Anthocyanin-rich extracts: Susceptibility to color loss by hydration and thermal degradation, influence of metal ions and endogenous copigments

Three anthocyanin-rich extracts from eggplant peel (EPP), pomegranate juice (PGJ), and blood orange juice (BOJ) were prepared and studied for their phenolic composition and coloring properties. BOJ and EPP anthocyanins (ACNs) were more resistant to water addition (reversible color loss) than PGJ ACNs, in agreement with differences in ACN glycosylation patterns and in the extracts' content in potent copigments. All extracts formed Al³⁺ complexes at pH 5 with concomitant reversion of the hydration equilibria and efficient color regeneration. EPP formed blue Al³⁺ complexes, while BOJ and PGJ formed purple complexes. Monitoring color loss and pigment degradation during thermal treatment at 50 °C showed that BOJ was the most stable extract, while EPP degraded rapidly, possibly because of the autoxidation of its caffeoyl derivatives. Both color loss and pigment degradation were substantially reduced upon Al³⁺–ACN binding, with BOJ again coming up as the most stable extract.