Extrusion Modification: Effect of Extrusion on the Functional Properties and Structure of Rice Protein

Modification of rice protein by extrusion technology can broaden the range of processing and applications for food and feed raw materials. In this study, rice protein was extruded at different screw speeds (100–250 rpm), extrusion temperatures (90–150 °C), and moisture contents (25–40%). Compared with an unextruded protein, the functional properties and structural properties of textured rice protein were evaluated. The results showed that, after extrusion, the solubility of protein was improved, by up to 19.76%, which was 45.23% higher than pre-extrusion; the water holding capacity of extruded rice protein was highest at 200 rpm, 130 °C, and 25%, which could be enhanced by 37.74%; the emulsion stability was enhanced by 152.82% at 200 rpm, 130 °C, and 35%. Under extrusion, the content of sulfhydryl and disulfide bonds of rice protein decreased significantly; the hydrogen bond content increased, and the ionic bond content decreased; the hydrophobic effect decrease, except at 200 rpm, 130 °C, and 40%. The microstructure changed significantly after extrusion, producing protein aggregates with a tight structure. No new characteristic peaks appeared after extrusion, but transformation occurred between the components of the secondary structure: β-sheet and β-turn angles to an α-helix structure toward the transformation, but β-sheet was still the main component. As a safe and efficient modification method, extrusion cooking can effectively improve the functional properties of rice protein to enrich the application of rice protein resources.