| The interactions of protein and polysaccharide have great influences on the structure and function of protein, which is often used as a means of protein structure modification. This paper is mainly aimed at the problems of egg white protein poor emulsifying properties. Ovalbumin(OVA) and Gum Arabic(GA) were used as models to study the interactions between protein and polysaccharides, and the interfacial adsorption behavior, emulsification, stability and rheological properties of their oil-in-water emulsion. And then the relationship of the interactions of protein-polysaccharides and the interfacial properties, emulsion stability were studied. Then the protein-polysaccharide complexes were used to emulsify thyme oil, and studied the stabilization and long-acting antibacterial effects of its interfacial adsorption structures on essential oil. This study aimed is to develop functional properties of egg protein, and further expand the application scope of egg protein in the food system. The main research contents and results are as follows:Firstly, the interactions and phase separation behaviors of OVA and polysaccharides were studied by turbidity method, then the ζ-potential and mean particle diameter of protein-polysaccharides complexes were analyzed. The results showed that: in the total concentration was 0.05%(protein/polysaccharide ratio was 2:1), with the decreasing of p H value, four different phases showed up: co-solubility phase region(p H > 4.61), soluble complex phase region(4.18 < p H < 4.61), insoluble complex phase region(2.4 < p H < 4.18), co-solubility phase region(p H < 2.4). At the p H 3.79, the turbidity value of the complexes was the highest(1.05). When the concentration of Na Cl increased from 0 m M to 60 m M, the turbidity value of the complexes dropped from 1.05 to 0.18, and the phase region of insoluble complexes became narrow. Under the same conditions, the effect of divalent ion on protein/polysaccharide interaction was stronger than that of monovalent ions, while the shielding abilities of different ions with the same charge were no difference. When the ratios of protein/polysaccharide increased from 1:1 to 24:1, the turbidity curve shifted to high p H values, and the maximum value came at protein/ polysaccharide ratio of 2:1. At first, the mean particle diameter of protein/polysaccharide complex coacervate changed very fast, and then slowed down. At p H of 3.7, the mean particle diameter of complex coacervate increased from 880 nm to 2300 nm in the initial 0 to 12 min, and then slowed down at 12 to 20 min.Secondly,the influence of the interactions between protein and polysaccharides on protein conformation and the rheological properties of complex coacervate were studied by ultraviolet spectrum, fluorescence spectrum, X-ray diffraction(XRD), fourier transform infrared spectroscopy(FT-IR), differential scanning calorimeter(DSC), rotational rheometer and other technical means. The results showed that: the protein-polysaccharide complex coacervate were mainly driven by the amino groups of OVA(at 1313cm-1) and the carboxyl group of GA(at 1609 cm-1 and 1420 cm-1), and the presence of GA changed the spatial structure of protein, the tyrosine residues of OVA shifted to a more hydrophilic environment, while the tryptophan residues to a more hydrophobic environment. At the same time, the denaturation temperature of protein increased from 72oC(control group) to 96 oC. As for the rearrangement of protein and polysaccharides, more ordered crystal structure were formed after the formation of protein-polysaccharide complex coacervate. The great changes of secondary structure of OVA were benefit for the interaction between protein and polysaccharides, and the α-helix and β-turn structure were the main domains for the interaction. Polysaccharides stabilized the structure of α-helix and β-turn, but had less influences on random coil structures and β-sheet. The complex coacervate showed higher viscosity and bigger thixotropic loop at p H 3.7 than 4.0.Thirdly,the influence of GA on the mean particle diameter, surface charge, the microstructure and interfacial adsorption characteristics of O/W emulsions stabilized by ovalbumin, evaluation of the stability and stabilization mechanism of emulsion exposed to environmental stress(storage time, p H value, salt concentration, heat treatment, oxidation) were studied. Rheological properties and the interaction between oil and the adsorption of proteins at interfaces were further determined, and the relationship between the emulsion stability and rheological properties was established. The results showed that at total concentration of 0.6%, OVA/GA ratio was 1:2, acidic p H values, GA covered quickly the surface of OVA-stabilized emulsion droplets, and the mean particle diameter of droplets distributed even(around 310 nm), and showed good stability at different storage time, p H value, heat treatment, oxidation and other environment stress. But the tolerance of Na Cl of emulsion were weak, the flocculation phenomenon occurred at Na Cl concentration above 20 m M. The results of the rheological properties showed the characteristics of Newton fluid(the emulsion viscosity decreased with the increasing of shear rate). The emulsion showed higher apparent viscosity at acid(p H 4.0 and p H 3.8) than that of neutral p H. Meanwhile, acidic emulsion showed better stability to the change of frequency.Fourthly, the effect of different adsorption layer structure on the stability of emulsion were studied, and the results showed that I and II emulsion showed smaller droplet size(around 330 nm) when the p H value was higher than 3.3, the mean particle diameter increased slightly after 1-week storage, which showed good storage stability. While III emulsion showed worse stability(obvious stratification showed after 7-days storage). I and II emulsions showed bad stabilities to salt concentration, the flocculation phenomenon occurred at Na Cl concentration above 30 m M. Although the particle size of III emulsion is larger, it showed good stability to salt concentration due to steric hindrance. However, III emulsion showed good stabilities to temperature(the mean particle diameter of emulsion showed no difference after heating for 30 min under 50 oC or 90 oC).The stabilization of bilayer and complexes emulsions to thyme oils with different oil phase were studied, the emulsion rates and emulsion retention rates of thyme oil by two kinds of emulsions were compared, and the antibacterial activities were studied. The results were as follow: bilayer emulsion of OVA-GA, when the ratio of sunflower oil and thyme oil was 1:1, the mean particle diameter of emulsion was smaller(around 310 nm), and the mean particle diameter only increased 20-30 nm after 1-week storage, which showed better stability. The emulsion rate of thyme oil was remained higher than 90%, and the retention rate of emulsion was still higher than 80% after 30-days storage. In the OVA/GA complexes emulsion, emulsion had smaller mean particle diameter in acid conditions; the smallest particle diameter was 275 nm. And after 1-week storage, the particle size increased by 10-15 nm, emulsion rate was more than 90%, and the retention rate of emulsion was higher than that of double bilayer emulsion after 30-days storage. Comparison of inhibition properties of bulk thyme oil, bilayer emulsion and complexes emulsion, and the results were as follow: bulk thyme oil and bilayer emulsion(p H 7.0 and p H 4.0) had the same minimal inhibitory concentrations. However, the minimal inhibitory concentration of bilayer emulsion(p H 3.5) and complexes emulsion(p H 4.0 and p H 3.5) was twice of them. This was because that the stability of interface structure slowed down the release rate of thyme oil. But, the stability of interface structure could prolong bacteriostatic of thyme oil with more long-lasting inhibition properties. |
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