Correlating structure and emulsification of soybean protein isolate: Synergism between low-pH-shifting treatment and ultrasonication improves emulsifying properties

The functional properties of a protein (particularly emulsification) are closely related to its conformation and structure. Herein, we investigated the effect of low-pH-shifting treatment (pH 7 → 2 → 7), either individually or in combination with ultrasonication (300 W, 10 min), on the structural and emulsifying properties of soy protein isolate (SPI). Compared with individual treatment, combined treatment more significantly improved the emulsifying properties of SPI by changing its spatial conformation. The two treatment methods were shown to exhibit both synergistic and sequence-dependent effects on the emulsifying properties of SPI. In addition, we analyzed correlations between the emulsification and structure using a mathematical model to directly and explicitly elucidate structure–emulsification relationships. Combined treatment alters protein structure in two ways: low-pH-shifting treatment exposes the hydrophobic groups present in the interior of the SPI, and ultrasonication breaks disulfide bonds, which increases β-sheet content, surface hydrophobicity, and sulfhydryl content. These three factors were found to be strongly positively correlated (> 0.9), and highly positively correlated with emulsifying properties (> 0.7). The SPI structure was found to affect its emulsifying properties, thereby providing a theoretical understanding of the emulsification mechanism.