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Antioxidant activity of South African red and white cultivar wines
Elizabeth Joubert and Dalene de Beer
Abstract
Background
Consumers and researchers are showing increasing interest in natural products such as wine and tea as its consumption is associated with a reduced risk of chronic diseases. These health benefits are attributed mainly to the antioxidant activity of its phenolic compounds. The "French Paradox", which refers to the low incidence of heart disease among the French population despite a high fat intake, is believed to be due to their high red wine consumption. Red wines consistently outperform other beverages such as green tea in various antioxidant tests (Benzie & Strain, 1999; Vinson et al., 1999).
The classical definition of an antioxidant given by Barry Halliwell (UK researcher) states: "An antioxidant is a substance that when present at low concentrations compared with those of an oxidisable substrate significantly delays or prevents oxidation of that substrate". Dietary antioxidants are reported to be effective against oxidation of DNA, proteins, lipids and lipid-containing structures such as low-density lipoprotein and membranes in the human body. It is these actions that contribute to their ability to prevent disease.
Several studies have investigated the antioxidant capacity of red, white and rosé wines from Europe and the Americas (Campos & Lissi, 1996; Simonetti et al., 1997; Landrault et al., 2001), but this is the first such study on South African wines (De Beer et al., 2003; De Beer et al., 2005). A project comparing the antioxidant capacity of some of the major cultivar wines produced by South Africa was conducted during 2000 and 2001. Cabernet Sauvignon (10), Ruby Cabernet (7), Pinotage (10), Shiraz (10) and Merlot (9) cultivar wines were selected from the 1998 vintage. The white cultivar wines were non-wooded and consisted of Sauvignon blanc (10), Chenin blanc (10), Chardonnay (10) and Colombar (10) wines from the 1999 vintage. All wines were purchased in January 2000 and sampled at the same time. Samples were frozen to minimise changes in phenolic composition until analysis.
Measurement and Implications of Antioxidant Activity of Wines
The antioxidant capacity of the wines was determined using two different in vitro (test tube) antioxidant tests, namely a free radical scavenging and a membrane lipid oxidation test. The use of different tests is important as antioxidants can act in several different ways in the body. Free radical scavenging tests measure the ability of an antioxidant to neutralise free radicals, which can damage lipids, DNA and proteins. The membrane lipid peroxidation test, on the other hand, measures the ability of antioxidants to protect the lipids in cell membranes against oxidation. Not only is the ability to scavenge free radicals important, but also whether the free radicals in the membrane can be accessed. Solubility of the antioxidant in the lipid and the three-dimensional structure of the antioxidant are of importance, amongst others. The changes that occur as a result of the oxidation of membrane lipids affect various normal cell functions and are associated with ageing. The antioxidant properties of wines, as measured using in vitro tests can not be directly used to predict their activity in the human body. Antioxidant compounds differ with respect to ease and rate of absorption into the bloodstream. Some researchers believe that antioxidants which are not absorbed in the bloodstream, however, may have beneficial effects in the small and large intestine. Antioxidants can undergo numerous metabolic structural changes, for instance in the liver, which affect their antioxidant activity in vivo (living cells). In some cases the metabolites are more potent antioxidants than the compounds present in the wine, whilst in other cases, they may be less potent.
Antioxidant Activity of South African Wines
The Pinotage and Merlot wines had the highest and Ruby Cabernet the lowest free radical scavenging capacity of the red cultivar wines (Figure 1). In the lipid peroxidation test, Merlot and Ruby Cabernet had the highest and lowest activity, respectively (Figure 1). Of the white cultivar wines, Chardonnay showed the highest and Chenin blanc the lowest activity of the white cultivar wines in both the free radical scavenging and lipid peroxidation tests, while Sauvignon blanc and Colombar wines had intermediate antioxidant activity (Figure 2). The differences in antioxidant capacity between the different cultivars of red or white wines were relatively small. The wines originated from different regions, were made using a variety of enological techniques and blending of up to 25% of a cultivar not stated on the wine label, are all factors which could play a role in masking the cultivar differences.
Phenolic Content of South African Wines
Phenolic compounds in wines are a diverse group comprising several sub-groups such as phenolic acids, flavanols, flavonols and anthocyanins (Figure 3). Structural differences of the compounds affect their antioxidant activity, e.g. flavanols generally have higher antioxidant activity than phenolic acids. Differences in antioxidant activity between the cultivar wines could be linked to their phenolic composition. Among the red cultivar wines, Ruby Cabernet wines with low antioxidant activity in both antioxidant tests had the lowest flavanol content and the second lowest flavonol and hydroxycinnamate contents (Figure 4). This cultivar, however, had the highest anthocyanin content. On the other hand, Merlot wines which showed high antioxidant activity in both antioxidant tests had the highest flavanol content and the second highest flavonol and hydroxycinnamate contents. From the results it seems that anthocyanin content does not contribute to the antioxidant activity as much as the other phenolic groups. Among the white cultivar wines, Chardonnay had the highest antioxidant activity and contained the highest amount of flavanols, flavonols and hydroxycinnamates (Figure 5). Chenin blanc wines, on the other hand, contained the lowest amount of these phenolic groups, therefore resulting in a low antioxidant activity.
Comparison of Red and White Wines
The red wines had free radical scavenging capacity values approximately 14 times that of white wines (Figures 1 and 2). The reason for this is the large difference between the total phenol content of red and white cultivar wines. Due to differences in dilution necessary for the test conditions, red and white wines could not be directly compared with regard to the inhibition of lipid peroxidation. Red wines caused a greater inhibition despite the fact that a lower concentration of total phenols (due to dilution) was present in the reaction mixture compared to that of white wines. Red wines have higher antioxidant activity than white wines, not only due to higher total phenol content, but also due to variations in different types of phenolic compounds. Red wines contain anthocyanins, providing the red colour, which is not found in white wines. Much higher amounts of flavanols and flavonols are also present in red wines due to fermentation in contact with the grape skins and seeds.
Future perspectives
The results of this study provide information on the average antioxidant activity of South African cultivar wines. The differences between cultivars were partly explained by differences in phenolic composition. More in-depth studies are underway on the contribution of individual phenolic compounds and especially polymeric phenolic compounds to the total antioxidant capacity of Pinotage wines. This cultivar was chosen as Pinotage is a South African cultivar with a unique phenolic profile. The effect of enological and viticultural factors on the total antioxidant capacity and the content of individual phenolic compounds of Pinotage wines will be investigated. This information will be used to set up guidelines for the production of Pinotage wine with optimal antioxidant capacity while retaining good sensory properties.
References
Benzie, I.F.F.; Strain, J.J. Ferric reducing/antioxidant power assay: direct measure of total antioxidant activity in biological fluids and modified version for simultaneous measurement of total antioxidant power and ascorbic acid concentration. Methods Enzymol. 1999, 299, 15-27.
Campos, A.M.; Lissi, E.A. Total antioxidant potential of Chilean wines. Nutr. Res. 1996, 16, 385-389.
De Beer, D.; Joubert, E.; Gelderblom, W.C.A.; Manley, M. Antioxidant activity of South African red and white cultivar wines: Free radical scavenging. J. Agric. Food Chem. 2003, 51, 902-909.
De Beer, D.; Joubert, E.; Gelderblom, W.C.A.; Manley, M. Antioxidant activity of South African red and white cultivar wines and selected phenolic compounds: In vitro inhibition of microsomal lipid peroxidation. Food Chem. 2005, 90, 869-577.
Landrault, N.; Poucheret, P.; Ravel, P.; Gasc, F.; Cros, G.; Teissedre, P-L. Antioxidant capacities and phenolics levels of French wines from different varieties and vintages. J. Agric. Food Chem. 2001, 49, 3341-3348.
Simonetti, P.; Pietta, P.; Testolin, G. Polyphenol content and total antioxidant potential of selected Italian wines. J. Agric. Food Chem. 1997, 45, 1152-1155.
Vinson, J.A.; Jang, J.; Yang, J.; Serry, M.; Proch, J.; Cai, S. Vitamins and especially flavonoids in common beverages are powerful in vitro antioxidants which enrich lower density lipoproteins and increase their oxidative resistance after ex vivo spiking in human blood plasma. J. Agric. Food Chem. 1999, 47, 2502-2504.
Acknowledgements
Financial support for the project by Winetech, NRF and THRIP is gratefully acknowledged. Our grateful thanks go to Dr W.C.A. Gelderblom, Dr M. Manley and personnel from both ARC Infruitec-Nietvoorbij and the Programme on Mycotoxins and Experimental Carcinogenesis at the MRC for their inputs during the project.
For further information contact Lizette Joubert at Tel.: (021) 809-3444, Fax: (021) 809-3592 or E-mail: JoubertL@arc.agric.za
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