Extruded soy protein as a novel emulsifier: Structure, interfacial activity and emulsifying property

Native soy protein has poor emulsifying property compared to relatively flexible proteins. In order to improve soy protein surface activity, protein was partially denatured under controlled heating and shearing condition to facilitate the formation of oil-water emulsions. For this purpose different feed moisture contents (18 and 25%) and extrusion temperatures (110, 130 and 160 °C) were used and their effect on structure and emulsifying properties of soy protein concentrate (SPC) were investigated. Results showed that extrusion treatment reduced the surface hydrophobicity (H0) and solubility of SPC. Protein solubility of extruded SPC increased with increasing the extrusion temperature. SDS-page represented the formation of polymer macromolecules with very large molecular weight during the extrusion process. The extruded protein had lower α-helix content compared to the native SPC. The proportion of β-turn structure for extruded SPC increased after the extrusion cooking. Results showed that protein aggregates were formed during the extrusion cooking. However, the mechanical forces during homogenization process disrupted the heat-induced aggregates. Disruption of aggregates during emulsification enabled SPC to adsorb rapidly at the oil/water interface. The proportion of random coil in extruded SPC structure substantially increased after the homogenization which improved the emulsifying activity (EAI) and stability indices (ESI) of SPC. Addition of extruded SPC significantly increased the surface charge of oil droplets during homogenization. Emulsions fabricated using extruded SPC at 25% feed moisture had smaller particle sizes than those extruded at 18% feed moisture content and consequently more emulsion stability was observed. The microstructural observations also showed that the oil droplets of emulsions prepared with native and extruded SPC had spherical shape. These results suggest that extruded SPC at 25% feed moisture and 110 °C can be considered as an effective emulsifier for food and other applications.