Unravelling the behaviour of curcumin nanoemulsions during in vitro digestion: effect of the surface charge

Oil-in-water nanoemulsions containing curcumin were prepared through high-pressure homogenization using corn oil and three different emulsifiers: Tween 20 (non-ionic), Sodium Dodecyl Sulphate (SDS, anionic) and DodecylTrimethylAmmonium Bromide (DTAB, cationic). A human gastric simulator was used as the in vitro digestion model (in which the stomach, duodenum, jejunum and ileum steps were performed) to evaluate the impact of surface charge on the digestion of the curcumin nanoemulsions. This model allowed the simulation of continuous peristaltic movements and consequently enabled a more mechanically realistic simulation of the dynamic digestion process than simple stirred vessel models. The emulsifier charge had a significant effect on the droplet size, particle electric charge and microstructure of curcumin nanoemulsions during the simulated digestion, which consequently influenced the free fatty acid release and curcumin bioavailability. The results showed the positively charged DTAB-stabilized emulsions to be the least stable during the digestion process, exhibiting the largest increase in droplet size and eventual phase separation. This also contributed to the low bioavailability of curcumin. Conversely, emulsions stabilized with Tween 20 showed retention of emulsion structure (high surface area) and greater free fatty acid production, which could explain the increased curcumin bioavailability. The emulsifier charge influenced the lipid digestion process and the bioavailability of the bioactive compound incorporated, probably by altering the ability of bile salts and digestive enzymes to adsorb onto the emulsion surfaces, thus altering the droplet size (and consequently the surface area) due to droplet breakup or coalescence within the digestive tract. The results of this work also highlighted the importance of subjecting the emulsions to a simulated gastric environment, since changes in pH, ionic strength, gastric enzyme activity and shear will impact the emulsion properties in the small-intestine. This manuscript has provided important insights into the effect of emulsifier charge on the behaviour of nanoemulsions during in vitro digestion, which is important to determine their functional performance, aiming at the optimization of nanoemulsion-based delivery systems to protect and release bioactive lipophilic compounds.