correlation between the phenolic content and dpph ...

Acta Alimentaria DOI: 10.1556/AAlim.2007.0005

CORRELATION BETWEEN THE PHENOLIC CONTENT AND DPPH RADICAL SCAVENGING ACTIVITY OF SELECTED CROATIAN WINES J. PILJAC-ŽEGARACa*, S. MARTINEZb, L. VALEKb, T. STIPČEVIĆc and K. KOVAČEVIĆ-GANIĆd a Department

of Molecular Biology, ‘Ruđer Bošković’ Institute, Bijenička c. 54, P.O. Box 180, HR-10000, Zagreb. Croatia b Department of Electrochemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, Savska c. 16/I, P.O. Box 177, HR-10000, Zagreb. Croatia c Department of Molecular Medicine, ‘Ruđer Bošković’ Institute, Bijenička c. 54, P.O. Box 180, HR-10000, Zagreb. Croatia d Department of Food Technology, Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, HR-10000, Zagreb. Croatia (Received: 4 April 2006; accepted: 29 November 2006)

The Folin-Ciocalteu (FC) test and cyclic voltammetry (CV) at a glassy carbon electrode were used to quantify phenolic antioxidants in a set of 17 Croatian wines and express them in gallic acid (GAE) and catechin equivalents (CE). The total phenolic index (TPI) values for red wines expressed in GAE ranged from 18.851 to 26.905 mM, while TPI for white wines ranged from 1.722 to 2.869 mM. The levels of phenolics derived from CV measurements were markedly lower than those of TPI, since these values include only those phenolic compounds that get oxidised up to 500 mV and contain orthodiphenol and triphenol groups. The free radical scavenging ability of the same set of wines was evaluated according to the Brand-Williams assay and expressed in equivalents of catechin, gallic acid, vitamin C and Trolox. Ivan Dolac barrique 2002 exhibited the highest antioxidant activity. The DPPH radical scavenging ability of the wines was also evaluated and correlated to the TPI values. Better correlation was observed between the TPI and the antioxidant activity for red wines (r2=0.826) as opposed to white wines (r2=0.686). The highest correlation (r2=0.970) was found between the TPI and the antioxidant activity measured when the whole set of samples was considered. Keywords: phenols, antioxidants, wine, cyclic voltammetry, DPPH

In addition to vitamins and minerals, the polyphenolic antioxidants of fruits and vegetables play an important role in prevention of human pathologies through a combined mechanism of oxidative stress reduction and free radical quenching. Thus, it has been shown that grape antioxidants: (i) slow down the natural process of thrombosis by inhibiting the aggregation of platelets (ALONSO et al., 2002); (ii) reduce the mortality * To whom correspondence should be addressed. Phone: ++385 (1) 45 60 987; fax: ++385 (1) 45 61 177; e-mail: [email protected] © 2007 Akadémiai Kiadó, Budapest

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from cardiovascular diseases – ‘French paradox’ (RENAUD & DE LORGERIL, 1992); (iii) exert a protective effect against other pathologies such as cancers and mutations associated with cell division and ageing, etc. (SOLEAS et al., 2002). Because of the increasing importance of naturally derived antioxidants and their activity, a wide range of methods is already available for the detection of phenolics in wine and must, including spectrophotometric (Folin-Ciocalteu, FC) (SINGLETON, 1974), chromatographic (HPLC) (VRHOVSEK et al., 2001), electrochemical (cyclic voltammetry and potentiometry) (KILMARTIN, 2001; MARTINEZ et al., 2005) and coulometric methods (ABDULIN et al., 2001; 2002). Antioxidant activity assays usually employ a free radical reaction with either DPPH (BRAND-WILLIAMS et al., 1995) or ABTS radical (total antioxidant status test, TAS) (SOLEAS et al., 1997), whereby the quenching potential of a sample is directly proportional to the change in colour detected spectrophotometrically. The aim of the work presented here was to compare the usefulness of two methods used to derive the phenolic indices (FC and CV) and a method which determines the antioxidant activity with respect to the DPPH radical for a set of monovarietal Croatian wines whose phenolic antioxidants’ content is unknown. For this purpose, four commonly employed standards, gallic acid, catechin, Trolox and vitamin C were used and the results were compared for a set of 17 wines. 1. Materials and methods 1.1. Wines Nine different white wines, seven different red wines (all made from Plavac mali grapes) and one rosé wine produced by the Badel 1862 wine cooperative, Zagreb, Croatia, were used for the study. All 17 monovarietal wines, available in high quantities on the market, were made from autochthonous Croatian grape cultivars (Vitis vinifera L.) suitable for the production of highest quality wines. The 2001, 2002 and 2003 vintages were included in the study with known low SO2 content ( Plavac Hvar > Ivan Dolac > Postup Potomje > Dingač > Plavac > Pelješac > Pošip Korčula > Rukatac > Pošip Smokvica >

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Benkovac rosé > Sauvignon Daruvar > Traminac Nespeš > Silvanac Zeleni > Graševina Križevci > Graševina Daruvar > Prigorska Kraljevina. 2.3. Correlation between phenolic composition and antioxidant activity The correlation coefficient between the TPI and the CV measures reported in Table 3 for the whole set of samples equals 0.935 and 0.830 for IA and Q500, respectively. Similar correlation coefficients were observed when IA and Q500 were compared to the DPPH radical scavenging ability, r2=0.914 and r2=0.893, respectively. Our results showed the highest degree of correlation (r2=0.970) between the TPI and the antioxidant activity measured by DPPH when the whole set of samples is considered. When the correlations between TPI and the antioxidant activity are compared for the set of red and white wines separately, it becomes clear that the TPI values are better correlated with the antioxidant activity for red wines (r2=0.826) as opposed to white wines (r2=0.686). The correlation coefficient observed between TPI and DPPH radical scavenging activity for white wines is markedly lower. This discrepancy may be explained by the fact that white wines do not contain polyphenolic compounds such as procyanidines or resveratrol, which exhibit good radical scavenging capacity and are mostly found in red grape skins. 3. Conclusion This study demonstrated the use of fast and effective spectrophotometric and cyclic voltammetry measurements in determination of the phenolic content and antioxidant activity of selected monovarietal Croatian wines. This was one of the few comparative studies reporting antioxidant activities and phenolic indices according to three analytical methods for such a large number of monovarietal wines made from native Croatian grapes. The results are specifically of regional importance, since they indicate that among the tested samples wines made from red Plavac mali grapes grown on the island of Hvar (Plavac Hvar and Ivan Dolac) and white Pošip grapes grown on the island of Korčula have the highest antioxidant activity. Correlations between the results indicate that FC method is more suitable for detection of total phenols, while cyclic voltammetry effectively detects only the low formal potential antioxidants. In comparing the results between methods, the highest degree of correlation (r2=0.970) was obtained between the TPI and the DPPH antiradical activity, when the whole set of samples is considered. * This research has been conducted as a part of the TEST program – technological research and implementation project financed by the Ministry of Science, Education and Sports of the Republic of Croatia. We thank the Badel 1862 wine cooperative, Zagreb, Croatia, for their wine donations.

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