Protection of Ascorbic Acid from Copper(II)−Catalyzed Oxidative Degradation in the Presence of Fruit Acids: Citric, Oxalic, Tartaric, Malic, Malonic, and Fumaric Acids
Vitamin C (ascorbic acid) is sensitive to oxygen and heat, and can be degraded during unsuitable conditions of cooking and preservation methods of food. The nutritional quality of food may be adversely affected due to transition metal−catalyzed oxidative degradation of ascorbic acid. The effect of Cu(II) complexes formed with protective organic acids widely found in fruits on the autoxidation of ascorbic acid was investigated, where ascorbic acid was quantified with the Cupric Reducing Antioxidant Capacity assay. The Cu(II)-catalyzed oxidation at pH 4.5 followed first-order kinetics with respect to ascorbic acid concentration. The rate constants of ascorbic acid oxidation increased with Cu(II) concentration for a fixed level of organic acid. The catalytic oxidation of ascorbic acid was inhibited in the presence of stable Cu(II)-organic acid binary complexes, and accordingly, the inhibitive potency of citric acid (highest) was followed by oxalic acid, malonic acid, malic acid, tartaric acid, and fumaric acid in this order. The rate-limiting step of ascorbic acid oxidation was the formation of a ternary transition complex between Cu(II), hydrogen ascorbate, and carboxylic acid. The inhibitive activity of Cu(II)-ligand complexes increased as the binary complex stability increased. The presence of fruit acids in foodstuffs may help protect nutritional ascorbic acid values by preventing metal−catalyzed oxidation reactions.