Effect of pH-shifting treatment on the structural and functional properties of soybean protein isolate and its interactions with (–)-epigallocatechin-3-gallate

Polyphenols are readily affected by environmental factors, such as heat and light, which cause them to degrade; however, the functional properties of polyphenols can be protected by complexing them with proteins. In this study, we investigate the effects of acid and alkali treatments on the functional properties of soybean protein isolate (SPI) and interactions between the treated protein and (−)-epigallocatechin-3-gallate (EGCG). The results show that alkali treatment (pH 12–7) affects the structure and functional performance of the protein more significantly than acid treatment (pH 2–7) because the protein structure unfolds, resulting in improved solubility, surface hydrophobicity, and emulsification. In addition, because alkali treatment can change the structure of a protein more than acid treatment, its structure loosens. The interactions between alkali-treated proteins and EGCG were also found to be stronger than those of acid-treated proteins and the control protein, and EGCG binds to a protein through van der Waals forces and hydrophobic interactions; these interactions protect EGCG from degradation, ensuring that its antioxidant properties are maintained and its bioavailability rate is improved.