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Phenolic compounds in South African red wines:
A preliminary study
Introduction
Phenolic compounds are major constituents of wine and have an impact on certain sensorial properties such as colour, mouth-feel characteristics and taste. Manipulation of the levels of these compounds in wine through the application of viticultural and oenological practices may therefore be a useful tool to improve wine quality. However, knowledge is lacking on exactly which polyphenols are responsible for specific sensorial observations and little is known about the polyphenol composition of South African red wines.
Different groups of phenolic compounds are found in wine, including benzoic and cinnamic acids, cinnamic acid esters, flavonols, flavan-3-ols and anthocyanins. Phenolic acids, though colourless in a dilute alcoholic solution, may become yellow due to oxidation and have an impact on wine colour. Flavonols are yellow pigments, extracted from grape skins during the wine-making process. Monomeric and polymeric flavan-3-ols are important contributors to the astringency and bitterness sensations of red wine ( Noble, 1990). The monomeric types found in wine are catechin, epicatechin, gallocatechin and epigallocatechin. These molecules polymerise in the grape and also in wine to form a variety of procyanidins and high molecular weight condensed polymers (tannins). Anthocyanins, red pigments extracted from grape skins during wine-making, form the basis of red wine colour. The five main anthocyanins that have been identified in grapes are malvidin, delphinidin, petunidin, peonidin and cyanidin. These pigments are found in the form of their respective monoglucosides in Vitis vinifera wine.
Cultivar associated differences in the levels of certain anthocyanins have been shown (Castia et al., 1992; Gonzalez-Neves et al., 2001). The same may be true for other phenolic compounds found in wine. It is possible that differences in the levels of certain colour and taste compounds may contribute to the distinctive sensorial characteristics expressed by wine from different cultivars.
A research project aimed at alleviating the lack of knowledge on the status of polyphenolic compounds in South African red wines was recently launched with industry support. This paper reports on preliminary findings on the levels of selected compounds in three red cultivar wines.
Materials and Methods
The wines used were obtained from an existing investigation, which explains why all material does not come from the same geographical region. Grapes (10 kg per sample) of Pinotage, Shiraz and Cabernet Sauvignon from the 2001 vintage were obtained from different regions and harvested at between 22øB and 29øB. Pinotage came from 15 vineyards in the coastal regions of the Western Cape region and Shiraz and Cabernet Sauvignon each from 15 vineyards in the warmer Robertson region. The grapes were processed and wines produced according to standard Nietvoorbij practices for small-scale red wine production.
The polyphenols in the wines were quantified by liquid chromatography according to a technique adapted from that of Waterhouse et al. (1999).
Results and Discussion
Some of the main differences in polyphenolic levels between the studied cultivars are presented. A clear difference between cultivars was observed in the mean concentration of protocatechuic acid (a benzoic acid), with Shiraz containing much higher levels relative to those of Cabernet Sauvignon and Pinotage (Fig. 1). The observed levels of protocatechuic acid in both Cabernet Sauvignon and Shiraz were below the detection threshold of this compound (Dadic & Belleau, 1973). Although this individual compound may therefore not have a sensory impact, its combined effect with other phenolic acids may (Gawel, 1998).
The mean level of the flavonol, quercetin-3-O-glucoside (Fig. 2), was also clearly higher in Shiraz than in Cabernet Sauvignon or Pinotage wines.
Due to their inherent yellow colour, the flavonols may to some extent have an impact on wine colour. Quercetin, which is found in sensorially significant concentrations in red wine, appears to elicit a bitter taste with weak astringency (Dadic & Belleau, 1973). The observed polyphenol levels (Fig. 1 and 2) seem to indicate unique benzoic acid and flavonol characteristics of Shiraz in comparison to Cabernet Sauvignon and Pinotage.
Levels of the anthocyanin, malvidin-3-O-glucoside (Fig. 3), was clearly the highest in the Pinotage wines, compared to Shiraz and Cabernet Sauvignon wines which have similar levels.
Pure anthocyanins have been reported to elicit a very mild, indistinct taste (Singleton & Trousdale, 1992). However, other studies have shown that anthocyanins do impact on the perception of bitterness and astringency in wine through their combination with flavan-3-ols and their polymers (Brossaud et al., 2001).
The mean concentration of a flavan-3-ol, procyanidin B1 (Fig 4), which is a dimer of catechin and epicatechin, was clearly higher in the Pinotage wines than in those of the other two cultivars. Pinotage also contained the highest mean concentration of polymeric phenols of the three cultivars (data not shown).
The high molecular weight polymeric flavan-3-ols are more astringent than the smaller oligomers, while the monomers (e.g. catechin), dimers (e.g. procyanidin B1) and trimers have a more bitter taste than the polymeric forms (Arnold et al., 1980; Peleg et al., 1999). The ratio of smaller oligomeric to larger condensed procyanidins in a wine could therefore influence the perceived ratio of bitterness to astringency. A change in the ratio of oligomeric to polymeric procyanidins due to polymerisation, takes place during ageing, which leads to the expression of a "softer" mouth-feel character by older wines in comparison to younger wines.
The monomeric, oligomeric and polymeric flavan-3-ols also play an important role in the stabilisation of wine colour. Anthocyanins are not stable and may degrade under certain conditions, leading to a loss of colour and therefore wine quality. However, pigments of young wines may be altered and stabilised during ageing by the reaction of anthocyanins with flavan-3-ols to yield complex, polymeric pigments. This reaction leads to a change in wine colour from the purple red of young wines to the tawny hue observed in older red wines. The ratio of anthocyanins to flavan-3-ols in any given wine could therefore have a marked impact on the final, stabilised colour of such a wine.
Another interesting observation was that Pinotage contained much higher levels of the cinnamic acid derivative, caftaric acid (Fig. 5), compared to the other two cultivars.
Although cinnamic acids and their derivatives are generally found in relatively low concentrations in wine, these compounds may contribute to bitter nuances in wine (Ong & Nagel, 1978; Gawel, 1998). The marked differences observed between Pinotage and the other two cultivars (Figs. 3 to 5) may probably be ascribed to genetic differences. However, the difference in climatic conditions could also have played a role, since the grapes of Pinotage were grown in a relatively cool climate, compared to the Cabernet Sauvignon and Shiraz grapes from a warmer climate.
Conclusions
Although relatively few wines were analysed, the data indicate that cultivar differences exist with respect to the levels of certain phenolic compounds. The observed differences may be due to genetic, as well as climatic factors. Analyses of more wines need to be done in order to confirm these preliminary findings. The sensorial impact of the polyphenols, responsible for the observed cultivar differences, on wine quality needs to be investigated in future studies. This may lead to the identification of those phenolic compounds that have an impact on specific wine quality parameters. Such impact components may eventually serve as a quantifiable measure of wine quality in the optimisation of viticultural and oenological techniques.
Acknowledgements
Financial support by Winetech is appreciated. For further information please contact Manda Rossouw at Tel.: (021) 809-3168, Fax.: (021) 809-3002 or E-mail: manda@infruit. agric.za
Literature cited
Arnold, R.A., Noble, A.C. & Singleton, V.L., 1980. Bitterness and astringency of phenolic fractions in wine. Am. Chem. Soc. 28, 678-680.
Brossaud, F., Cheynier, V. & Noble, A.C., 2001. Bitterness and astringency of grape and wine polyphenols. Austr. J. Grape Wine Res. 7, 33-39.
Castia, T., Franco, M.A., Mattivi, F., Muggiolu, G., Sferlazzo, G. & Versini, G., 1992. Characterization of grapes cultivated in Sardinia: Chemometric methods applied to the anthocyanic fraction. Sc. Aliments 12, 239-255.
Dadic, M. & Belleau, G., 1973. Polyphenols and beer flavor. Proc. Am. Soc. Brew. Chem. 4, 107-114.
Gawel, R., 1998. Red wine astringency: a review. Aus. J. Grape Wine Res. 4, 74-95.
Gonzalez-Neves, G., Gomez-Cordoves, C. & Barreiro, L., 2001. Anthocyanic composition of Tannat, Cabernet Sauvignon and Merlot young red wines from Uruguay. J. Wine Res. 12, 125-133.
Noble, A.C., 1990. Bitterness and astringency in wine. In: Rousell, R., (ed.). Bitterness in Food and Beverages; R. Rousell, (ed). Elsevier Science, Amsterdam. pp. 145-158.
Ong, B.Y. & Nagel, C.W.,1978. Hydroxycinnamic acid-tartaric acid ester content in mature grapes and during the maturation of white riesling grapes. Am. J. Enol. Vitic. 29, 277-281.
Peleg, H., Gacon, K., Schlich, P. & Noble, A.C., 1999. Bitterness and astringency of flavan-3-ol monomers, dimers and trimers. J. Sci. Food Agric. 79, 1123-1128.
Singleton, V.L. & Trousdale, E.K., 1992. Anthocyanin-tannin interactions explaining differences in polymeric phenols between white and red wines. Am. J. Enol. Vitic. 43, 63-70.
Waterhouse, A.L., Price, S.F. & McCord, J.D., 1999. Reversed-phase high-performance liquid chromatography methods of analysis of wine polyphenols. Methods Enzymol. 299, 113-121.
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