Antioxidative Properties of Acyl Ascorbates on Lipid Oxidation in Bulk, Disperse and Microcapsule Systems

Abstract

Acyl ascorbates were synthesized through the condensation of L-ascorbic acid with various fatty acids using an immobilized lipase in water-miscible organic solvent, and their antioxidative properties on lipid oxidation in bulk, disperse and microcapsule systems were examined. The optimal conditions for enzymatic synthesis in a batch reaction were determined and the continuous production of acyl ascorbate was conducted using a continuous stirred tank reactor and a plug flow reactor. The oxidative stability of lipid with saturated acyl ascorbate in bulk system increased with increasing acyl chain length, whereas that in the oil-in-water emulsion and microcapsule system depended on the acyl chain length and the concentration of the ascorbate.Keywords: acyl ascorbate, antioxidative emulsifier, continuous production, immobilized lipase, lipid oxidation IntroductionLipid, which is one of the main components in food, is susceptible to oxidation, resulting in the generation of undesirable flavors and toxic products such as hydroperoxides. Therefore, lipid oxidation has received much attention due to its involvement in food deterioration and the relevance of lipid peroxidation in vivo to membrane damage, cancer, heart disease and aging (Cosgrove et al., 1987). Lipid oxidation is a complicated process consisting of three steps: initiation, propagation and termination, and the reaction mechanism has been elucidated in detail (Endo, 1998;Frankel, 1991;Porter, 1986). The addition of an antioxidant is one way to suppress oxidation, as the antioxidant may lengthen the oxidation induction period at very low concentrations (Gwo et al., 1985).These are usually phenolic compounds such as tocopherols, butylated hydroxyanisole, butylated hydroxytoluene and propyl gallate, and are used to inhibit the chain reaction by scavenging free radicals generated from lipid molecules. l-Ascorbic acid is a water-soluble compound also known as vitamin C. It is used widely as an additive in foods and cosmetics owing to its strong reducing ability. Its lipophilic derivatives are acylated with a long-chain fatty acid, such as palmitic acid, and used in lipid-rich foods (Enomoto et al., 1991). These are mainly synthesized by a chemical procedure (Tanaka and Yamamoto, 1966), and are edible surfactants because each substrate is eaten daily. The antitumor activity and metastasis-inhibitory effects of 6-O-palmitoyl ascorbate have been reported (Miwa and Yamazaki, 1986;Nagao et al., 1996). In addition, the 6-O-and 2-O-monoesters of ascorbic acid with phosphate or fatty acid are known to have physiological activity equivalent to unmodified ascorbic acid (Inagaki and Kawaguchi, 1966). Acyl ascorbate consists of an ascorbyl moiety as the hydrophilic group and an acyl residue as the hydrophobic group, and therefore is a promising emulsifier with both reductivity and surface activity. However, palmitoyl and stearoyl ascorbates, which are commercially available, are highly water insoluble, preventing their use as an emulsifier in oil-in-water emulsions. (Fig...

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