the previous year (10% compared to 5%), although one cannot simply ascribe this to the "cooler season". However, this state of affairs was not reflected in the average iBMP concentrations. For example, the Durbanville area showed a drastically lower average than the previous year. The cooler regions (Constantia, Elgin and Hermanus) nevertheless had the highest iBMP levels, with relatively high values also to be found in a few Stellenbosch and Paarl wines.
General
A variation in averages from one vintage to the next occurred within a region and may possibly be ascribed, inter alia, to the fact that the wines did not come from the same producers throughout. For the two seasons the highest iBMP concentrations (between 10 and 14 ng/L) occurred in the wines from the cool regions, namely Elgin, Constantia and Durbanville. Relatively high values were also observed in the Stellenbosch and Paarl regions. The lowest average iBMP values occurred in the warmer Robertson, Worcester and Olifants River regions. The above trends are to be expected, both theoretically and judging from experience. The data of the Olifants River (2002 season) and Durbanville (2003 season) regions are exceptions for which there is no simple explanation.
It should be borne in mind that some of the wines in this investigation might have contained a percentage of other cultivar wines, as allowed by the South African certification system. One nevertheless assumes that blends would have been the exception and in instances where they did occur, that the percentage of other cultivars would have been very low, in order to prevent a possible lack of Sauvignon blanc character to be further masked.
Although methoxypyrazines are not the only role players, they are important grape quality parameters and it is necessary for viticultural and oenological practices to be optimised so as to produce maximum quality Sauvignon blanc wines (Marais, 1998; Marais et al., 1999; Marais, 2001). The choice of optimal terrain for the cultivation of Sauvignon blanc is of cardinal importance in a warm country such as South Africa, and it goes without saying that the coolest localities must be selected.
Summary
The results of this investigation show iBMP concentrations in a representative spectrum of Sauvignon blanc wines, produced in South Africa over two seasons (2002 and 2003). The average and highest values give some indication of what may be expected in the respective climatic regions. It is clear that on the whole, the average iBMP levels were particularly low in both seasons. This indicates that Sauvignon blanc is being cultivated in the wrong localities and/or that viticultural and oenological practices were not optimal. There were a few exceptions from certain cool localities, however, indicating that Sauvignon blanc shows potential in these areas. In certain instances iBMP averages varied from vintage to vintage within a region, therefore the highest averages should be taken as the potential of that particular region. Although the wines used in this study were as representative as possible, it could be that specific terroirs with potentially high levels of iBMP were not taken into account. Further investigations will be required in this regard.
Literature references
Allen, M. S., Lacey, M. J., Harris, R. L. N. & Brown, W. V., 1988. Sauvignon blanc varietal aroma. Aust. Grapegrower & Winemaker 292, 51-56.
Buttery, R. G., Seifert, R. M., Guadagni, D. G. & Ling, L. C., 1969. Characterization of some volatile constituents of bell peppers. J. Agric. Food Chem. 17, 1322-1327.
Kotseridis, Y., Anocibar Beloqui, A., Bertrand, A. & Doazan, J. P., 1998. An analytical method for studying the volatile compounds of Merlot noir clone wines. Am. J. Enol. Vitic. 49, 44-48.
Lacey, M. J., Allen, M. S., Harris, R. L. N. & Brown, W. V., 1991. Methoxypyrazines in Sauvignon blanc grapes and wines. Am. J. Enol. Vitic. 42, 103-108.
Le Roux, E. G., 1974. 'n Klimaatsindeling van die Suidwes-Kaaplandse wynbougebiede. M.Sc. Thesis, Stellenbosch University, Stellenbosch, South Africa.
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., 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., 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.
Martin, G., 2003. Belangrike wyndruiftipes in Suid-Afrika. Wynboer 170, 8-11.
Acknowledgement
Financial support by Winetech and donation of the wines by the South African National Wine Show Association.
Further information may be obtained from: Dr. Johann Marais, tel. (021) 809 3096, e-mail: johannm@infruit.agric.za
2-METHOXY-3-ISOBUTYLPYRAZINE LEVELS IN A SPECTRUM OF SOUTH AFRICAN SAUVIGNON BLANC WINES
Sauvignon blanc is one of the most important white varieties in South Africa. Its typical varietal aroma is characterised by different nuances, described as grassy, green pepper-like, asparagus-like, herbaceous, fig, gooseberry and tropical fruits. Methoxypyrazines are responsible for the typical grassy and green pepper nuances. The most important methoxypyrazine in Sauvignon blanc grapes and wines is 2-methoxy-3-isobutylpyrazine (iBMP), because it appears in much higher concentrations than the other methoxypyrazines. A spectrum of Sauvignon blanc wines, of which the origin and number were as far as possible representative of the plantings in the South African wine industry, was obtained from the 2002 and 2003 Veritas Wine Show. In total, 88 wines from the 2002 vintage and 115 wines from the 2003 vintage were obtained. IBMP was extracted from the wines and analysed by gas chromatography/mass spectrometry. The results of this survey displayed typical iBMP concentrations in Sauvignon blanc wines, produced in different regions over two seasons. The average and highest values gave an indication of what can be expected in terms of iBMP levels in the various climatic regions. Average values in each region were in general relatively low (1 to 3 ng/L), although a few values between 10 and 14 ng/L were obtained in the cooler regions, such as Elgin, Constantia and Durbanville.
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