Ternary anthocyanin-iron ion-pectin interactions: Structure-dependent formation and spectral properties as potential Tools for more stable blue colorants

Anthocyanins, while known for their color and suggested health benefits, face limited use as natural food colorants due to poor stability. The focus of the presented work is to study ternary pectin-metal ion-anthocyanin interactions (PIrA) as a function of anthocyanin structure and the impact of such interaction on the spectral properties, as a potential avenue for utilization as colorants. Apple pectin, an anionic polysaccharide, was enriched with iron ions to form molecular anchoring sites for anthocyanins. The complexation of selected anthocyanins, differing in the number of hydroxyls and size of the conjugated sugar, with the iron-enriched pectin was studied in single and multi-component models in comparison to systems without iron enrichment or enriched simultaneously with iron and anthocyanins. Anthocyanins with three hydroxyl groups on their B-ring, as delphinidin-3-glucoside (D3G) and delphinidin-3-rutinoside (D3R), resulted in higher wavelength of the maximal absorbance (λ_max) upon PIrA complexation compared to anthocyanins with two hydroxyl groups on their B-ring, as cyanidin-3-glucoside (C3G) and cyanidin-3-rutinoside (C3R) (578, 574, 560, 566 for D3R, D3G, C3R and C3G, respectively). The anthocyanins affinity to the iron-enriched pectin was also structure dependent. Dissociation constants (K_d) presented significantly higher values for cyanidins than delphinidins (0.9, 0.5, 27.0, and 10.8 μM for D3R, D3G, C3R, and C3G, respectively). Finally, PIrA complexation improved anthocyanin stability. Empiric color degradation rates (K [1/day]) for PIrA complexes were also structure dependent (−2.5 ± 0.1*10⁻², −4.7 ± 0.4*10⁻², −4.8 ± 0.5*10⁻² for D3G, D3R, and blackcurrant extract, respectively). PIrA complexation was found to be structure-dependent and to improve blue color stability.