Physicochemical properties and oxidative stability of fish oil nanoemulsions as affected by hydrophilic lipophilic balance, surfactant to oil ratio and storage temperature

Abstract

Fish oil nanoemulsions, stabilized with different mixtures of non-ionic surfactants (Tween 80, Span 80), were prepared by high intensity ultrasound method. Physicochemical properties and storage stability of nanoemulsions were studied under the effects of surfactant to oil ratio (SOR 0.5⿿1.5), hydrophilic lipophilic balance (HLB 9⿿15) and temperature (4 and 25°C) for 1 month. Nanoemulsions of an average droplet size of 82nm were obtained after sonication for 10min. An increase in SOR and HLB led to a decrease in particle size and optical density. Conventional emulsions were more oxidatively stable than respective nanoemulsions. Ostwald ripening rate and oxidative instability of nanoemulsions stored at 25°C were lower than those measured at 4°C. Increased oxidative stability at higher temperatures was attributed to the decreased solubility of oxygen at higher temperatures as well as to the possible differences in the molecular orientations of the fatty acids within the droplets at different temperatures. Ostwald ripening was the major destabilizing mechanism during storage of nanoemulsions at low temperature and in the early stage of nanoemulsion storage at higher temperature. After 2 weeks, other mechanisms such as flocculation and coalescence revealed more important roles. Incorporating α-tocopherol into nanoemulsions fairly increased the chemical stability of unsaturated fatty acids maybe because of its partitioning mainly at the interface. However, after two weeks it was not so efficient. Morphological studies confirmed the results obtained by laser diffraction technique.