In order to compete internationally, the South African wine industry and Nietvoorbij have made a huge financial investment to prioritise, launch and operate a research programme about the terrain specificity of cultivars. An important part of the research was a project concerning microclimate. Sauvignon blanc was used in this study – and others – due to the cultivar’s climatic sensitivity. Sauvignon blanc is currently one of the most important white cultivars in South Africa and the total plantings have increased by 90%, from 2618 hectares in 1986 to 4966 hectares in 1998. Although high quality Sauvignon blanc wines are being produced locally, the biggest percentage consists of neutral wines that do not display the typical characteristics of the cultivar. This study was therefore undertaken firstly to identify microclimatic parameters which influence grape composition and wine quality, and secondly to apply this knowledge so that the quality of Sauvignon blanc wines may be improved, thus making South Africa more competitive on international markets.It is important to identify the impact components of Sauvignon blanc. Once these are recognised, they may be applied to guide viticultural and oenological practices so that optimal grape and wine quality is obtained. The most important aroma components of Sauvignon blanc may be divided into two groups, namely those which are present in the grape (monoterpenes, norisoprenoids, methoxypyrazines) and those which are formed during fermentation (mercapto components, esters and higher alcohols). The most important impact component is 2-methoxy-3-isobutylpyrazine (ibMP), which is responsible for the typical grassy and green peppery flavours of Sauvignon blanc.This article is an abridged and popular version of published work on the same topic (Marais, Hunter & Haasbroek, 1999). More information as well as references to other studies on this topic can be found in the said publication.
MATERIAL AND METHODS
The techniques were described in full (Marais et al., 1999) and may be summarised as follows. The trial was conducted on Sauvignon blanc over five years (1995-1999). Control grapes (normal sunlight exposure) were compared to shaded grapes (natural shading). Microclimatic parameters, namely temperature and light radiation, were monitored in the vicinity of the bunches on an ongoing basis and linked to impact components of Sauvignon blanc, namely 2-methoxy-3-isobutylpyrazine (green peppery character), monoterpenes and norisoprenoids (fruity, flowery character) in the grapes. Wines were made according to standard white wine practices and wine quality (fruitiness, asparagus/green pepper aroma intensities) determined sensorially. The study investigated the effect of grape ripeness (from veraison to harvest), different regions (the cool Elgin, moderate Stellenbosch and warmer Robertson areas), as well as seasons (the cool 1997 season versus the warmer 1996, 1998 and 1999 seasons).
RESULTS AND DISCUSSION
Data, indicating typical trends of ripeness, vintages and regions, are given in Figures 1 to 7. The concentrations of total monoterpenes (the sum of individually determined monoterpenes) increased in line with an increase in light exposure, as well as an increase in ripeness between veraison (approximately 16B) and harvest (approximately 21B) (Fig. 1). Total bound aroma components (monoterpenes and norisoprenoids; also determined individually) showed similar trends (Data not shown). In certain instances total monoterpene, as well as total bound aroma concentrations, peaked about a week before ripeness, followed by a decrease in concentrations.
The effect of microclimate on ibMP concentration shows an inverse trend to monoterpenes and norisoprenoids, namely a decrease with an increase in light exposure, as well as with an increase in ripeness between veraison and harvest (Fig. 2).
Average temperatures inside the canopy in the vicinity of the bunches show the expected differences between the warmer Robertson, more moderate Stellenbosch and cooler Elgin areas (Fig. 3). They also indicate subtle differences between treatments, namely lower average maximum temperatures in the shade treatments. Seeing that average temperatures do not always indicate obvious trends, it is important to look at outlier temperatures as well (Fig 4). Here the differences between regions and vintages were more prominent, which probably better explains the big differences in wine styles between regions and vintages. Variation in temperature over a season and between day and night can have an important effect on the development or degradation of aroma components. With regard to light radiation, the trends within the vines were as expected, while light radiation trends above the vines were affected by cloudiness and/or pollution (Fig. 5). With ibMP being so light sensitive, it goes without saying that light radiation is a very important factor in the determination of Sauvignon blanc wine quality.
Different climatic conditions manifest differently in aroma component levels and consequently in grape and wine quality. Certain aroma component concentrations are higher in cooler regions (Fig. 6), as well as in cooler seasons (Fig’s. 1 and 2). It is clear that wine styles, with regard to the contribution made by the particular aroma components measured (Fig. 6), may differ significantly between grapes that were exposed to the sun and grapes that were in the shade, as well as between the warm Robertson and cooler Elgin areas. This was indeed the case if one looks at the sensorial data of the wines from the regions in question (Fig. 7). The wine styles of the shaded grapes tended to be more green pepper-like, while the wine styles of the grapes that had been exposed to the sun tended to be more fruity. Furthermore the Robertson wines displayed the more tropical, fruity style and the Elgin wines the more green pepper/asparagus style. This does not imply that the quality of one style is better than that of the other. There is a place in the market for both styles, on condition that the Sauvignon blanc character should always be recognisable. In this regard an attempt was made to define optimum ripeness, namely: “Optimum ripeness for maximum Sauvignon blanc wine quality is that stage where sufficient ibMP is still present in the grapes to obtain the desired green pepper aroma, and where it is complemented by sufficiently developed concentrations of other fruity/flowery aromas.” The implication is that Sauvignon blanc grapes that are harvested too early, may display a non-complex, one-sided grassy character, and grapes that are harvested too late, especially in very warm vintages and regions, may seem too neutral.
SUMMARY AND CONCLUSIONS
Light radiation, temperature, monoterpene and norisoprenoid concentrations decrease and ibMP concentration increases with an increase in canopy density. Monoterpene and norisoprenoid concentrations increase and ibMP concentration decreases during ripening. Monoterpene, norisoprenoid and ibMP concentrations are higher in grapes from cooler localities and cooler seasons. There is an obvious relationship between trends in aroma concentration and microclimatic factors, and in future temperature and light radiation may be used to predict wine quality. Two styles of Sauvignon blanc, namely the green pepper/asparagus style and the fruity/tropical style, are possible and within certain limits, it is up to the viticulturist and wine-maker to decide which style they want to produce. In order to produce high quality Sauvignon blanc grapes and wines in South Africa, it is recommended that “cool” localities should be identified in each region. Sauvignon blanc should definitely not be planted anywhere. Furthermore, it is possible to obtain optimum light/shade balance in a locality – and therefore optimum flavour balance – by applying the correct canopy management. It is then up to the wine-maker to ensure that the potential of Sauvignon blanc grapes is further exploited.
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.