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Relationship between grape colour and wine quality
By: Johann Marais and Francois October, ARC Infruitec-Nietvoorbij, Stellenbosch
Key words: Grape colour, wine quality, Pinotage, Shiraz, Cabernet Sauvignon
Introduction
Grape colour is an integral and important part of red grape and red wine quality. It is generally accepted that an increase in grape colour coincides with an improvement in phenol structure, an increase in aroma intensity and an increase in wine quality. The colour of red or black grapes is caused by the presence of anthocyanins in the grape skins. In the search for objective, easily quantifiable quality parameters, colour is therefore a logical option and has been investigated as a possible grape and wine quality parameter for many years. It has the benefit of being measurable in the vineyard even before vinification and all its concomitant variables takes place. Research about the relationship between grape colour and wine quality has already been done in Australia and promising results have been obtained (Iland, 1987; Holgate, 2000). To further facilitate the measurement of grape colour, colour charts for the visual assessment of grape juice have been developed, which should be able to replace the more expensive spectrophotometric assessment of colour (Mollah & Reynolds, 2002). Grape colour has even been used as a parameter for remuneration of producers for their grapes (Barnett, 2004). In a preliminary local investigation good correlations have been obtained between Pinotage grape colour and wine quality (Marais et al., 2001). These results were pertinent to one season only, however. The purpose of this study was to determine the relationship between grape colour and wine quality in Pinotage, Shiraz and Cabernet Sauvignon from different areas and across various seasons.
Material and methods
The procedure of Iland et al. (2000) was followed and briefly entails the following: Grapes from different areas (20 kg/vineyard block) were obtained over four seasons (2001 to 2004) and immediately frozen at -20°C. The grapes were representative of the respective vineyard blocks and an attempt was made to harvest them at approximately 24°B. Pinotage (140 samples) was obtained from the Western Cape and Shiraz (93 samples) and Cabernet Sauvignon (92 samples) from the Robertson area. Frozen berries, representative of the total sample, were removed from a few bunches (100 berries/sample), weighed and homogenised. Each sample was then extracted with 10 mL 50% ethanol and the colour extract analysed spectrophotometrically at A520 nm. The values were expressed as milligram anthocyanins per gram of berry weight. The rest of the grapes were thawed, crushed and fermented at 24°C with yeast strain WE 14. Further treatments followed standard small scale red wine vinification procedures, as applied at Nietvoorbij.
The wines were not subjected to malolactic fermentation, but were bottled directly after fermentation and stored at 0°C. Approximately three months after bottling the wines were evaluated sensorially for wine colour intensity, berry/plum intensity and overall wine quality (for Pinotage), for wine colour intensity, berry intensity, spice intensity and overall wine quality (for Shiraz) and for wine colour intensity, vegetative intensity, berry intensity and overall wine quality (for Cabernet Sauvignon) by a panel of six experienced judges. The wine characteristics were judged using a 10 cm line scale. At the same time wine colour was analysed spectrophotometrically at A420 nm and A520 nm and expressed as modified wine colour density (at pH 3,5 and with the effect of SO2 having been removed). The research was done in duplicate.
The statistical relationship between grape colour and wine quality was determined for each cultivar over the four seasons. In finetuning the data, allowance was made for a window of 2°B (between 23°B and 25°B, as well as between 24°B and 26°B) and only the data between these margins used.
Results and discussion
Where all the samples were taken into account, the degree of ripeness varied by 7°B. Correlations between grape colour and wine quality were consequently influenced by degree of ripeness. A relatively narrow window of 2°B was subsequently selected to eliminate, inasmuch as possible, the effect of ripeness on the relationship between grape colour and wine quality. The margins between 23°B and 25°B, as well as between 24°B and 26°B, were selected in view of the fact that these represent the degree of ripeness at which red grapes are normally harvested.
The data from certain individual vintages showed excellent correlations for all three cultivars (data not shown). In instances where the data from all the vintages were combined, good correlations occurred in isolated instances only (Table 1). The trends showed, however, that the darker the grape colour, the more intense the aroma and the better the wine quality. In all three cultivars there was a fairly good correlation between grape and wine colour as determined spectrophotometrically.
There was a weak relationship between the sought-after berry/plum character of Pinotage and wine quality. Moreover, there wasn't any manifestation of a relationship between grape colour and the intensity of spiciness in Shiraz. In both instances this may indicate that the wines did not have the specific characteristics, or that the panel was divided about what truly constitutes berry/plum character and spiciness. In some instances berry characteristics were evident in Shiraz, however. There was a good relationship between grape colour and the intensity of vegetative aroma in Cabernet Sauvignon in the wines made from grapes with a lower degree of ripeness (between 23°B and 25°B). This also had a positive effect on overall wine quality. Vegetative character should in this instance not be seen as negative, but as representative of the positive herbaceous nuances. The wines from the riper grapes (between 24°B and 26°B) displayed more berry-like flavours, but once again this was not reflected in the relationship between grape colour and berry intensity.
As far as the relationship between grape colour and overall wine quality is concerned, Shiraz showed the most potential and statistically significant correlations were obtained. Using the models in question (Figures 1 and 2) it is theoretically possible to predict wine quality based on grape colour to a 66% level of accuracy (in the case of all the wines from the Robertson region) and a 71% level of accuracy (in the case of the Graham Beck - Robertson wines only).
An attempt was made to correlate berry weight and wine quality parameters, but no clear relationships were obtained. This corresponds with results obtained in Australia (Holgate, 2000).
Conclusions and recommendations
The determination of grape colour is a simple technique that can easily be done in the vineyard before harvesting the grapes. This study makes a contribution to the investigations regarding objective quality parameters that may be used to define wine quality. The results illustrate that it is theoretically possible to predict wine quality by looking at grape colour. It also raises the option of identifying specific grape blocks that perform poorly and to decide which viticultural practices will be required to improve matters.
Grape colour can therefore be a good guideline for wine quality, but according to the results obtained under the conditions of this study, it appears that grape colour cannot always or exclusively be used as a quality parameter. Too many additional factors come into play during ripening. At this stage colour should therefore be regarded as complementary to other parameters. It is necessary for this investigation to be extended to more regions and to cover more seasons in order to create improved prediction models.
Literature references
Barnett, T., 2004. Berry color as a wine quality indicator at Brown Brothers Winery. Wines & Vines 85, 38-41.
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.
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.
Marais, J., Iland, P. & Swart, E., 2001. Exploring the relationships between Pinotage grape colour and wine quality - Preliminary results. Wynboer 139, 20-22.
Mollah, M. & Reynolds, J., 2002. Visual colour assessment of red winegrapes. Wine Industry Journal 17, 76-80.
Acknowledgements
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