Designing of nanocapsules of curcumin using native casein micelles and their physicochemical, structural, stability and bioavailability analysis

Casein (CN), the major milk protein, is a nanovehicle to deliver nutrients like calcium, phosphorus and protein. CN has been used in various forms like caseinates and reassembled micelles, to encapsulate bio-actives for their efficient delivery, although the inherent properties of CN like heat stability, functionality and digestibility are lost in these methods. Hence, present study focuses on utilization of native CN to nanoencapsulate model hydrophobic compound ‘Curcumin’ so as to protect it from degradation during processing and digestion, along with preserving the inherent properties of CN. The method is based on pH manipulation towards alkaline side assisted by solvent and mechanical stirring to facilitate opening of CN micelles to expose hydrophobic interior of micelles. This leads to hydrophobic interaction between the inner core and the curcumin which is also lipophilic in nature. It was then followed by bringing the pH to neutral sides, thereby subsequent closing to regain the original nanostructure of CN micelles with curcumin trapped inside it. The solution so prepared was dried to obtain powdered nanocapsules that have an Z-avg & zeta potential (230.00 ± 1.50 nm, − 20.40 ± 0.48 mV) similar to CN micelles in buffalo skim milk (229.00 ± 1.13 nm, − 20.20 ± 0.86 mV). Transmission electron microscopy images of CN micelles with and without encapsulation were not significantly different again restating the fact that native structure was restored. The characteristic absorbance peak of FTIR around 3508 cm^− 1, which corresponds to the –OH stretching vibration of curcumin, disappeared after encapsulation, due to its reduced stretching and bending, confirming the binding to hydrophobic core of micelles. The encapsulation efficiency of the nanocapsules developed was excellent: 98.65 ± 0.35% in the liquid sample and 99.12 ± 0.12% for the dried formulation. The in vitro digestion studies concluded that approximately 95% of compound released under intestinal conditions; hence, curcumin was released with good bioavailability. Therefore, the curcumin was not only prevented from degradation under environmental and digestibility conditions but also the native structure of micelles was preserved. These types of nanocapsules have the potential to be incorporated in various food products to design functional foods that will have bioactive properties of the encapsulated compound as well as enhanced protein content due to CN.

Graphical abstract