Non-covalent interaction of complex plant protein and betanin: Mechanism of improving thermal stability of betanin

Betanin (BN) is a promising purple-red natural pigment, but its application is hindered by thermal instability. The complex plant protein (RP-SPI) constructed by rice protein (RP) and soy protein isolate (SPI) was used as raw material to investigate the thermal protection effect and protective mechanism of RP-SPI on BN. The results showed that RP-SPI significantly improved the thermal stability of BN, and the retention rate of BN increased from 5.63% to 48.08% when heated at 80 °C for 60 min. Subsequently, the fluorescence spectroscopy, isothermal titration calorimetry (ITC), and molecular docking were utilized to investigate the interaction mechanism between RP-SPI and BN. The results indicated that the main interaction between RP-SPI and BN was hydrogen bond. The interaction changed the secondary structure and reduced surface hydrophobicity of the protein. Meanwhile, morphological observation showed that RP-SPI was a flake protein with a network structure, and a large amount of aggregation occurred after binding with BN, resulting in a significant increase in particle size. Taking together, the non-covalent interaction is the key to the thermal protection of RP-SPI on BN. Betanin bound to RP-SPI through hydrogen bonds and acted as a bridge connecting the surrounding proteins.