Ascorbic acid improves the antioxidant activity of European grape juices by improving the juices’ ability to inhibit lipid peroxidation of human LDL in vitro

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Abstract
Antioxidant activities of red and white European grape juices towards copper induced lipid oxidation of human low‐density lipoproteins (LDL) were examined in vitro. LDL lipid peroxidation was assessed spectrophotometrically by monitoring the development of conjugated lipid hydroperoxides at 234 nm. Red grape juice concentrate inhibited lipid peroxidation of LDL by prolonging the lag phase by 2.7 times relative to a control when evaluated at a total phenolic concentration of 10 μM gallic acid equivalents (GAE). Both red grape juices tested blocked lipid peroxidation of LDL at 20 μM GAE. White grape juice exerted prooxidant activity at 5–20 μM GAE. The antioxidant activity, inhibition of lipid peroxidation of LDL in vitro, was correlated with the juices’ levels of total phenols (r > 0.98, P < 0.01), anthocyanins (r > 0.99, P < 0.01), flavan‐3‐ols (r > 0.97, P < 0.05), and hydroxybenzoates (r > 0.96, P < 0.05) when the phenolic composition of each grape juices was analysed by HPLC. 5 μM ascorbic acid alone did not exert antioxidant activity towards LDL, but combinations of 5 μM ascorbic acid with 5 μM GAE juice phenols eliminated the prooxidant activity of white grape juice, and significantly improved the antioxidant activities of red grape juices.