Abstract: The study investigated the effect of different extrusion parameters: temperature, material moisture of raw material, and screw speed on the conformation of soybean protein. After being extruded, soy protein isolate (SPI) was measured for protein solubility, zeta potential, particle size, protein subunits and protein secondary structure. Additionally, the formation mechanism of fibrous soybean protein was uncovered. The results showed that with an increase in extrusion temperature, the natural structure of SPI was destroyed, the internal groups were exposed, and the content of disulfide bonds increased, promoting the formation of protein aggregates. Too high or too low temperature was not conducive to the formation and stability of protein aggregates. At too low or too high raw material moisture content, a relatively complete extrudate was difficult to form at the extrusion mouth, while at raw material moisture contents of 20%–22%, the extrudate was in the best state due to the protective effect of water, and a uniform SPI aggregate was formed. Low screw speed (below 130 r/min) could lead to insufficient mechanical energy input, so that the protein was not completely denatured and depolymerized. While at high screw speed (above 140 r/min), the effect of high shear force promotes the destruction of the original natural structure of proteins, resulting in the formation of large protein aggregates. In summary, under appropriate extrusion conditions, the optimal extrudate quality can be obtained, while extreme extrusion conditions make extrusion molding difficult or result in nonuniform texture. This study can provide a theoretical basis for optimizing the quality of fibrous soybean protein.

Key words: twin-screw extrusion; soy protein; extrusion parameters; structure