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Acyl arbutin was synthesized through the condensation of arbutin with a saturated fatty acid (C6-18) by the immobilized lipase in a batch reaction. The conversion at 10 and 20 g/l-solvent of immobilized lipase reached 45% over 2 d, but the initial reaction rate per amount of immobilized lipase decreased at 20 g/l-solvent. The radical scavenging activity of acyl arbutin in an ethanol solution was independent of the acyl chain length, although the rate constant, k, estimated for the oxidation of methyl linoleate in a bulk system with acyl arbutin by using the Weibull equation, decreased as the acyl chain length increased. This indicates the antioxidative ability of acyl arbutin with a long acyl chain to be due to its lipophilicity. Furthermore, it is suggested that dodecanoyl arbutin mainly acted on the interface between the oil and water phases in an O/W emulsion, and effectively suppressed the oxidation induced at the interface.

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 ...