Structural and emulsifying properties of protein recovered from walnut meal as affected by alkaline extraction pH

This study investigated the regulatory effects of alkaline extraction pH (8.0-11.0) on the composition, structure, and emulsifying properties of walnut protein (WP). As the pH increased from 8 to 9-11, the protein recovery yield increased sharply from 55.46% to 79.80%-85.26% attributed to the increased solubility of glutelin and globulin. In contrast, the proportion of albumin was higher at pH 8 than at pH 9-11. Given that albumin possesses inherently weak emulsifying capacity, this compositional difference provided a basis for the pH-dependent variation in emulsifying properties. Structural analysis revealed that with increasing pH, the WP structure became looser and underwent unfolding. Surface hydrophobicity increased continuously from pH 8 to pH 10, peaked at pH 10, and then slightly decreased at pH 11 due to enhanced intermolecular interactions. Meanwhile, the absolute ζ-potential and conformational flexibility reached their maximum at pH 11. As the extraction pH increased, the ability of WP to reduce interfacial tension was enhanced. The droplet size (d_4,3) of the stabilized emulsions decreased from 68.0 μm (pH 8-E) to 5.53 μm (pH 11-E), and the flocculation index dropped from 833% to 189%. In summary, over the pH range of 8 to 11, synergistic changes in the composition and structure of WP drove the improvement in emulsifying properties.