Riboflavin-photosensitized lipid oxidation in oil-in-water nanoemulsions: effects of oxygen scavengers and metal chelator.

This study examined how droplet size affects riboflavin-photosensitized lipid oxidation in oil-in-water emulsions and evaluated the roles of sodium azide (SA), superoxide dismutase (SOD), and ethylenediaminetetraacetic acid (EDTA) under light exposure. A model oil phase containing octadecane as the carrier oil and methyl linoleate as the oxidizable lipid substrate was used, while sodium dodecyl sulfate (SDS) served as a model anionic surfactant. Small- and large-droplet O/W (O/W) emulsions were produced by high-pressure homogenization at 20,000 psi and 1,500 psi, respectively and the emulsion was incubated 32 °C. The particle size of small and large droplets was determined 62.1 nm and 190 nm, respectively. The ζ-potential for small and large droplets was determined -73.3 and -90.8 mV. In the presence of riboflavin, light exposure markedly promoted lipid oxidation in both emulsion systems, as indicated by increased lipid hydroperoxide and headspace hexanal formation. The rate of lipid oxidation was higher in small-droplet emulsions than in large-droplet emulsions during photooxidation, suggesting that reduced droplet size increased oxidative susceptibility. However, SA did not significantly inhibit lipid hydroperoxide or hexanal formation, whereas SOD reduced lipid hydroperoxide accumulation but did not inhibit headspace hexanal formation. In contrast, EDTA strongly inhibited hexanal formation in both small- and large-droplet emulsions, indicating that transition metal-catalyzed hydroperoxide decomposition played an important role in secondary volatile oxidation. Overall, droplet size emerged as a key structural factor affecting riboflavin-photosensitized lipid oxidation in O/W emulsions, whereas metal chelation was particularly effective in limiting secondary volatile oxidation.