Acylated Anthocyanins as Natural Colors : reactivity in aqueous solution, metal binding, stabilization for food applications

Anthocyanins are ubiquitous plant pigments that exhibit bright colors from red to blue. Thus, they are good candidates to replace the synthetic food colors. However, the low stability of anthocyanin colors is a real hurdle to their industrial applications, especially under near neutral conditions required to express the blue color. A promising perspective is to resort to anthocyanins acylated by p-hydroxycinnamic acids, as these pigments develop colorstabilizing mechanisms (intramolecular copigmentation, self-association) based on strong stacking interactions between the anthocyanidin chromophore and the acyl residues. Therefore, this work investigates the structural transformations of acylated anthocyanins (proton transfer, water addition), their affinity to metal ions and their resistance to thermal degradation in the presence or absence of added metal ions. To that purpose, kinetic and thermodynamic studies by UV-visible spectroscopy are combined with the identification of degradation products by UPLC-DAD/MS. The impact of the acyl residues (number, location, type) was deciphered from a series of isolated pigments from red cabbage and purple sweet potato. With the former, the acyl residue bound to the external glucose of the sophorose moiety provides a) optimal protection against attacks by H2O, H2O2 and sulfite, b) improved affinity for metal ions, c) enhanced resistance against thermal degradation (for anthocyanins and their metal complexes). By contrast, caffeic acid, whether free or as an acyl residue (in purple sweet potato), accelerates the degradation of anthocyanins in spite of stabilizing the color. Under moderate heating at pH 7, red cabbage anthocyanins were degraded into acylsophoroses, phloroglucinaldehyde-2-O-glucoside, protocatechuic acid, 3,5,7trihydroxycoumarin derivatives, and 2,4,6-trihydroxyphenylacetic acid derivatives. Intramolecular acyl migration was also evidenced. The anionic base, a major colored form at pH 7, appears most vulnerable to autoxidation. The hydrogen peroxide thus produced is further involved in anthocyanin degradation. Overall, the tight binding of acylated anthocyanins to iron and aluminum ions and possibly the addition of natural antioxidants (e.g., N-acetylcysteine) are promising perspectives for the development of stable natural blue colors.