The oxidation processes of linoleic acid (LA) in the presence of L-ascorbic acid or saturated acyl L-ascorbate additives were measured at various temperatures and molar ratios of the additive to LA. Higher oxidative stability of LA was observed at higher additive levels for all additives. The addition of the ascorbates lengthened the induction period for the oxidation of LA. An autocatalytic kinetic rate equation was used to model the oxidation processes of LA mixed with the ascorbates, and the dependence of the rate constant, k, on acyl-chain carbon number was determined. At any temperature, the use of ascorbate additives decreased the k value for LA, and there was a slight tendency for k values to decrease with increasing acyl-chain length. The apparent activation energy, E a , and the frequency factor, k 0 , for the rate constant were determined from Arrhenius plots. The calculated E a and k 0 values also decreased with increasing ascorbate acyl-chain length.Paper no. J10440 in JAOCS 82, 389-392 (May 2005).KEY WORDS: Apparent activation energy, autocatalytic-type kinetics, frequency factor, rate constant, saturated acyl L-ascorbate.Lipid oxidation in foods causes rancidity and nutritional deterioration. Oxidation is a complicated process including the steps of initiation, propagation, and termination. The kinetics of the oxidation in a bulk lipid have been extensively studied (1-3), and the entire lipid oxidation process has been described by a kinetic expression of the autocatalytic type in terms of the fraction of unoxidized substrate (2). In previous studies, we also applied the expression to the entire oxidation process of n-6 FFA, their esters, and acylglycerols, and we evaluated their kinetic parameters (4-6). L-Ascorbic acid is a hydrophilic vitamin with high reducing activity due to its enediol-lactone resonant structure; it is widely used as an additive in foods and cosmetics. Its lipophilic derivatives with long acyl chains such as 6-O-palmitoyl and 6-O-stearoyl L-ascorbates are also used as an additive in foods rich in lipids. Recently, some studies have appeared on the lipase-catalyzed synthesis of acyl ascorbates in an organic solvent (7,8). We also synthesized various 6-O-acyl ascorbates by using an immobilized lipase and reported their antioxidative abilities (9-12). Although there have been some studies of the antioxidative ability of palmitoyl ascorbate for the oxidation of oils (13-17), the application of the acyl ascorbates with medium chain lengths has not been reported.In this study, we analyzed the oxidation processes of linoleic acid (LA) in the presence of L-ascorbic acid or saturated acyl L-ascorbate by applying a kinetic equation of the autocatalytic type to the processes. The dependence of the rate constant on the acyl-chain carbon number of the ascorbates at various temperatures was investigated. Furthermore, the apparent activation energy, E a , and the frequency factor, k 0 , for the rate constant were determined, and the effects of the carbon number on them were demonstrated. EXP...

The oxidation processes of linoleic acid encapsulated with gum arabic or maltodextrin at various weight ratios by spray-drying were analyzed using the model in which the free energy of activation for the rate constant of the autocatalytic type kinetics was assumed to obey a Gaussian distribution. The model could well express the oxidation processes, and the rate constant corresponding to the mean value of the free energy of activation, k, was greater for linoleic acid encapsulated at the higher weight ratio. Emulsions of linoleic acid and maltodextrin solution with different diameters were spray-dried to prepare the microcapsules. The oxidation processes of linoleic acid within the microcapsules were also calculated using the model. The k value was smaller for the emulsion with a smaller diameter.

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Two polyunsaturated fatty acids (PUFAs) or their esters were mixed, and their oxidation processes were measured at 659 C and ca. 0% relative humidity. Except when a PUFA ester was mixed with a free PUFA, the oxidation of the less-oxidative PUFA was promoted as its content in the mixture decreased, while the oxidation of the more-oxidative PUFA was delayed with a decrease in its content. A kinetic model is proposed whereby a PUFA acts as the diluent for another PUFA, and the oxidation rate of the PUFA is proportional to the product of the unoxidized PUFA concentration and the sum of the concentrations of the oxidized PUFA and the other oxidized PUFA. This model well expressed the oxidation processes of the PUFAs in their mixed system.

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The oxidation process of a polyunsaturated acylglycerol mixed with a saturated or unsaturated acylglycerol was measured at 65˚C and ca. 0% relative humidity. When two polyunsaturated acylglycerols were mixed, their oxidation processes were expressed by the kinetic model in which one acylglycerol acts as a diluent for another acylglycerol, and the oxidation product of the acylglycerol participates in the oxidation of another acylglycerol. However, the oxidation process of 1-monolinolein mixed with a saturated or monounsaturated acylglycerol was not expressed by the model but obeyed the kinetics of an autocatalytic type. The rate constant in the mixed system was greater than the constant which was predicted under the assumption that the saturated or monounsaturated acylglycerol merely acted as a diluent.