Heat-induced Aggregation Of Soy Proteins And Its Mechaism

Heat treatments are important modification methods in industry, which affects the functionality of proteins greatly. Functionality shows close relationship with aggregation of proteins. Studies on thermal treatments under atmospheric pressure did not reflect the actual fact in food production system, but hydrothermal cooking, which associate high temperature with pressure (121℃, 0.1MPa) in lab-scale could stimulate the fact in production to some extent. Additionally, the heat-induced aggregation of soy proteins is almostly focused on the level of glycinin andβ-conglycinin. However, there is less literature concerning about heat-induced aggregation of soybean protein influenced by hydrothermal cooking or at subunit level. The contents contain four parts, functional properties and heat-induced aggregation of soy protein affected by thermal treatment/hydrothermal cooking, structure and mechanism of the aggregates induced by thermal treatment/hydrothermal cooking, isolation and purification of soybean protein subunits, heat-induced aggregation of single-subunit and composite-subunits system and its interaction model. Main results are as follows:1. Functional properties and heat-induced aggregation of soy protein in different concentrations by thermal treatment/hydrothermal cooking were compared systemically. Thermal treatment exhibited higher water holding capacity, apparent viscosity and gelling-forming ability,while hydrothermal cooking showed higher solubility (80%-95%) and emulsifying ability index(200-275 m2/g). For hydrothermal cooking, more hydrophobic bonds were involved and induced less hydrophobicity.Two kinds of aggregates were different in particle size, zeta-potential and involvement of subunits. Most of aggregates induced by thermal treatment could be broken up by 2-mercaptoethanol while it was not the case for hydrothermal cooking.2. The subunit profile, structure property and inter-molecular interaction of soluble aggregates induced by thermal treatment/hydrothermal cooking were studied systemically, and mechanisms of the aggregation were also deduced. The results of subunit profile showed: Thermal treatment induced more aggregates with high density/high sedimentation coefficient while hydrothermal cooking induced more aggregates with high density/low sedimentation. The results of structure showed: thermal treatment make the microenvironment of phenylalanine more exposed to water, while hydrothermal cooking makes that of tryptophan and tyrosine more exposed to water, which resulted in the former of higher surface hydrophobicity.β-sheets decreased and random coiling content andα-helix cotent increased concomitantly both for two treatment and hydrothermal cooking made H-bonds of inter-molecularβ-sheets strengthen. The results of intermolecular interaction showed: during thermal treatment, aggregates were formed and disulfide bonds were formed resulting from oxidation of -SH simultaneously. As for hydrothermal cooking, all disulfide bonds were disrupted and exposed hydrophobic group aggregates firstly and then disulfide bonds were formed resulting from -SS-/-SH exchange reaction subsequently during the discharge of pressure, buried in the interior of aggregates.3. Isolation and purification of soybean protein subunits were investigated systemically. The physicochemical and functional properties of subunits were also studied. Based on former researchers? work, a new chromatographic isolation method which can obtain large amount of glycinin subunit had been developed, in which DEAE-Sepharose fast flow column chromatograph were adopted. Three constitute subunits ofβ-conglycinin were isolated firstly by combining ion exchange chromatography with immobilized-metal affinity chromatography (IMAC) in relatively large quantity. The contents of acidic amino acids in the subunits were consistent with their eluting order in DEAE-chromatography.α'αexhibited highest emulsifying activity index under neutral or alkali pH, the foamability and foam stability ofβwere better thanα'α.4. The heat-induced aggregation and raleted factors of soy protein subunits were investigated systemically. The thermal stability of the subunits were ranked as follows:α'α/ acidic subunits>β>basic subunits. The morphology of heat-induced aggregates was much different from each other. Zeta-potential of the subunits were almostly stable during the heating process, the exposed degree of hydrophobic groups in different subunits are correlated closely with the heating temperature and time which created different balance between electrostatic repulsion and hydrophobic interaction, and resulted in different aggregation behavior. The heat-induced aggregation of acidic subunit andα'αinfluenced by ionic strength is also in accordance with the principle mentioned above.5. The heat-induced aggregation ofα,α',βwith basic subunits was studied systemically, and their interaction model were also proposed.αandα' showed higher ability in inhibiting aggregation on basic subunits thanβ.βwas easy to form soluble aggregate when concentration ofβ-conglycinin was high. On the contrary, it tended to form insoluble aggregate when concentration ofβ-conglycinin was low. The common ground of the interaction betweenαα'/βwith basic subunit were as follows:αα'/βattracted basic subunits by electrostatic interactions before thermal treatment. Composites composed of basic subunits andαα'/βwere formed through hydrophobic interaction after thermal treatment, and the electrostatic repulsion among composites would inhibit the increasing volume of aggregates.The differece betweenαα'-basic subunits composites andβ-basic subunits composites were as follows:βformed more hydrophobic bonds with basic subunits thanαα', which resulted in more surface hydrophobicity ofαα'-basic subunits composites.More minus charges and the flexible and hydrophilic peptide chains (extending area ofαα') distributing in the surface ofαα'-basic subunit composites can paly a more important role in inhibiting the hydrophocobity-inducd aggregation than the minus charges in the surface ofβ-basic subunit composites.