Walnut is an important woody oil crop. Walnut meal left after oil extraction is rich in high-quality protein, but it is underutilized, causing a waste of resources. Bioactive peptides derived from walnut meal protein have the potential for food development and are a promising ingredient in functional foods owing to their diverse bioactivities such as antioxidant, cardiovascular protective and memory improving activities. The effective utilization of walnut meal protein, for example by developing bioactive peptides is of great significance to realize the comprehensive utilization of walnut meal and to improve the added value of products. This review summarizes previous studies on the preparation, purification, identification and biological activity of walnut peptides in order to provide a theoretical basis for the high-value utilization of walnut meal protein resources and in-depth studies of walnut protein peptides.

In this study, a mixture of ultrasound or microwave pretreated walnut meal protein peptides and CaCl₂ was used for the preparation of walnut peptide-calcium chelate. The effects of different pretreatments on the calcium chelating capacity, structural changes and stability of walnut meal protein peptides were analyzed. The results showed that compared with walnut meal protein peptide-calcium chelate (WPP-Ca), the chelation rates of ultrasound-pretreated walnut meal protein peptide-calcium chelate (UP-WPP-Ca) and microwave-pretreated walnut meal protein peptide-calcium chelate (MP-WPPCa) were enhanced, which indicated that ultrasound and microwave pretreatments improved the calcium-chelating capacity of the peptides effectively. Using ultraviolet-visible (UV-Vis) absorption spectroscopy and Fourier transform infrared (FTIR) spectroscopy, it was found that ultrasound and microwave pretreatments mainly affected the calcium ion binding sites such as amino groups, carbonyl groups, carboxyl groups, amide bonds and carboxylate groups of walnut meal protein peptides. The results of X-ray diffraction (XRD) showed that ultrasound and microwave treatments changed the molecular arrangement of walnut meal protein peptides, thereby making the structure of walnut peptide-calcium chelate more ordered. Fluorescence spectroscopy showed that ultrasonic and microwave treatments promoted the chelation between aromatic amino acids and calcium ions. In addition, UP-WPP-Ca and MP-WPP-Ca showed good stability toward different pH values, temperatures, and gastrointestinal digestion. In short, ultrasonic and microwave pretreatments can improve the calcium-chelating capacity and stability of walnut meal protein peptides, which is of guiding significance for the processing of walnut peptide-calcium chelate and the development of calcium supplements.