Evaluating the role of metal ions in the bathochromic and hyperchromic responses of cyanidin derivatives in acidic and alkaline pH

• Trivalent metal ions caused significant changes on anthocyanin spectral characteristics.
• Anthocyanin response depended on structure, metal electron organization, and pH.
• Largest bathochromic shifts resulted from Fe3+ ≈ Ga3+ > Al3+ > Cr3+ >> Mg2+.
• As pH increased, less metal ion was needed to produce a response.
• Bathochromic and hyperchromic shifts were greatest in pH 6 and 5, respectively.

In many food products, colorants derived from natural sources are increasingly popular due to consumer demand. Anthocyanins are one class of versatile and abundant naturally occurring chromophores that produce different hues in nature, especially with metal ions and other copigments assisting. The effects of chelation of metal ions (Mg2+, Al3+, Cr3+, Fe3+, and Ga3+) in factorial excesses to anthocyanin concentration (0–500×) on the spectral characteristics (380–700 nm) of cyanidin and acylated cyanidin derivatives were evaluated to better understand the color evolution of anthocyanin-metal chelates in pH 3–8. In all pH, anthocyanins exhibited bathochromic and hyperchromic shifts. Largest bathochromic shifts most often occurred in pH 6; while largest hyperchromic shifts occurred in pH 5. Divalent Mg2+ showed no observable effect on anthocyanin color while trivalent metal ions caused bathochromic shifts and hue changes. Generally, bathochromic shifts on anthocyanins were greatest with more electron rich metal ions (Fe3+ ≈ Ga3+ > Al3+ > Cr3+).