The amphilic antioxidant, such as acyl ascorbate, would be expected to be a useful food additive. In this study, the antioxidative action of acyl ascorbate was examined, because knowledge of its behavior in aqueous solution is required for its effective use for food. First, the degradation kinetics of ()-catechin in aqueous solution at various pHs and temperatures were empirically expressed by the Weibull equation. The rate constant, k, was the lower at the lower pH and temperature. The temperature-dependency of the k value was analyzed on the basis of the Arrhenius equation, and it was indicated that the enthalpy-entropy compensation held during the degradation. Second, octanoyl L-ascorbate was synthesized by the condensation of L-ascorbic and octanoic acids in acetone using immobilized lipase from Candida antarctica, and the degradation kinetics of catechin in aqueous solution in the presence of octanoyl ascorbate was examined. The dependency of the k value in the catechin solution with octanoyl ascorbate on the concentration of the ascorbate was obviously dif ferent from that with ascorbic acid. That is, the degradation was suppressed at low octanoyl ascorbate concentrations, while proceeded fast at a high concentration, suggesting the formation of the micelle for octanoyl ascorbate at high concentrations.

The degradation kinetics of catechin in aqueous solution in the presence of l-ascorbic acid or octanoyl l-ascorbate, which was synthesized through the immobilized-lipase-catalyzed condensation of ascorbic and octanoic acids in acetonitrile, and of various organic acids were expressed by a first-order equation. The stability of catechin was reduced by the addition of ascorbic acid or octanoyl ascorbate at a concentration of 10 mmol/L, although the rate constant, k, for degradation in the presence of ascorbic acid or octanoyl ascorbate decreased with increasing citric acid concentration. Malic, lactic and sorbic acids were also effective for decelerating the oxidative degradation by ascorbic acid and octanoyl ascorbate, but their suppressive effects on the degradation of catechin differed between the solutions containing ascorbic acid and octanoyl ascorbate. This indicates that octanoyl ascorbate acts on catechin with a different mechanism from that of ascorbic acid.Keywords: catechin, octanoyl ascorbate, organic acid, degradation kinetics, first-order equation *To whom correspondence should be addressed. E-mail: wysyk@hiro.kindai.ac.jp IntroductionCatechins are known constituents of green tea and have physiological functions, including antioxidative activity (Huang and Frankel, 1997) and the ability to suppress adipocyte differentiation (Furuyashiki et al., 2004). Drinks containing catechins also tend to contain acidic additives, such as water-soluble organic acids, with the most common acidifier being citric acid. The stability of tea catechins in aqueous solution has been investigated by many researchers (Chen et al., 1998;Friedman and Jürgens, 2000;Tanaka and Kouno, 2003). More recently, Aoshima and Ayabe studied methods to prevent the deterioration of catechin-enriched green tea (Aoshima and Ayabe, 2007). Ortiz et al. examined the effects of storage relative humidity and addition of other ingredients, such as citric and ascorbic acids, on catechin stability in green tea powder (Ortiz et al., 2008). l-Ascorbic acid is a water-soluble vitamin known as vitamin C and is widely used as an additive in foods and beverages due to its strong reducing ability. 6-O-Acyl ascorbate, which is synthesized through the condensation of ascorbic acid and fatty acid using an immobilized lipase in an organic solvent, has both higher and lower suppressive activities than ascorbic acid, depending on their concentrations in aqueous solution (Watanabe et al., 2009). In this study, the effects of the coexistence of octanoyl ascorbate, which is one of the most soluble acyl ascorbates in water, and citric acid on the degradation of catechin was compared with the case of ascorbic acid. (+)-Catechin solution, not green tea extracts, was selected as a simple system to investigate this, as complex reactions, such as the intermolecular coupled-oxidation between catechol ring and hydroxy o-quinone of catechins, may occur for solutions with mixed catechins (Tanaka and Kouno, 2003). Materials and MethodsMaterials (+)-Catechin (purity: 98%) was ...