INFLUENCE OF MOLECULAR EXCHANGE PROCESSES ON THE FATE OF EMULSIFIED LIPID NANOPARTICLES IN OIL-IN-WATER EMULSIONS

The migration of hydrophobic substances, such as colors flavors, antioxidants, pro-oxidants, or vitamins, from one location to another in foods impacts their formation, stability, and quality. The impacts of molecular exchange processes - Ostwald ripening (OR) and compositional ripening (CR) - on the nanoparticles in oil-in-water emulsions were therefore investigated to learn the fates of the emulsified lipid nanoparticles in food products containing multiple kinds of hydrophobic ingredient. Firstly, the impact of oil physical states on OR process in nanoemulsions was examined. When the initial particle sizes were similar, the mean particle size of liquid lipid droplets significantly increased during storage while that of solid lipid particles maintained relatively stable, which was attributed to the solidification of the solid particles that retard molecular exchange. Secondly, the effects of oil water-solubility, the initial oil droplet composition, and oil droplet physical state on both OR and CR processes in mixed oil-in-water nanoemulsions were investigated when the initial particle sizes were similar. Two stages of droplet growth were observed after mixing the nanoemulsions containing oils with different water-solubility: (i) an initial rapid growth associated with exchange of oil between droplets; (ii) a later slow growth after all the droplets had similar compositions. The initial stage occurred more quickly when the oil droplets were mixed with higher water-solubility droplets, rather than mixed with lower water-solubility droplets, which had shown that higher water-solubility of oil favored the CR process. In addition, changes in droplet growth and composition during storage was observed to be depended on the initial mixing ratio of two kinds of droplets. The larger the proportion of higher water-solubility droplets present, the faster the rate of droplet growth, which had shown that higher water-solubility of oil favored the OR process. The final droplet composition depended on the initial ratio of the different droplets. Furthermore, in the mixed nanoemulsions containing two kinds of oil droplets, the solidification of one kind of oil droplets retarded molecular exchange but promoted droplet growth in the mixed nanoemulsions. The promoted droplet growth was attributed to the increased difference in particle size which was induced by the unbalanced molecular exchange. Finally, the effects of oil droplet physical state and oil droplet composition on molecular exchange processes between droplets when a nanoemulsion (around 0.2 µm in diameter) was added to an emulsion containing larger droplets (around 40µm in diameter) were investigated. Two groups of droplets were observed in the droplet size distribution after mixing: small droplets group corresponding to the nanodroplets in nanoemulsions and large droplets group corresponding to the droplets in emulsions. The volume ratio of droplets groups in the mixed systems had a significant change after mixing. When liquid nanodroplets and liquid large droplets with similar or higher water-solubility of oil were mixed, the mass transfer of oil molecules was observed to be significant from nanodroplets to large droplets during mixing, which had suggested that the molecular exchanges were initially dominated by OR process and droplet coalescence. As a result, the addition of nanodroplets had significantly promoted the droplet growth of large droplets in this case. The present of CR process had shown some impact on retarding this type of droplet growth when nanodroplets were added to emulsions with larger and higher water-solubility droplets. When solid nanoparticles and liquid large droplets with higher water-solubility of oil were mixed, the mass transfer of oil molecules was observed to be significant from large droplets to small droplets during mixing, which had suggested that the molecular exchanges were initially dominated by CR process. In this case, the solidification of nanoparticles may have retarded the diffusion of inside molecules and therefore large droplets kept losing molecules. Furthermore, some compositional exchanges were observed in the mixed systems with different oil compositions after mixing, and the solidification of nanoparticles have shown a delay effect on compositional exchange. Surprisingly, there was observed to be a limited amount of transfer of oil molecules between the droplets under the effects of droplet ripening process in all the mixed systems during the storage time, which have indicated that the efficiency of mass transfer between oil droplets in these mixed systems was relatively low after mixing. Our results provide a better understanding of molecular exchange processes in mixed emulsions containing nanoparticles and other emulsion droplets which may be useful for optimizing the formulation of emulsified lipid nanoparticles for food products containing multiple kinds of hydrophobic ingredient