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Effect Of Berry Size, Sunlight Exposure And Ripeness On Chenin Blanc Wine Quality
Johann Marais, Danie van Schalkwyk and Francois October
ARC Infruitec-Nietvoorbij, Stellenbosch
Key words: Berry size, sunlight exposure, shade, ripeness, wine quality, Chenin blanc
This study is part of a broader investigation into Chenin blanc. Aspects such as lees contact after fermentation and aging potential of Chenin blanc wine will be discussed in future articles.
INTRODUCTION
Chenin blanc plantings and harvested tonnage amount to approximately 18% of the total in South Africa. Based on this prominent role played by Chenin blanc in the South African wine industry and as a result of competition with other white cultivars and other wine producing countries on overseas markets, there is renewed interest in Chenin blanc as a white wine cultivar. Recent local and international competitions have shown that this cultivar, when cultivated and treated properly, is able to produce high quality wines. With neutral cultivars such as Chenin blanc depending mostly on the presence of fermentation flavourants, viticultural and enological factors that promote the development of positive fermentation components are therefore bound to play an important role. In this study the effects of berry size, exposure to sunlight, shading and degree of ripeness on the wine quality of Chenin blanc were investigated. Berry size and grape composition may be influenced by irrigation amongst others (MaCarthy, 1977; Myburgh, 2003), while sunlight exposure and shading have a significant effect on grape composition and wine quality (Marais et al., 1992; Marais et al., 1999).
MATERIAL AND METHODS
The grapes were obtained from the Stellenbosch region (Villiera) in 2002 and from the Robertson (ARC experiment farm) and Riebeek-West (Dagbreek) regions in 2004. Differences in berry size were manipulated by pruning on Villiera and by irrigation in Robertson and Riebeek-West. In the case of irrigation (small berry treatment) moisture stress was applied after berry set and irrigation only took place at a tensiometer reading of 90 kPa. Grapes from the small berry treatments and the large berry treatments (grapes exposed to sunlight and shaded grapes) were harvested at both 21°B and 24°B. In total there were eight treatments, namely 1) = Small berries/Sun/21°B; 2) = Small berries/Sun/24°B; 3) = Small berries/Shade/21°B; 4) = Small berries/Shade /24°B; 5) = Large berries/Sun/21°B; 6) = Large berries/Sun/24°B; 7) = Large berries/Shade/21°B; 8) = Large berries/Shade/24°B. Berry size (mass and volume) was monitored in all the treatments. In all instances three crates (60kg) of grapes were used per treatment and wine was made according to standard Nietvoorbij small-scale white wine vinification techniques. Total acid concentrations of the treatments were, where required, adjusted to 7 g/l. The wines were analysed gas chromatographically for esters, as well as sensorially evaluated for fruitiness/fermentation bouquet and overall wine quality by a panel of six experienced judges. The study was done in triplicate.
Table 1. Summary of the overall effects of berry size, sunlight exposure, shading and ripeness on Chenin blanc wine quality (2002 and 2004 seasons).
Comparison
between small and large berries (regardless of ripeness, sunlight
exposure and shading) Ester
concentrations Wine
quality No
obvious difference between large and small berry treatments Trend
of higher values in small vis à vis large berry treatments
(specifically in Riebeek-West trial) Comparison
between 21oB and
24oB (regardless
of sunlight exposure and shading) Berry
size Ester
concentrations Wine
quality No
obvious difference between 21oB
and 24oB Obvious
trend of higher values at 24oB
compared to 21oB Trend of
higher values at 24oB
compared to 21oB
Comparison
between sunlight exposure and shading (regardless of ripeness) Berry
size Ester
concentrations Wine
quality Trend
of predominant bigger berries in the shade than in the sun Trend
of predominant higher values in the shade than in the sun
Trend
of predominant higher values in the shade than in the sun
RESULTS AND DISCUSSION
During the study problems were repeatedly experienced with rot and incomplete ripening and therefore the grapes of certain treatments could not always be obtained. Certain trends could nevertheless be observed in the results that were available (Figures 1 to 3). A summary of these trends is given in Table 1. The viticultural manipulations (pruning in the Villiera vineyard and irrigation in the Riebeek-West and Robertson vineyards) induced the expected differences in berry size and may therefore be considered successful. Although ester concentrations and wine quality varied among the different berry size treatments, the trend at the Riebeek-West trial was that the small berry treatments produced higher quality wines.
With regard to the comparison between grapes that were exposed to sunlight and shaded grapes, the latter treatment resulted almost throughout in bigger berries and the corresponding wines had higher ester levels and a higher wine quality.
CONCLUSIONS AND RECOMMENDATIONS
It goes without saying that grapes should not be harvested too early or too overripe. Judging from the conditions of this study Chenin blanc appears to be optimally ripe at between 21°B and 24°B, but probably nearer to the higher degree of ripeness. An attempt should be made to decrease berry size either by pruning or irrigation, since smaller berries seem to produce higher quality wines. This is due to the fact that the smaller berries may contain higher concentrations of flavourants and precursors thereof. Furthermore direct sunlight radiation on grapes should be avoided, because grapes that ripen under indirect light conditions produce higher quality wines. To a large extent the results of this study confirm expectations based on actual experience. It is necessary for the study to be repeated over more seasons to confirm the trends that have been observed.
MaCarthy, M.G., 1977. The effect of transient water deficit on berry development of cv. Shiraz (Vitis vinifera L.). Aust. J. Grape Wine Res. 3, 102-108.
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., Van Wyk, C.J. & Rapp, A., 1992. Effect of sunlight and shade on norisoprenoid levels in maturing Weisser Riesling and Chenin blanc grapes and Weisser Riesling wines. S. Afr. J. Enol. Vitic. 13, 23 - 32.
Myburgh, P.A., 2003. Response of Vitis vinifera L. cv. Sultanina to level of soil water depletion under semi-arid conditions. S. Afr. J. Enol. Vitic. 24, 16 -24.
Further queries may be addressed to: Dr Johann Marais, tel. (021) 809-3096, email MaraisJ@arc.agric.za or Danie van Schalkwyk, tel. (021) 809-3156, email VSchalkwykD@arc.agric.za
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