Formation, structure and rheological properties of soy protein gels

Keywords: soy protein isolate, glycinin,β-conglycinin, heat denaturation, gelation, network structure, rheology, permeability measurements, microscopy, pH, ionic strength, emulsified oil dropletsThis study was performed to understand the factors determining heat-induced formation and properties of soy protein gels the relations between gel properties and network structure in order to support application of soy proteins in food products. Three soy protein preparations were used: soy protein isolate, which is a mixture of soy proteins, purified glycinin and aβ-conglycinin rich fraction. Glycinin andβ-conglycinin are the main proteins in soybeans. Protein denaturation was studied by differential scanning calorimetry. Rheological properties of the soy protein gels were investigated in small and large deformation tests. Information on coarseness of the network structure was obtained by permeability measurements and confocal scanning laser microscopy.Heat denaturation proved to be a prerequisite for gel formation at all conditions of pH and ionic strength studied.β-Conglycinin gels were formed at temperatures of about 55-70°C and glycinin gels at about 70-95°C. Soy protein isolate gels were formed on heat denaturation ofβ-conglycinin at pH lower than 6 and on heat denaturation of glycinin at pH higher than 6. On further heating at 90 or 95°C, gels became stiffer, which was explained by further incorporation of protein in the network and, at pH 7 and 7.6, by the occurrence of rearrangements in the network structure. Gel stiffening on cooling was thermoreversibel and did not involve covalent bond formation and rearrangements.Gel properties like stiffness, fracture behaviour and water holding capacity strongly depend on conditions during gel formation, such as pH, salt concentration, protein concentration, heating conditions and addition of oil droplets. Also the type of protein, glycinin orβ-conglycinin, and their mixing ratio affect gel properties. The differences in gel properties could, for a large part, be related to differences in the network structure of the gels. The most important structural characteristics are pore size, thickness and curvature of the strands. Another factor determining gel properties is the amount of protein incorporated in the network. At pH > 5, less protein (mainly acidic polypeptides) participated in network formation than at lower pH values.