Assembly, characterization and application of ovotransferrin fibrils
Creators: name: Wei, Zihao
Publisher: name: No Publisher Supplied
Year: 2019

description: Ovotransferrin (OVT), which accounts for about 12% of egg white proteins, attracts more and more research interests especially during these years due to multiple biological benefits and excellent solubility. However, since extraction procedures of OVT are very complicated and extraction yield of ovotransferrin is very low, research about OVT has not been widely investigated yet. At present, most of OVT studies focus on extraction and structural characterization, and application of OVT in constructing food polymeric structures and delivery systems has seldom been done. The overall aim of my PhD thesis was to assemble, characterize and subsequently apply OVT fibrils in emulsion delivery systems, which may help to construct food polymeric structures and delivery systems using OVT as building blocks. Firstly, the optimal environmental condition (pH, temperature, ionic strength, etc) for OVT fibrillation was explored. Atomic force microscopy (AFM) revealed that OVT fibrils consisted of both long and short fibrils. By decoupling hydrolysis and self-assembly, it was demonstrated that both intact monomers and peptides were the building blocks of OVT fibrils. Cicrular dichroism results indicated that internal structures of OVT amyloid fibrils could be stacked β-sheet. OVT amyloid fibrils had no in vitro cytotoxicity, suggesting great application potential. Secondly, digestion and stability of OVT fibrils were investigated. Gastrointestinal digestion of OVT nanofibrils was characterized by thioflavin T (ThT) fluorescence and AFM. Most of OVT nanofibrils were disrupted during gastrointestinal digestion, and some OVT nanofibrils showed resistance to proteolytic digestion in vitro. OVT fibrils were stable over a wide pH range, and OVT fibrils possessed excellent stability against environmental stresses such as frozen storage−lyophilization−rehydration. The excellent stability indicated that OVT fibrils were stable food polymeric structures and could be applied in design of nutraceutical delivery systems.Thirdly, impact of many other food ingredients in complex food systems on OVT fibrillation was studied using tools such as ThT fluorescence, SDS-PAGE and AFM. The investigated food ingredients included polyols, saccharides and polyphenols. In terms of effects of polyols on OVT fibrillation, the presence of glycerol or sorbitol could reduce the rate of OVT fibrillation, and slowdown of OVT fibrillation was strongly dependent on concentration of glycerol or sorbitol. Regarding impact of Maillard reaction, glycation could suppress fibrillation of ovotransferrin, and glucosylation exerted stronger inhibitory impact on fibril formation than lactosylation. Glycation decreased average contour length of ovotransferrin fibrils, When it came to influence of polyphenols, the bound polyphenols (EGCG and GA) could inhibit OVT nanofibrillation, and higher level of complexation of OVT with more polyphenols showed stronger fibril-inhibitory activity. Covalent bound polyphenols exerted stronger inhibitory influence on OVT nanofibrillation than corresponding non-covalent bound polyphenols. Lastly, OVT fibrils were applied in emulsion delivery systems. OVT fibrils were verified as effective Pickering emulsifiers, and visual apperance confirmed that OVT fibrils could stabilize Pickering emulsions with high emulsified phase volume and stability index at different fibril concentrations and oil fractions. OVT fibrils could be employed to fabricate stable Pickering emulsions at various ionic strengths (0–1000 mM) and pHs (2–7). OVT fibril-stabilized Pickering emulsions could provide curcumin protection, and the distinction in curcumin protection was related to ionic strengths and pHs of emulsions. As demonstrated in TIM-1 and pH-stat digestion models, OVT fibril-stabilized Pickering emulsions could increase curcumin bioaccessibility. To improve freeze-thaw stability of Pickering emulsion systems, organogel-based Pickering emulsions stabilized by OVT fibrils were developed. Organogel-based Pickering emulsions stabilized by OVT fibrils had excellent storage stability, and freeze-thaw stability of organogel-based Pickering emulsions stabilized by OVT fibrils was better than that of conventional Pickering emulsions (without organogel) stabilized by OVT fibrils. In comparison with organogel, organogel-based Pickering emulsion could improve both extent of lipolysis and hesperidin bioaccessibility. The acquired knowledge in this thesis may facilitate assembly, characterization as well as application of food protein fibrils.