Curcumin encapsulation in zein-rhamnolipid composite nanoparticles using a pH-driven method

Zein is widely used to encapsulate non-polar bioactives due to its ability to form protein nanoparticles with hydrophobic interiors. In this study, a simple pH-driven method, which does not use organic solvents, was employed to fabricate zein-rhamnolipid composite nanoparticles. An alkaline zein solution (pH 12.0) was mixed with an acidic rhamnolipid solution (pH 2.0), which led to the spontaneous formation of composite nanoparticles (pH 7.0). The possibility of using these nanoparticles to encapsulate and deliver curcumin was then studied. Nanoparticle properties, such as size and charge, were mainly determined by the mass ratio of zein-to-rhamnolipid used. The nanoparticles formed were spherical and relatively small (d ≈ 100 nm) according to dynamic light scattering and transmission electron microscopy. Nanoparticle suspensions remained stable from pH 5 to 9 but aggregated under more acidic conditions. At neutral pH, the nanoparticles were stable at low ionic strengths (<100 mM NaCl), but aggregated at higher electrolyte levels. Moreover, they remained stable when incubated at relatively low temperatures (37 or 55 °C) but aggregated at higher temperatures (90 °C). Curcumin was loaded into the nanoparticles using the pH-shift method, which led to a high encapsulation efficiency. The nanoparticles were able to effectively protect the curcumin from degradation during one-month storage at both 25 and 37 °C. The food-grade nanoparticles fabricated in this study may be suitable as delivery systems for hydrophobic nutraceuticals in functional foods and beverages.