Improvement of heat-induced nanofibrils formation of soy protein isolate through NaCl and microwave

A novel method involving microwave heating (MH) at 85 °C can be applied to induce the self-assembly of soy protein isolate (SPI) into SPI-nanofibrils at low pH and in the presence of NaCl. The effect of heating type (MH and conventional heating (CH)), heating times (0, 30, 60, and 90 min), and NaCl (0 and 160 mM) on the formation and physicochemical properties of SPI-nanofibrils at pH 2.0 and 85 °C were investigated. The SPI-nanofibrils were characterized through thioflavin T (Th T) fluorescence, intrinsic fluorescence emission spectroscopy, atomic force microscopy (AFM), transmission electron microscopy (TEM), surface charge density, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and Fourier transform infrared spectrometer (FTIR). The sample treated with MH for 30 min in the presence of NaCl had the highest potential to form SPI-nanofibrils as evidenced by Th T. The AFM (998 nm) and TEM (1573 nm) results demonstrated that the longest SPI-nanofibrils were formed in the sample treated with MH for 60 min in the presence of NaCl. MH and NaCl procedures had a significant effect on the level of β-sheet and led to increasing its level (35.3%). The results of SDS-PAGE indicated that progressive polypeptide hydrolysis occurred upon heating, and the polypeptide hydrolysis for the samples treated with MH was much more severe than the samples treated with CH. From the surface charge density results, MH and NaCl led to decreasing electrostatic repulsion. Hence, these phenomena might increase the SPI-nanofibrils level by the electrostatic interaction. Finally, the data suggest that MH and NaCl not only can enhance the potential for SPI-nanofibrils formation but also increase the length of SPI-nanofibrils.