Storage stability and multi-spectroscopy analysis of the ternary complex induced by mulberry anthocyanin extract interacting with whey protein isolate and rutin under acidic conditions

In the present study, we have investigated the relevant mechanisms underlying the stabilizing effects of whey protein isolate (WPI) and rutin (Ru) on the mulberry anthocyanin extract (MAE) solution at pH 3.6 during storage. The results demonstrated that there was no pronounced difference in color between the MAE solutions when adding WPI and Ru separately and together. However, the combined use of WPI and Ru reduced the total anthocyanin degradation rate by 16.2% and 18.3% compared to WPI and Ru used individually, respectively, after 9 days of storage at 40 °C. Fluorescence, UV–visible, Fourier transform infrared, and circular dichroism spectroscopies, as well as particle size and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, indicated that the formation of binary and ternary complexes involving β-lactoglobulin (β-LG), cyanidin-3-O-glucoside (C3G), and Ru. The primary interaction between β-LG and C3G was electrostatic interaction, with an enthalpy change (ΔH) of −25.02 kJ mol−1 and an entropy change (ΔS) of 7.89 J mol−1 K−1 at 298 K, whereas the interactions between β-LG and Ru involved hydrogen bonding and van der Waals force, with ΔH of −124.80 kJ mol−1 and ΔS of −291.72 J mol−1 K−1 at 298 K. Furthermore, a looser, more open, and disordered structure and larger particle size were observed in protein after the addition of C3G/Ru. The presence of Ru in the ternary complex favored the development of these characteristics. Overall, to protect anthocyanins, β-LG and Ru could exert complexation and copigmentation effects on C3G, respectively. The findings of this study provide a foundation for employing ternary complexes to increase the storage stability of anthocyanins.