| Whey protein isolate (WPI) is particularly important food ingredient thatis extensively utilized as a functional food material in food processing, because of itshigh nutritional value and sensory characteristics. Increasing interest is directedtoward modifying whey proteins to enhance functionality and thereby add value tothe proteins. Cross-linking of protein molecule could improve the functionalproperties. Phenolic compounds could induce cross-linking reaction between proteinmolecules. In the present study, the main objective was to investigate the effect ofcross-linking reaction induced by (-)-epigallocatechin-3-gallate (EGCG) on thefunctional properties of WPI. Furthermore, the physiochemical characterization,structure and functional properties of modified WPI were analysized. The mainresults are as follows:(1) The cross-linking reaction induced by EGCG and its effects on functionalproperties of WPI were analysized. WPI was covalently modified by EGCG on thealkaline condition. The SDS-PAGE and gel filtration chromatography on SepharcylS-200indicated the cross-linking of WPI components induced byEGCG. Two-dimensional gel electrophoresis (2-DE) analysis was used to separatethe WPI derivatives. Spot detection and matching were performed using the PDQuest8.0.1software. The main WPI derivative was identified as the dimmer ofβ-lactoglubin by the matrix-assisted laser desorption/ionization time-of-flight massspectrometry (MALDI-TOF MS) after digestion by trypsin. The modification of WPIinduced by EGCG enhanced the emulsifying and foaming properties.(2) The effects of cross-linking reaction on the structure of β-lactoglobulin werestudied. β-lactoglobulin was covalently modified with EGCG on the alkalinecondition. Amino acid analysis showed the contents of cysteine and lysine inmodified β-lactoglobulin were higher than those of native β-lactoglobulin. This resultindicated that-SH and-NH2groups were the specific binding site in the chain ofβ-lactoglobulin. The modification reduced the surface hydrophobicity ofβ-lactoglobulin. The conformational changes of modified β-lactoglobulin wereinvestigated using Fourier transform infrared (FT-IR) spectroscopy. The bands of amide I and II showed distinct downshifts. The deconvoluted spectrum in the amide Iregion of modified β-lactoglobulin in comparison to native β-lactoglobulin revealedthe increased contents of α-helix and β-sheets structures, and the decreased contentsof unordered and turn structure.(3) The characteristics of dimmer of β-lactoglobulin were evaluated. Thedimmer was purified using the column chromatography on Sephadex G100. Themolecular mass was36729.26Da, as revealed by MALDI-TOF MS. The UVabsorption spectra of dimmer showed two UV absorption bands centered on about230nm and275nm, which indicated the modification involved the phenolic groups.Intrinsic fluorescence intensity of dimmer was decreased as compared with that ofnative β-lactoglobulin, which illustrated that the tertiary structure of the protein wasaltered. FT-IR analysis showed that the contents of α-helix, β-sheets, turn andunordered structures were19.25%,41.30%,19.76%and19.67%, respectively. Theresults of differential scanning calorimetry (DSC) showed the denaturationtemperature (Td) of dimmer increased as compared with native β-lactoglobulin. Thisrevealed that cross-linking enhanced the thermal stability of β-lactoglobulin.(4) The effects of cross-linking reaction induced by EGCG on the functionalproperties of β-lactoglobulin were investigated. The modification improved theemulsifying activity and emulsifying stability. The foaming activity was increased,while the foaming stability was decreased. Compared with the control, the emulsionstabilized by modified β-lactoglobulin showed smaller droplet size and better sizedistribution. EGCG treatment induced more ability to β-lactoglobulin to be adsorbedat the oil-water interface. Furthermore, the modification increased the apparentviscosity of the emulsion. |
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