Effects of soluble aggregates sizes on rheological properties of soybean protein isolate under high temperature

The rheological properties of five concentrated soluble soybean protein isolates (SPI, 600 g/kg) with different average particle sizes, sulfhydryl content, and surface hydrophobicity were investigated at two different temperatures, i.e., 60 °C and 140 °C. The dynamic rheological results at 60 °C exhibited protein aggregation as the most significant factor affecting the storage and loss modulus. Time-sweep test at a high temperature (140 °C) and the temperature sweep test (30 °C–150 °C) exhibited that the SPI aggregates with large size need more energy to enter the flow state, and the protein interactions among these aggregates may also be hindered in the thermo-mechanical process. Furthermore, in order to exploring the protein interactions, N-Ethylmaleimide (NEM), sodium dodecyl sulfate (SDS), or urea were added to five soluble SPI samples during time-sweep experiments at 140 °C. The large particle sizes of pre-aggregated soy proteins may be an obstacle to further interactions (disulfide bonds, hydrophobic interactions, and hydrogen bonds) between protein molecules in the thermo-mechanical process. Proteins easily formed thermally irreversible aggregates via hydrophobic interactions or hydrogen bonds when disulfide bonds were hindered, and hydrogen bonds were indispensable forces for maintaining the viscoelasticity of the soy protein system during the extrusion-like condition (140 °C, 80% strain, 10 Hz frequency). The reaction behavior of soy protein under extrusion-like conditions can be used to understand the protein structural changes during the extrusion processes and produce consumer-popular meat analogs.