Improvement of foaming and emulsifying properties of SPI via weak-base synchronized membrane separation: Insight from tuning the interfacial flexibility

Preparation of soy proteins with high functionalities is significant for enhancing their utilization. Here, we propose a green and mild weak-base synchronous membrane separation (MS) technique to enrich soy protein isolate (SPI), with two traditional extraction methods, alkali extraction-isoelectric point precipitation (AE) and salt extraction (SE), as the control. The interfacial properties of SPI were investigated by Cryo-SEM, interfacial rheology (dilatational and shear), and Langmuir-Blodgett films combined with AFM. Results showed that MS extraction significantly reduced the disulfide bond content of SPI, which induced partial depolymerization of the protein and exposed the internal hydrophobic groups, and thus leading to the reduction of particle size as well as the increase of surface hydrophobicity (H0). These structural properties conferred MS-SPI with a faster interfacial adsorption rate, followed by easier unfolding and rearrangement after anchoring to the interface. Finally, the flexible and denser interfacial layers with high mechanical properties were formed through strong in-plane interactions among SPI, which could effectively prevent the rupture of oil droplets and air bubbles. Therefore, the emulsification stability and foamability of MS-SPI could be maintained for up to ∼252 min and ∼232%, which was at least 2.4 times and 1.2 times than that of AE-SPI and SE-SPI, respectively. These findings demonstrated that MS extraction was an effective approach for broadening the applications of soy proteins in the food ingredient field.