Anthocyanins with catechol (cyanidin) or pyrogallol (delphinidin) moieties on the B-ring are known to chelate metals, resulting in bluing effects, mainly at pH ≤ 6. Metal interaction with petunidin, an O-methylated anthocyanidin, has not been well documented. In this study, we investigated metal chelation of petunidin derivatives in a wide pH range and its effects on color and stability. Purple potato and black goji extracts containing >80% acylated petunidin derivatives (25 µM) were combined with Al3+ or Fe3+ at 0 µM to 1500 µM in buffers of pH 3-10. Small metal ion concentrations triggered bathochromic shifts (up to ~80nm) at an alkaline pH, resulting in vivid blue hues (hab 200°-310°). Fe3+ caused a larger bathochromic shift than Al3+, producing green colors at pH 8-9. Generally, metal ions increased the color stability and half-life of petunidin derivatives in a dose-dependent manner, particularly at pH 8. Petunidin derivative metal chelates produced a wide range of colors with enhanced stability.
Keywords: anthocyanin; black goji (Lycium ruthenicum Murr.); metal chelation; natural colorant; purple potato (Solanum tuberosum L. subsp. andigenum).