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Exploring the relationships between Pinotage grape colour and wine quality - Preliminary results
As a result of its South African uniqueness and distinctive berry/ plum character, Pinotage is increasingly in demand locally and overseas. Knowledge about the phenolic composition of Pinotage grapes and wine, especially with respect to colour, is lacking. Research on grape colour, as an objective measure to predict red wine quality, is in progress in Australia and promising results have been obtained (Francis et al, 1999; Holgate, 2000). A similar approach was followed in a local, preliminary study on Pinotage. The purpose was to determine the relationship between grape colour and wine characteristics and quality.
Material and Methods
Grape processing and wine-making
Sixteen Pinotage grape samples (16 - 20 kg per sample) from different climatic regions in South Africa (2000 season) were harvested between 17øB and 26øB. The samples were immediately frozen at -20øC until processing. Still in the frozen state, berries were carefully removed from the stems. Three representative portions (100 berries each) were selected from the large pool of berries. One lot was used to determine juice øB and the other two kept frozen at -20øC until further processing for berry colour and seed phenolics analyses. The rest of the berries were mixed and divided into three equally representative parts (three replicates) and each replicate was crushed and fermented at 25øC in 10? plastic buckets, using Saccharomyces cerevisiae yeast strain WE 14. The juice/skins of each sample was mixed (punching-down of the cap) three times per day. Further processing was according to standard small-scale winemaking techniques, used at the Nietvoorbij experimental cellar.
One of the 100 berry lots of each sample was processed for the determination of grape colour. Still in the frozen state, the 100 berries were weighed and homogenised (Iland et al, 2000).
Colour assessment
Grape colour (red pigments of berries) was measured for each homogenised sample, after extraction with 50% EtOH, at A520 nm and expressed as mg anthocyanins per gram berry weight (Iland et al, 2000). Wine colour was measured at A420 nm and A520 nm, the spectral values summed and expressed as modified wine colour density (at pH 3,5 and with any effects of SO2 removed) (Somers & Evans, 1977; Iland et al, 2000).
Sensory evaluation
Wines were sensorially evaluated for cherry intensity, berry/plum intensity and overall wine quality by a panel of seven experienced judges. Evaluation of the quality parameters was done by means of the line-method.
Statistical analysis
Statistical relationships between variables were determined by linear regression analysis.
Results and Discussion
Correlations between grape colour and wine colour, cherry intensity, berry/plum intensity and overall wine quality are shown in Figures 1 to 4. These findings confirmed similar results reported previously (Somers & Evans, 1974; Iland, 1987; Francis et al, 1999; Holgate, 2000). Grape colour, which increases with an increase in grape ripeness, was significantly, positively correlated with modified wine colour density
(Fig 1).
It is further evident that the cherry intensity of the wine decreased with an increase in grape ripeness (Fig 2) or was masked by the concomitant increase in berry/plum intensity (Fig 3). Both flavours contribute to the typical character of Pinotage, of which the overall wine quality was highly positively correlated with grape colour (Fig 4). Although these flavours were measured in the wines, they would reflect the changes that are occurring in the grapes during ripening. At harvest, grape ripeness of the 16 samples analysed (one replicate of each sample) varied between 170B and 260B. It is clear that an increase in overall wine quality coincided with an increase in grape ripeness (Fig 5). It can therefore easily be reasoned that grape ripeness may also be a suitable parameter for the prediction of optimum wine quality. However, when seven samples, harvested between 23,20B and 24,40B, are compared, it is clear that wine quality can differ significantly, even when the grapes are harvested at almost similar ripening stages (Fig 6). This may be ascribed to different grape compositions, caused by different climatic conditions and viticultural practices. In the latter case (Fig 6), the R2 value indicates that the berry colour index accounts for 78% of the variation in wine quality.
Conclusions
Preliminary results on Pinotage indicated that a prediction of wine quality, either in terms of cultivar-specific characteristics (e g berry/plum intensity) or overall quality, from grape colour is indeed possible. Further research is necessary to confirm these results and to apply it to other red wine cultivars as well. These aspects will be investigated as part of a project on the phenolics of red grapes and wines, which will commence at Nietvoorbij during the 2001 season. The possibility to use grape colour assessment as a tool to compensate the grape producer accordingly, should also be investigated.
Acknowledgements: Technical contributions by D van Schalkwyk and A Louw are appreciated.
Literature Cited
FRANCIS, I.L., ILAND, P.G., CYNKAR, W.U., KWIATOWSKI, M., WILLIAMS, P.J., ARMSTRONG, H., BOTTING, D.G., GAWEL, R. & RYAN, C., 1999. Assessing quality with the G-G assay. In : BLAIR, R.J., SAS, A.N., HAYES, P.F. & HOJ, P.B. (eds.). Proc. 10th Aust. Wine Ind. Tech. Conf., 2-5 Aug. 1998, Sydney, Australia. pp. 104-108.
HOLGATE, A., 2000. Berry colour index assay - application and interpretation of the assay as an objective measure of red wine grape quality in a commercial vineyard. Proc. 5th Int. Symp. Cool Climate Vitic. & Enol. 16-20 January 2000, Melbourne, Australia. In press.
ILAND, P., 1987. Predicting red wine colour from grape analysis. The Aust. Grapegrower & Winemaker 285, 29.
ILAND, P., EWART, A., SITTERS, J., MARKIDES, A. & BRUER, N., 2000. Techniques for chemical analysis and quality monitoring during winemaking. Patrick Iland Wine Promotions, Campbelltown, Adelaide, Australia.
SOMERS, T.C. & EVANS, M.E., 1974. Wine quality: Correlations with colour density and anthocyanin equilibria in a group of young red wines. J. Sci. Food Agric. 25, 1369-1379.
SOMERS, T.C. & EVANS, M.E., 1977. Spectral evaluation of young red wines: Anthocyanin equilibria, total phenolics, free and molecular SO2 , "Chemical Age". J. Sci. Food Agric. 28, 279-287.
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