Kafirin protein and its applications in nanoencapsulation, pickering emulsion and electrospun fiber

Prolamines from grains have attracted intensive attention due to their potential to satisfy the demand for environmental friendly, abundantly available and cost effective biomaterials. Kafirin, the major prolamine protein from sorghum grain, attracts our research interests due to its unique properties: solubility in aqueous alcohol, highly hydrophobic and non-allergenic nature and reduced susceptibility to proteolytic degradation. Therefore, kafirin has the potential to serve as a novel building block for food grade delivery vehicles. However, research efforts to clarify its functionality related physicochemical properties as well as fulfilling its application potentials are scarce. My Ph.D thesis exploits these research objectives by first investigating the structure, morphology and self-assembly behavior of kafirin, which are highly relevant in guiding its applications thereafter. Results suggested that the α-helix content of kafirin decreased with the increase of solvent polarity. Kafirin took stretched and extended conformations in aqueous alcohol solutions. Under the solvent evaporation procedure, kafirin self-assembled into particles with defined shapes, both protein concentration and solvent polarity affecting the self-assembly behavior. Secondly, kafirin protein was assembled into nanoparticles for encapsulation and to enhance cellular uptake of hydrophobic nutraceutics. The formation mechanism as well as its cellular uptake fate during oral consumption was elucidated. Curcumin loaded kafirin nanoparticles (cc-kaf) as well as cc-kaf with CM-chitosan coating (cc-kaf/CMC) was prepared by an anti-solvent method. Both formulations exhibited solid spherical morphology with the nanoparticle dimension of 100-300 nm. The encapsulation efficiency and loading efficiency were 55.0 ± 1.1% and 5.0 ± 0.1% for cc-kaf and increased to 86.1±2.1% and 6.1±0.2% after introducing CM-chitosan. Hydrogen bonding, electrostatic attraction and hydrophobic interaction were found to be the major driving forces for particle formation and curcumin encapsulation. The particulate formulations improved the photo stability, dissolution profile in simulated gastrointestinal fluids and the cellular uptake efficiency of curcumin. In the following section, kafirin nanoparticles were utilized as a novel Pickering emulsion stabilizer to stabilize oil-in-water Pickering emulsions (KPEs). Effects of particle concentration, oil phase ratio and ionic strength on the microstructure and rheological properties of KPEs were elucidated. Observation of oil-water interfacial structure suggested that kafirin nanoparticles were capable of stabilizing emulsion system with long-term coalescence stability, and they anchored at the emulsion droplet interface as individual or aggregated ones with low surface coverage. In terms of the processing stability, KPEs were less stable under alkaline or body temperature conditions. Meanwhile, KPEs offered a protective effect to encapsulate and retard the lipid oxidation rate compared to Tween 80 stabilized emulsions (TE). However, due to hydrolysis by pepsin, KPE could not survive the gastric digestion process. After escaping the gastric digestion, the lipid digestion extent of KPE in intestinal fluid fell in between that of bulk oil and TE. In another research attempt, the kafirin nanoparticles were introduced into the double emulsion system by stabilizing the outer interfacial layer of the double emulsion. The osmotic pressure gradient-driven swelling was found to be the major challenge for long-term stability of the as prepared double emulsion (KDE) during storage and processing. Under simulated gastric digestion, KDE underwent structural collapse and its lipid digestion profile in simulated intestinal fluid followed a similar trend as the kafirin particles-stabilized single Pickering emulsion. To address the observed poor storage stability and premature release of the oil phase after oral intake, we immobilized KPE within an alginate based hydrogel matrix. Through combined effects of increase in bulk viscosity, deposition of alginate polymer onto the interface and the pepsin inhibition effect of alginate, the processing as well as gastric digestion stability of Pickering emulsions were improved. Lastly, electrospinning of kafirin protein was carried out and polycarprolactone was blended to obtain fiber mats with tunable mechanical, swelling and release properties. Compared to the hydrophobic surface of neat PCL fiber mat, KAF/PCL fiber mats showed hydrophilic surface characteristics, and the swelling property was composition dependent. The fiber mats evolved from brittle ones to flexible ones as the relative content of PCL increased. Drug encapsulated within KAF/PCL fiber mats were found to be diffusion controlled. While amorphous regions of kafirin dominated the release rate, PCL functioned as hydrophobic skeleton to maintain the 3D scaffold of the fiber matrix. The fabricated KAF/PCL fiber mats opened a new application field for the underutilized cereal protein and could serve as practical candidates for wound healing and tissue engineering scaffolds.Ph.D.Includes bibliographical referencesby Jie Xia