Discovery and characterization of novel anthocyanin-metal complex for blue food coloring

The Search for natural blue colorants to broaden the palette in the food industry remains an ongoing challenge. We hypothesized that anthocyanin-metal complex could be a promising option for blue food coloring. This study investigated the stability and color property of various anthocyanins to form complexes with metal ions, revealing that the stability of complexes is influenced by hydrophilicity, binding affinity, and steric hindrance effects. On this basis, a complex consisting of petunidin-3-O-(trans-p-coumaroyl) rutinoside-5-O glucoside (Pet3(cou)R5G) and Fe(III) with good stability and coloration properties was discovered. Structural characterization by Job's plot (3,1 stoichiometry), mass spectrometry ([M]²⁻ at m/z 1424.4604), Fourier transform infrared spectroscopy (FeO bond at 548 cm⁻¹) and circular dichroism spectroscopy confirmed octahedral coordination. Notably, this complex exhibited a blue hue distinct from brilliant blue FCF, and demonstrated superior stability in candy. Overall, our research advances anthocyanin-based blue colorant development by elucidating structure-stability relationships and practical applications.