The fate of acylated anthocyanins in mildly heated neutral solution

In neutral solution, anthocyanins acylated by hydroxycinnamic acids typically exhibit attractive blue colors and a higher resistance to color loss compared to their nonacylated homologs. However, they remain vulnerable to a poorly understood combination of oxidative and hydrolytic reactions that strongly contribute to color loss and limits their industrial applications. In this work, the thermal degradation of isolated red cabbage anthocyanins (0, 1 or 2 acyl groups) at pH 7 was investigated by UPLC-DAD-MS (low- and high-resolution). Non-oxidative alterations, including deacylation and intramolecular acyl transfer, were observed and found very dependent on the number and position of the acyl group(s) as well as on the presence of iron ions. At intermediate and advanced thermal degradation, several oxidative mechanisms were evidenced that lead to protocatechuic acid, phloroglucinaldehyde 2-O-glucoside, acylglycosides and derivatives of 2,4,6-trihydroxyphenylacetic acid and 3,5,7-trihydroxycoumarin. Based on the product distribution observed and on the impact of added Fe²⁺ ions and H₂O₂, possible degradation mechanisms are discussed. They likely start with a one- or two-electron transfer from the anionic base (a major colored form in neutral solution) to O₂. The hydrogen peroxide produced could then further react as an electrophile with the anionic base and/or the hemiketal (major colorless hydrated form). This contribution to understanding the degradation mechanisms of anthocyanins around neutrality can open up new stabilization strategies to extend the range of their food applications to neutral media.