Preparation, structural changes and functional properties of the covalent complexes of almond protein and phloretin

Proteins and polyphenols exhibit distinct biological activities and functional properties. A comprehensive investigation into the formation mechanisms, structures, and functional properties of protein-polyphenol complexes will deepen our understanding of their interactions and establish a theoretical foundation and technical support for development of novel functional foods and pharmaceutical products. The almond protein-phloretin (AP-PHL) covalent complex was synthesized through the covalent binding of hydroxyl radicals to phloretin (PHL), utilizing almond protein (AP) as the raw material. Ultraviolet absorption spectroscopy (UV), fluorescence spectroscopy (FS), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (RS), transmission electron microscopy (TEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA) were employed to characterize the AP-PHL complex. Additionally, its properties, including emulsification characteristics and antioxidant activity, were analyzed. The results indicated that the hydrophobic groups in hydroxyl radical-treated AP relocated to a hydrophilic environment and interacted with PHL, thereby forming a stable complex. TEM results indicated that AP formed clusters within the central region of PHL. Additionally, UV and FS analyses revealed that the maximum absorption wavelength of AP-PHL shifted from 287.5 nm to 258 nm and 280 nm, respectively. As the PHL concentration increased, the fluorescence intensity gradually decreased, accompanied by a slight redshift. FTIR and RS analyses revealed that modifications in functional groups (e.g., -CH₃, =CH₂, CO, CC, CO) were implicated in the interaction between AP and PHL. Such structural modifications, along with other changes, enhanced the functional properties of AP-PHL, including thermal stability, water solubility, and emulsification, thereby indicating its substantial potential for applications in food and pharmaceuticals.