Comparison of Emulsion and Nanoemulsion Delivery Systems: The Chemical Stability of Curcumin Decreases as Oil Droplet Size Decreases

The water dispersibility, chemical stability, and bioaccessibility of curcumin, a labile hydrophobic nutraceutical, can be enhanced by incorporating it inside the oil droplets of oil-in-water emulsions or nanoemulsions. In these multiphase systems, the curcumin remains relatively stable to degradation when surrounded by oil but degrades rapidly when surrounded by water. We hypothesized that the size of the lipid droplets would therefore impact the stability of encapsulated curcumin by altering the surface area of oil exposed to water. The effect of droplet surface area on the kinetics of curcumin degradation was therefore studied by producing emulsions with different mean droplet diameters (d32) and therefore different specific surface areas (AS): large (d32 = 20.9 μm; AS = 300 m2 kg–1); medium (d32 = 2.53 μm; AS = 2500 m2 kg–1); small (d32 = 0.26 μm; AS = 24,000 m2 kg–1); and very small (d32 = 0.083 μm; AS = 80,000 m2 kg–1) emulsions. All the emulsions initially had milky-yellow appearances and were relatively stable to aggregation during the course of the experiments. However, rapid creaming was observed in the large and medium emulsions because of their relatively large droplet size. The emulsions all exhibited some color fading during storage, with the rate of curcumin degradation increasing with decreasing droplet size. For instance, the percentage of curcumin remaining in the emulsions after 17 days storage was 91.4 ± 1.5 > 77.3 ± 6.6 > 66.7 ± 1.9 ≫ 30.6 ± 2.8% for the large, medium, small, and very small emulsions, respectively. The more rapid chemical degradation of the curcumin in the smaller droplets can be attributed to the fact that curcumin exchange between the interior and exterior of the droplets occurs more rapidly as the droplet dimensions decrease. Our results indicate that the droplet size plays a critical role in the degradation of curcumin encapsulated in emulsions, which may have important consequences for the formulation of curcumin-enriched foods and beverages with enhanced bioactivity. In particular, it suggests that emulsions are more effective at chemically stabilizing curcumin than nanoemulsions.