Study On Tea Polyphenol-protein Nano-complexes

(-)-Epigallocatechin-3-gallate (EGCG), a kind of polyphenol compounds, is the majorcontent of tea polyphenol. It plays several important physiological roles including antioxidant,anticancer, anti-mutation, etc. However, EGCG are very sensitive to moisture, heat, oxidation,and light due to their unsaturated chemical structures, thus limiting its commercialapplications. The astringency of EGCG further affects its application in food industry.Encapsulation is an effective approach to improve the physical and chemical stability ofEGCG by the self-assembly between macromolecular protein and EGCG forming thecore-shell nanocomplexes. Casein is a type of linear protein that is unique in milk. Owing tothe rich proline in structure, casein has a strong affinity with polyphenols and consequentlyacts as a stable and controlled delivery for EGCG. Besides, glycosylation of casein via Millardreaction in mild condition may further improve the solubility and emulsification of casein,and thereby increase the encapsulation efficiency of EGCG. This study fabricated a new kindof EGCG nano-carrier based on dextran glycosylated casein. With the encapsulationefficiency, retention rate and the average particle size as the indices, the influences of caseinor glycosylated casein binding on the EGCG encapsulation efficiency, storage stability and invitro release were thoroughly investigated. Subsequently, these finding were correlated withthe interaction mechanisms between EGCG and protein. The protective effect of linoleic acidby EGCG-protein nanocomplexes was also carried out.With the encapsulation efficiency (EE), retention rate and average particle size as theindices, the influences of casein or glycosylated casein binding on the EGCG encapsulationeffect, storage stability and in vitro release were thoroughly investigated by means of dynamiclight dynamic (DLS) and high performance liquid chromatography (HPLC). The optimalformulation for nanocarriers was obtained. Results showed that when casein concentrationwas up to2.5mg/mL, protein solution pH value was7.0, and the EGCG/casein mole ratiowas10:1, the EE of EGCG in EGCG-casein nano-complexes was up to86%with the averageparticle size of88nm. During the10days storage, the average particle size fluctuated, and theretention rate decreased to65%. In the case of glycosylated casein, the EE of EGCG canexceed93%with the average size of72nm at the conditions of glycosylated caseinconcentration>2.5mg/mL, protein solution pH6.5-7.5, and EGCG/glycosylated casein moleratio64:1. The glycosylated casein also possessed high retention ability to EGCG,maintaining the retention rate around70%during the10days storage. The in vitro digestionexperiment was performed to evaluate the controlled-release effect of native and glycosylatedcasein on encapsulated EGCG. It was found that the EGCG exhibited a slow and sustainedrelease when binding to glycosylated casein in comparison with that binding to native casein.At the same time, the degradation of EGCG was effectively inhibited by glycosylated casein,confirming a better protection of EGCG. These findings demonstrated that glycosylated caseinhad a higher loading of EGCG than casein, and could be used as a promising and effectivenano-carrier for EGCG.Based on fluorescence spectroscopy, circular dichroism (CD), and Fourier transform infrared spectrum (FTIR) techniques, the influence of EGCG on the protein conformation wasinvestigated. Meanwhile, the quenching effect of EGCG on the instrinsic fluorescence ofprotein was determined to elucidate the interaction between EGCG and protein as well astheir binding constant and binding force. Fluorescence spectrometry showed that thequenching type was static quenching, and the main interaction was driven by hydrogenbonding, depending on temperature. CD and FTIR demonstrated that EGCG mainlyinteracted with the amino acid of protein, but exerted little influence on its secondarystructure. Both small angle x-ray scattering and transmission electron microscopy confirmedthat glycosylated casein can form a core-shell structure in solution, thus enhancing thehydrophilicity of casein and increasing the contact area with EGCG. EGCG-proteinnano-complexes had a good protection to linoleic acid. The strong interface antioxidantactivity was attributed to the synergistic effects between glycosylated casein and EGCG. Thisencapsulation technique for EGCG can provided a new possibility to improve the storagestability of linoleic acid.