Examination of molecular mechanism for the enhanced thermal stability of anthocyanins by metal cations and polysaccharides

Anthocyanins exhibit colour variation over wide pH range but the colour stability is relatively low at the physiological pH. To improve the stability of anthocyanins in neutral to weakly acidic pH region, effects of metal cations and polysaccharides on the colour stability of cyanidin-3-glucoside (C3G) were examined by ultraviolet-visible and resonance Raman spectroscopies. C3G was thermally stabilized by the addition of Fe3+ but formed aggregation. However, further addition of anionic polysaccharides enhanced the thermal stability of C3G without aggregation. Similar stabilisation was confirmed for delphinidin-3-glucoside (D3G) but not for pelargonidin-3-glucoside. The stability of anthocyanins considerably varied depending on pHs and kinds of metal cations, polysaccharides and buffer molecules. The characteristic resonance Raman bands of C3G-Fe3+ and D3G-Fe3+ complexes were significantly affected by the addition of alginate, 18O/16O-isotope substitution, and Fe2+/Fe3+-replacement. These results suggest that alginate associates with C3G through Fe3+ to form a stable complex, which enhances the thermal stability of C3G.