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Antioxidants in wines
Steven J. Langford* and Gordon J. Troup+
Introduction

In the first AIM-digest article on this topic (1), only the antioxidants having the highest concentrations in red wines, and in ‘aged’ white wines (those with skin and seed, and/or oak exposure, ~ a year old) were discussed. These are catechin in its various forms and polymers, found mainly in the grapeseed coat, and the anthocyanins (colourants) found mainly in the skins. These molecules are all polyphenols: they contain more than one aromatic (resonant) ring, with at least one hydroxyl group attached to the rings, which induces their antioxidant behaviour. Some diagrams were given in the previous article. There are a number of monophenols which occur in wines, and these will be discussed in the next section. They are also free-radical scavenging antioxidants, but not as efficient as the polyphenols, and there is experimental evidence for this. For example, the monophenolic alcohol tyrosol, I, which occurs in wines(and olive oil), acts as a simple, but weak antioxidant when compared to the polyphenol catechin. This moderate weak effect was borne out by an experiment on three white wines containing tyrosol at the concentrations 28mg/L, 30mg/L and 11mg/L (2). With the particular antioxidant assay used, only the wine which contained catechin at a concentration of 55mg/L showed any significant antioxidant action: the other two wines contained much less catechin.

Monophenols in wine

The main other monophenols in wines are: gallic acid, II, from seed coats and from oak. The gallate part can attach to catechins, thus making them even more efficient antioxidants. Then, caffeic acid, III, ferulic acid, IV, and coumaric acid, V. These last three occur in grape juice, usually as tartrates, and are the major phenolics in ‘young’ white wines (no skin and seed, no oak exposure, less than 6 months old). As can be seen from the figures, their chemical structure is very similar, containing both a phenol portion and an ‘alkene’ ( or ‘unsaturated’) portion. Like tyrosol, these are much smaller molecules than the catechins and anthocyanins, and therefore can probably get to parts of the body that the bigger molecules cannot, by different transport mechanisms.

The stilbenes

The major stilbenes found in wines are resveratrol,VI, and pterostilben, VII. These not only have phenolic rings, but also a double bond in the middle of the ‘dumbell’, similar to III -V, which makes them both very efficient antioxidants. The reason for their special action lies with 2 events. One, a free radical generated at the phenolic oxygen is stabilised by its delocalisation around the aromatic ring, the double bond, and the second ring. Two, The unsaturated portion acts very similarly to the (aliphatic) parts of cell walls which are for the most part unsaturated. Some harmful effects of (unstabilised) free radicals are attributed to reactions at these parts of the cell walls, causing a breakdown.There are over 100 papers about the anticarcinogenic action of resveratrol: the development of cancer is mediated by free radicals. When pterostilben is observed in wines, the concentration is about 0.1 times that of resveratrol.

Flavonols

Flavonols are polyphenols similar to the catechins, but with a slightly different structure. The main one is quercitin, VIII, which also resembles some anthocyanins.

Discussion

We need now to look at the concentrations of these various molecules in red and white wines. The data here presented are taken from the paper by Waterhouse (3), and are only a typical representative summary, since each wine is unique. However, there are things we can be sure of: ‘young’ white wines will have a smaller concentration of catechins and oak gallates than ‘aged’ whites, and both these whites will have a much smaller concentration of catechins and anthocyanins than reds, oaked or not.

Two independent studies (4, 5) on white wines have shown that people drinking only young white wines do not receive the protective effect of red wines, and in fact that the antioxidant capacity of the blood serum is decreased, not increased! This agrees with the results of (2) quoted above. As pointed out in the article by Stockley (6) in AIM, alcohol has a pro-oxidant action, which is overcome by the phenolics in the wine: see also (7). Insufficient antioxidants in the ‘young’ whites cannot do this alone: a supplement to the diet (red wine, grapeseed extract) may be needed.

The generic names for the various phenol types have been omitted or simplified.

The comments made above about the differences between red and white wine antioxidant content are clearly illustrated. However, an Australian group based at University of Western Sydney believes it can increase the production of resveratrol in red wines by a factor of ~75. Let us hope that the success comes without a change for the worse in wine quality!

Steven J. Langford is from the School of Chemistry and Gordon J. Troup is from the School of Physics and Materials Engineering at Monash University, Clayton 3800,Victoria, Australia.

REFERENCES1. Hewitt DG and Troup GJ: Oxidation, free radicals and antioxidants in wines.AIM-dig. 2002:10 (4) 9-11. 2. Cui J, Tosaki A et al.: Cardioprotective abilities of white wine, in ‘Alcohol and wine in health and disease’ (eds. Das DK and Ursini F), Annals of New York Acad. Sci. 2002: 957 308-16. 3. Waterhouse AL ibid.2002: 957 21-36. 4. Van Velden DP, Mansvelt EPG et al., ibid. 2002:957 337-40. 5. Fuhrman B, Lavy A and Aviram M: Consumption of red wine with meals reduces the susceptibility of human plasma and LDL to undergo lipid peroxidation. Am.J.Clin. Nutr. 1965: 61 549-554. 6. Stockley C: Potential cardiovascular benefits of moderate wine consumption. AIM-dig. 2002: 11 (2) 10-11. 7. Sato M Maulik N and Das DK: Cardioprotection with alcohol. Annals NY Acad.Sci. 2002: 957 122-135.

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