Comparative analysis of walnut (Juglans sigillata) protein quality from defatted meal: Traditional versus green oil extraction technologies

Walnut meal is a protein-rich by-product of oil extraction. The extraction method may influence the quality of the protein within walnut meal and determine its potential application value. However, comparative studies on the characteristics of walnut meal protein under different oil extraction methods, particularly emerging green methods, remain scarce. This study analyzed the structural, functional and digestive properties of walnut protein (WP) isolated from meals defatted using three methods: pressing extraction (PE, traditional), supercritical fluid extraction (SFE, green), and aqueous extraction (AE, green). PE induced extensive structural unfolding and disruption to intermolecular forces, yielding WP with the highest solubility and superior emulsifying properties. In contrast, SFE preserved the native, compact protein structure, yielding WP with superior water-holding capacity and the strongest gelling ability. AE-WP exhibited intermediate structural characteristics and the highest in vitro protein digestibility, indicating that moderate denaturation benefited protein digestion. In terms of peptide release during protein digestion, the looser structures of SFE-WP and AE-WP facilitated the release of low-molecular-weight and bioactive peptides. Furthermore, strong correlations were found between structural characteristics (e.g., surface hydrophobicity, secondary structure, aggregation state and intermolecular forces) and functional/digestive performance, providing a scientific structure-property relationship for the targeted use of proteins in walnut meals.