Preparation, structural characterisation, absorption and calcium transport studies of walnut peptide calcium chelate

In this study, a walnut peptide (WP) with calcium-binding capacity was prepared using a combination of alkalase and neutrase. The conditions for the preparation of walnut peptide calcium chelate (WP-Ca) were optimised (a peptide/calcium chloride ratio of 1 : 4 for 70 min at 50 °C and pH 9.5). Fractionation <i>via</i> ultrafiltration showed that peptides with a size <1 kDa demonstrated the highest calcium binding capacity at 106.4 mg g^-1. Scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, zeta potential and other analyses were performed to characterize WP-Ca. The combined results indicate that calcium binds by interacting with the carboxyl oxygen, hydroxyl oxygen and amino nitrogen of walnut peptides to form WP-Ca. The chelate showed good gastrointestinal stability. Furthermore, using the Caco-2 cell monolayer model, WP-Ca was shown to significantly increase calcium bioavailability and effectively reverse the inhibitory effects of dietary factors (phytates and phosphates) on calcium absorption. The results provide a scientific basis for developing novel calcium supplements and high-value walnut utilisation.