Control Of Soy Protein Thermal Aggregation Behavior And Structural Characterization Of Soy Protein Aggregates

Heat treatment is commonly used in soy protein processing. The structural and functionalproperties of the products are determined by the thermal aggregation behavior of soy protein.In this work, the thermal aggregation behaviors of β-conglycinin (7S) and glycinin (11S) themain fractions in soy protein were studied, the structure of the aggregates formed duringheating was characterized, the relation between protein solubility and protein aggregation wasdiscussed, and the soy protein aggregate particles which were able to carry bio-activemolecules were prepared. Main results are as follows:1) The thermal aggregation kinetics of soybean7S and11S were studied with size exclusionchromatography and low-angle light scattering (SEC-LALS), and the structure of the solubleand insoluble aggregates was analyzed by small-angle X-ray scattering (SAXS). TheLumry-Eyring nucleated polymerization (LENP) was used to elucidate the thermalaggregation behaviors of7S and11S. It was found that7S and11S possessed differentthermal aggregation behaviors. With limited aggregation,7S soluble aggregates had lesscompact structure grew via monomer addition. On the contrary, the aggregation of11S wasnot restricted. Condensation between the aggregates occurred in the thermal aggregation of11S. Significant increase of the particle size and density in the core turned the11S aggregatesinto insoluble materials.7S-11S complex was formed, when11S was heated with7S. In thiscase, the aggregation of11S was restricted and terminated at the polymerization stage in thepresence of7S. As a result, the solubility loss of11S caused by heating was recovered. Andhydrophobic interaction played an important role in the preferential interaction between7Sand11S.2) In order to reveal the mechanism that the thermal aggregation of soy protein could besuppress by artificial molecular chaperone, SDS and soybean11S were used to perform themodel test. It was found that SDS-11S complex was formed under the interaction between thehydrophobic groups on these two molecules at neutral pH. The exposed hydrophobic groupson the surface of11S were covered, and plenty of charged groups which could produce strongelectrostatic barriers were brought to the surface of11S. As a result, the11S particles werehard to approach to each other and the thermal aggregation could not occur. The aggregation tendency of protein was determined by the competition between the hydrophobic attractionand electrostatic repulsion. Shielding the exposed hydrophobic residues and employing morehydrophilic groups on the surface of the protein molecules is an effective strategy forimproving the solubility of soy protein during processing.3) The effect of physical treatment on the solubility of soy protein was studied. It was foundthat the solubility of basic polypeptides was greatly improved by hydrothermal cooking(HTC). It was suggested that higher pH of the dispersion, stronger steam pressure, longertreated time, less protein content and ionic strength of the dispersion were favorable forincreasing the solubility of basic polypeptides after HTC with alkaline conditions. Thestructure of insoluble aggregates unfolded under the effect of high temperature and high steampressure. As this effect was removed, the protein molecules rearranged under the guidance ofits primary structure and the surrounding conditions. Proper environment conditions were ableto decrease the reaggregation extent of these protein molecules and improve the solubilizationeffect of HTC.4) The soybean11S aggregate particles were prepared by heating11S dispersion at pH nearits pI. The mean particle size of these particles was157nm, and the particle size PdI was0.105. The yield of these particles was about80%. Hydrophobic interaction and hydrogenbonds were the main driving force for the formation of the11S aggregate particles. After driedand redispered, the particles maintained good dispersiveness and thermal stability underseveral pH conditions. In addition, these11S aggregate particles could weaken theprotein-based gel network.5) The soy protein aggregate particles were prepared by homogenizing the soy protein gelinduced by gluconic acid-δ-lactone (GDL). The particle size and colloidal stability weredetermined by the pH and ionic strength of the dispersion during homogenizing. Stabledispersed particles appeared when homogenizing was conducted at pH far from the pI of soyprotein. Microbial transglutaminase (MTGase) was entrapped in these particles. Whendispersed in the denatured soy protein isolates (SPI) dispersions, gels were formed with theseMTGase entrapped particles. It suggested that these particles could be used as a bio-activemolecule carrier in food processing.