Linoleic acid was emulsified with gum arabic or maltodextrin at various weight ratios of the acid to the polysaccharide in the presence or absence of a small-molecule emulsifier. The emulsions were spray-dried to produce microcapsules. Emulsions prepared with gum arabic were smaller in droplet size and more stable than those prepared with maltodextrin, and linoleic acid in a gum arabic-based microcapsule was also most resistant to oxidation than that in a maltodextrin-based microcapsule. Although the oil droplet size in the emulsion with maltodextrin decreased and the emulsion stability was improved by addition of a small-molecule emulsifier to linoleic acid, the oxidative stability of the encapsulated linoleic acid was not significantly improved. Encapsulated linoleic acid of small droplet size oxidized more slowly than that of large droplet size.

Methyl linoleate was encapsulated with gum arabic by two drying methods, hot-air-drying and freezedrying. The oxidation of methyl linoleate encapsulated by both methods depended on the relative humidity during storage. Methyl linoleate encapsulated by freeze-drying was more slowly oxidized than that encapsulated by hot-air-drying at any relative humidity. The initial fraction of nonencapsulated lipid in the hot-air-dried microcapsule was about 1%, and the fraction increased quickly in the early stage of storage at a high relative humidity. On the other hand, the fraction of nonencapsulated lipid in the freeze-dried microcapsule was about 10%, but it did not change during storage at any relative humidity. Scanning electron micrographic observation of microcapsules prepared by hot-air-drying and freeze-drying showed that their morphologies were greatly different. These results suggested that the state of the lipid encapsulated by freeze-drying was different from that encapsulated by hot-air-drying.

6-O-Palmitoyl L-ascorbate was added to linoleic acid at various molar ratios of the ascorbate to the acid, the mixtures were emulsified with a maltodextrin or gum arabic solution, and the emulsions were spray-dried to produce microcapsules. At higher molar ratios, the oil droplets in the emulsions were smaller, and the oxidative stabilities of the encapsulated linoleic acid were higher for both the maltodextrin- and gum arabic-based microcapsules. 6-O-Capryloyl, caproyl, and lauroyl L-ascorbates, which were synthesized through lipase-catalyzed condensation in acetone, were also used for the microencapsulation of linoleic acid. Except for capryloyl L-ascorbate, the addition of a saturated acyl ascorbate, especially caproyl ascorbate, to linoleic acid was effective for preparing oil droplets of small particle diameter and for suppressing the oxidation of the encapsulated linoleic acid.

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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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