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Antioxidant activity and its relation to the content of phenolic compounds in seven varieties of potato. Introduction
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Antioxidant activity and its relation to the content of phenolic compounds in seven varieties of potato Introduction Diets rich in fruits and vegetables have been associated with lower incidence of cancers and heart disease, which may be related to the antioxidant activity of molecules present in these foods. Phenolic compounds are potent antioxidants in vitro, of which flavonoids are of particular interest for their potential positive impact on health. Potatoes are one of the most consumed staple foods in the world and therefore an important source of phenolics. In Ireland it is the third most important crop with a farm gate value of approximately 45million euros annually. In this work we evaluated the scavenging capabilities of methanolic solutions extracted from the skin and flesh of seven varieties of potato against the stable radical DPPH and analyzed its relationship to the content of total phenolics and total flavonoids. We also applied a HPLC method to quantify glycoalkaloids. These compounds can be harmful to human health in quantities above 20mg/100g of tuber. Results Results for total phenolics, total flavonoids and antioxidant activity are shown in figures 1-2. Figure 3 shows the correlation between total phenolics and total flavonoids (yellow dots), total phenolics and antioxidant activity (violet dots) and total flavonoids and antioxidant activity (blue dots). Ratio between total flavonoids and total phenolics is displayed in Table 1. Figure 4 shows a HPLC chromatogram of the two main glycoalkaloids found in potatoes, alpha-solanine and alpha-chaconine. All varieties showed a higher concentration of total phenolics and total flavonoids and greater antioxidant activity in the skin than in the flesh. Statistical analysis showed a significant effect of tissue (skin or flesh) at p<0.0001 on total phenolics, total flavonoids and antioxidant activity. The effect of variety was significant at p<0.0001 for total phenolics and p<0.001 for antioxidant activity. It was not significant for total flavonoids in the skin but was significant in the flesh at p<0.05. Of all varieties examined Setanta showed the highest levels of total phenolics in both skin and flesh. Total flavonoids and antioxidant activity were highest in the flesh of Setanta. There is a reasonably good correlation between total phenolics and total flavonoids; total phenolics and antioxidant activity; and total flavonoids and antioxidant activity; with the latter showing the best fit. This may be due to flavonoids having a higher antioxidant activity than other phenolic compounds. The highest proportion of total flavonoids as compared with total phenolics was found in Axona and Orla for skin and flesh tissues respectively. Materials and methods Skin and flesh of three tubers per variety were ground in liquid nitrogen and extracted in triplicate. Solutions of 80% methanol in water and 10% acetic acid in water with sodium bisulfite were used to extract phenolic compounds and glycoalkaloids respectively. Total phenolics were determined by reaction with Folin-Ciocalteu reagent according to Singleton etal1.Results were expressed as gallic acid equivalents (GAE). Total flavonoids were determined by reaction with sodium nitrite and aluminium chloride according to Marinova etal2. Results were expressed as catechin equivalents (CE). Glycoalkaloids were determined by HPLC with UV detection after SPE clean-up according to Knuthsen et al3. Antioxidant activity was evaluated by reaction with the stable radical DPPH according to Goupy et al4. Results were expressed as trolox equivalents (TEAC). Statistical analysis was carried out with SAS 9.1.3. using a linear mixed model with Tukey adjustment. Conclusions The methodology proposed has proven suitable for the study of phenolic content and the antioxidant activity of skin and flesh tissue in potatoes, allowing for the comparison among varieties of this tuber. The present work is part of a four year study of 62 varieties of potato grown over two seasons in two different locations. Valcarcel, J.1; Reilly, K1; Gaffney, M1; Brunton, N2; O’Brien, N3. 1Horticulture Development Unit, Teagasc Research Centre, Kinsealy, Dublin, Ireland, 2Teagasc-Ashtown Food Research Centre, Dublin, Ireland, 3School of Food and Nutritional Sciences, U.C.C., Cork, Ireland. Contact:jesus.barros@teagasc.ie, kim.reilly@teagasc.ie Acknowledgements The authors would like to thank Teagasc and the Walsh Fellowship Programme for funding this work, as well colleagues at Ashtown Food Research Centre and Teagasc Carlow for assistance. Table 1: Ratio total flavonoids to total phenolics Alpha-chaconine Alpha-solanine Fig 4. HPLC chromatogram of the main glycoalkaloids in potato skin. • References • Singleton, V. L.; Rossi, J. A., Jr., Colorimetry of Total Phenolics with Phosphomolybdic-Phosphotungstic Acid Reagents. Am. J. Enol. Vitic. 1965, 16, (3), 144-158. • D. Marinova, F. R., M. Atanassova, Total phenolics and total flavonoids in Bulgarian fruits and vegetables. Journal of the University of Chemical Technology and Metallurgy 2005, 40, (3), 255-260. • Knuthsen, P.; Jensen, U.; Schmidt, B.; Larsen, I. K., Glycoalkaloids in potatoes: Content of glycoalkaloids in potatoes for consumption. Journal of Food Composition and Analysis 2009, 22, (6), 577-581. • 4. Pascale, G.; Mireille, H.; Patrick, B.; Marie Josèphe, A., Antioxidant composition and activity of barley (Hordeum vulgare) and malt extracts and of isolated phenolic compounds. Journal of the Science of Food and Agriculture 1999, 79, (12), 1625-1634.
