Stability Of Soy Protein-rutin Complex Emulsion And Its Application In Controlled Release Of β-carotene

Soybean protein isolate(SPI),one of the resources of food protein with functional properties, is widely used as a proteinaceous emulsifier in food emulsions. However, soy protein-stabilized emulsions are more sensitive to environmental factors, and commercial SPI product canrarely remain in the native state, because heat treatment is almost inevitable in SPI processing. In this study, SPI and rutin particles as emulsifiers could competitively adsorb at the oil/water interface and form mixed compact interfacial layers. These processings could improve emulsion stability, and help for encapsulating and slowly releasing of β-carotene in these complex emulsions.First, the adsorption of NSP(native soy protein) and HSP(heated soy protein) at the oil/water interface during emulsification was studied. Protein samples were prepared by heating protein solutions at concentration of 1- 5%(w/v), and were then diluted to 0.3%(w/v). The results showed that soy proteins those had been heated at higher concentrations generated smaller droplet size of emulsion. Increase in homogenizer rotating speed resulted in higher protein adsorption percentages and lower surface loads at the oil/water interface. Surface loads for both unheated and heated soy proteins were linearly correlated with the unadsorbed proteins’ equilibrium concentration at varying rotating speed. With the elevation in NaCl addition level, protein adsorption percentage and surface loads of emulsions increased, while the lower droplet sizes were obtained at the ionic strength 0.1 M. The aggregates and non-aggregates displayed different adsorption behaviors when rotating speed or NaCl concentration varied.Next, interactions between SPI and rutin particles were investigated by fluorescent spectrometry, interfacial tensiometry and isothermal titration calorimetry. Interfacial rheological analysis was also used to evaluate the adsorption behavior of SPI-rutin mixtures at the O/W interface. The average hydraulic radius of rutin particle was 29.9 nm in this experiment. Rutin could bind with aromatic amino acid residues of SPI due to hydrophobic interaction, forming a SPI- rutin binary complex. The fluorescence quenching of SPI induced by rutin was in the form of dynamic collision quenching. ITC experiments indicated that the interaction between SPI and rutin in aqueous solution was an exothermic reaction, which could be spontaneous. The addition of rutin reduced interfacial tension of NSP at the O/W interface, while that of HSP was less influenced. This was probably because rutin particles could competitively adsorb at the O/W interface with NSP, and form mixed compact interfacial layers. While HSP had stronger adsorption capacity, rutin particles could not replace it at the interface. So, interfacial tension in HSP samples was only dependent on the adsorption amount of HSP. Besides, the addition of rutin reduced storage modulus of NSP.The effects of rutin on SPI emulsifying properties were studied. We compared the differences between NSP-rutin emulsion and HSP-rutin emulsion, including emulsion microstructure, droplet size distribution, zeta potential values, protein/rutin adsorption amount at O/W interface, emulsion rheological behavior, etc. The addition of rutin reduced the oil droplet size slightly, which was consistent with the observations made using CLSM. The zeta potential of SPI emulsions increased with the increasing of rutin addition. In the case of NSP emulsion, rutin particles could competitively adsorb into the oil-droplet interface with high interfacial accumulation. The initial interfacial protein of more than 42.1% was displaced when rutin concentration increased to 0.2 mg/m L. Steady shear viscosities for complex emulsions decreased with increasing shear rate,suggesting a shear-thinning behavior. The viscosity of all HSP samples was higher than that of NSP samples. NSP emulsion’s viscosity became more sensitive to shear force when rutin addition increased. SDS-PAGE electrophoresis showed all the protein subunits were present in the oil phase, indicating there was no significant selecting in replacing of SPI to rutin.To improve the stability of soy protein emulsions using rutin particles is the most important purpose of this study. We investigated the effects of rutin additive on the physical(storage stability, pH stability, ion stability, freeze-thaw stability, etc) and oxidative stability(fat oxidation) of emulsions stabilized by NSP and HSP. How rutin particles influence structural properties of mixed interfacial layers and stability of emulsion was discussed, and the underlying mechanism was proposed. The formation of NSP-rutin mixed interfacial layers resulted in evident improvement of physical and oxidative stability, except freeze-thaw stability. However, the stability of HSP emulsions showed no remarkable improvement due to lower interfacial accumulation and physical location of rutin. In case of HSP emulsion, protein aggregation led to heterogeneous and porous interfacial film. Rutin particles were believed to mainly adsorb outside protein layer, and could not contact with oil-water interface directly. Therefore, rutin could not play the role of stabilizing effectively in HSP emulsions.Considering the good stability of NSP-rutin emulsion, β-carotene delivery system was prepared based on NSP-rutin emulsion. The capacities of NSP-rutin emulsion on enhancing stability and controlled release of β-carotene were investigated. The results showed that the encapsulating of β-carotene in oil droplet improved significantly the storage stability, light and heat stability of β-carotene, and did not influence the forming of emulsion and oil droplet size. β-carotene could be released slowly in simulated gastrointestinal digestion. These findings would provide a new idea to the application of SPI-food particle complex emulsions.