| Soy protein is one of the vegetable protein resource with high yield and low price. Over the past few decades, there are a lot of researches on improving functional properties of soy protein through structural modification, such as emulsifying properties, foaming properties and gelation. Even though the functional properties of soy protein are improved after structural modification, there is no commercial soy protein products with high emulsifying properties, which could be comparable to large molecular emulsifier such as sodium caseinate. The objective of this study was to improve the emulsifying properties of soy protein through hydrolysis and glycation. Furthermore, through the study on the rheological properties of the protein, the mechanism of interactions between emulsifiers and structural characteristics of emulsifiers at the interface was further clarified, which could provide a theoretical basis and guidance for the better development and application of soybean protein as surfactant and colloidal stabilizers.Firstly, the effects of the combination of enzyme hydrolysis and glycosylation on the emulsifying properties and interfacial properties of soybean protein were studied. The results showed that the large molecular weight fractions(Mw > 30 kDa) of soybean peptides exhibited better emulsifying properties while the emulsifying properties of soybean peptides with small molecular weight were poor. Results of soy peptides glycation showed that, with the increase of the molecular weight of the sugar, the emulsifying stability of the soybean peptide increased gradually. The addition of sugars could reduce the surface hydrophobicity and the net charge of soybean peptide, reducing the repulsion between the chains, increasing the interfacial adsorption amount. The surface activity measurement revealed that the soy peptide–dextran conjugates were closely packed and that each molecule occupied a small area of the interface. It was further confirmed that the soy peptide–dextran conjugates formed a thick adsorbed layer at the oil–water interface, as observed in the confocal laser scanning micrographs. The interfacial layer of soy peptides was rheologically complex with broad linear viscoelastic region and strong elastic modulus, and the soy peptide–dextran conjugates might form multilayer adsorption at the interface. This study suggested that the improved surface properties of the soy peptide–dextran conjugates were a result of the strong membrane formed by the closely packed molecular and multilayer adsorption at the interface, which provided steric hindrance to flocculation.Based on the purpose of preserving the large molecular weight of soy protein hydrolysates, effects of selectively hydrolysis(reducing the 11 S and keeping the 7S) on emulsifying and interfacial properties of soy protein were investigated. The emulsion stabilizing mechanism and interfacial properties of limited hydrolysis products of soy protein, selective hydrolysis products, 7S and SPI were also studied. The results showed that the emulsifying properties of soy protein were greatly improved through selective hydrolysis, which was comparable to 7S and sodium caseinate. Proteins adsorb at the oil/water interface and generate stable emulsion as efficient emulsifiers in two ways. First, the structure of the protein is flexible(e.g., sodium caseinate), the interfacial shear modulus of the protein is small, but with large adsorption amount, adsorbed with their long axis orientated perpendicular to the interface, allowing the interfacial film to sustain larger deformations without structural fracture. Second, globular proteins have a relatively low molecular weight(e.g., 7S), allowing them to form high viscoelastic interfacial films to protect the oil droplets against flocculation and coalescence. The selective hydrolysis product, RG, contains both globular and flexible structures, which could generate a high viscoelastic interfacial film, and also with large adsorption amounts, facilitating the formation of stable emulsions.In order to investigate whether the emulsifying properties of 7S and RG could be further improved, the effects of covalent and non-covalent binding of dextran on the emulsifying properties of 7S and reducing-glycinin(RG) protein were studied. The results showed that no matter whether bounded with dextran, the particle size of RG increased, the surface net charge was reduced, the particle size of emulsion was small, but under the condition of acidic and high salt concentration, the emulsion formed by RG was slightly unstable than that of sodium caseinate. Emulsions prepared by 7S, regardless of whether bonded with dextran, were stable than that of sodium caseinate under acidic conditions. However, in high salt concentration, emulsions preparation by sodium caseinate were more stable than that of the 7S-dextran conjugates.In order to improve the stability of 7S near the isoelectric point, soluble soy polysaccharides(SSPS) and gum arabic(GA) was chosen to complex with 7S through electrostatic interaction. The effects of soluble soy polysaccharides(SSPS) and gum arabic(GA) on the interfacial shear rheological properties of ?-conglycinin(7S) at both the air/water and oil/water interfaces were investigated. The results showed that the adsorption behavior at the two interfaces were quite different. For 7S–SSPS, the adsorption behavior was mainly dominated by 7S, with a lower adsorption rate than that for 7S alone at the air/water interface. Conversely, the adsorption behavior was mainly dominated by SSPS at low adsorption times but was later dominated by 7S adsorption at the oil/water interface. Emulsion stability measurement showed that the 7Sbased emulsion was stabilized against flocculation by SSPS even under acidic conditions, which might be attributed to the bounded SSPS molecule stabilized the oil droplets effectively by steric stabilization. Addition of GA did not improve emulsion stability under acidic conditions because GA dominated the adsorption layer and did not provide steric stabilization at the oil/water interface. Surprisingly, with or without polysaccharides, 7S could form an interfacial film with a high elastic modulus under all conditions, even near the isoelectric point, which indicated that flocculation might be mainly due to the electrostatic attraction between the emulsion droplets.At last, a preliminary study on the application of 7S and RG in oil powder was carried out. Hydrogenated coconut oil was used as the core material, and RG as the emulsifier. When the protein content reached 6% or higher than 6%, the emulsion before spray drying, the oil powder and emulsion after dissolving, showed small particle size, uniform distribution characteristics, and the o emulsion after dissolving was stable after 6 h storage at room temperature, which indicated that the RG has good encapsulation properties. The addition of monoglyceride had competitive effects with RG, which causing a significant decrease in the elastic modulus of the interfacial film. 7S and monoglyceride together could form emulsion with small average droplet size around 300 nm. However, most of the oil powder of 7S produced by spray drying appeared to be bonded, which might be related to the partial denaturation of 7S in the spray drying process. This study would provide a new idea for the preparation and application of high emulsifying soy protein products. |
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