Curcumin is a bioactive phytochemical that can be utilized as a nutraceutical or pharmaceutical in functional foods, supplements, and medicines. However, the application of curcumin as a nutraceutical in commercial food and beverage products is currently limited by its low water-solubility, chemical instability, and poor oral bioavailability. In this study, all-natural colloidal delivery systems were developed to overcome these challenges, which consisted of saponin-coated curcumin nanoparticles formed using a pH-driven loading method. The physicochemical and structural properties of the curcumin nanoparticles formed using this process were characterized, including particle size distribution, surface potential, morphology, encapsulation efficiency, and loading capacity. Fourier transform infrared spectroscopy and X-ray diffraction indicated that curcumin was present in the nanoparticles in an amorphous form. The curcumin nanoparticles were unstable to aggregation at low pH values (200 mM), which was attributed to a reduction in electrostatic repulsion between them. However, they were stable at higher pH values (3 to 8) and lower NaCl levels (0 to 200 mM), due to a stronger electrostatic repulsion between them. They also exhibited good stability during refrigerated storage (4 °C) or after conversion into a powdered form (lyophilized). A simulated gastrointestinal tract study demonstrated that the in vitro bioaccessibility was around 3.3-fold higher for curcumin nanoparticles than for free curcumin. Furthermore, oral administration to Sprague Dawley rats indicated that the in vivo bioavailability was around 8.9-fold higher for curcumin nanoparticles than for free curcumin. These results have important implications for the development of curcumin-enriched functional foods, supplements, and drugs.

The poor water solubility and bioavailability of curcumin can be improved by encapsulating it into liposomes. However, the existing encapsulation technologies, such as the thin film method and the ethanol injection method, are complex and require the use of organic solvents. In this study, an organic solvent-free and easily scalable encapsulation technique was studied by utilizing the pH-dependent solubility properties of curcumin. Phospholipid was dissolved in water to form liposomes. Curcumin was deprotonated and dissolved under alkaline conditions and then encapsulated into the liposomes after acidification. Morphological observation and X-ray diffraction analysis confirmed that curcumin liposomes had been successfully prepared. Curcumin liposomes prepared by the pH-driven method were stable during storage. During in vitro digestion, curcumin liposomes prepared by the pH-driven method showed similar bioaccessibility to those prepared by the thin film method and higher bioaccessibility than those prepared by the ethanol injection method. The pH-driven method, which is organic solvent-free and easily scalable for industrial production, is thus a promising method for the preparation of curcumin liposomes.