Preparation and Functional Properties of Glycosylated Walnut Peptides with Different Molecular Masses

Three walnut peptides with different molecular masses were prepared from walnut meal protein by sequential enzymatic hydrolysis and ultrafiltration, and separately glycosylated with five sugars (glucose, sucrose, lactose, maltodextrin and dextran). The reaction conditions were optimized based on emulsifying properties and anti-lipid oxidation capacity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), Fourier transform infrared (FTIR) spectroscopy and circular dichroism (CD) spectroscopy were used to characterize the glycosylated peptides, their physicochemical properties were studied, and their emulsion stability was determined by laser confocal microscopy (LCM). The results showed that the optimal conditions for the glycosylation reaction between walnut peptide with molecular mass greater than 30 kDa and maltodextrin were as follows: ratio of walnut peptide to maltodextrin 1:2, reaction time 8 h, reaction temperature 80 ℃, and peptide concentration 15 mg/mL. SDS-PAGE confirmed the formation of a covalent complex between the walnut peptide and maltodextrin through glycosylation reaction. FTIR spectroscopy showed that sugar molecules were connected to walnut peptide molecules by covalent bonds. CD spectroscopy showed that the peptide’s structure changed. The proportion of α-helix increased from 4.85% to 6.43%, the proportion of antiparallelism increased from 30.16% to 35.91%, the proportion of parallelism increased from 3.28% to 4.33%, the proportion of β-turn decreased from 23.21% to 20.77%, and the proportion of random coil decreased from 33.59% to 31.22%. The glycosylation modification did not improve the Fe²⁺ chelating capacity, but increased the solubility, emulsifying capacity to (83.24 ± 1.64) m²/g, emulsion stability to (218.49 ± 3.55) min, water absorption capacity to (3.92 ± 0.36) g/g, oil absorption capacity to (3.17 ± 0.24) g/g, surface hydrophobicity to 251.05 ± 6.91, 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging capacity to (86.74 ± 2.14)%, and anti-lipid oxidation capacity to (62.17 ± 3.51)%. The results of LCM showed that the encapsulation capacity of walnut peptides for walnut oil was enhanced after glycosylation, suggesting a potential application for improving the stability of emulsions. The glycosylation modification of walnut peptides provides a new idea for the processing and utilization of walnut meal by-products and provides a reference for the application of glycosylation modification in food development.