Copigmentation of cyanidin 3-O-glucoside with phenolics: Thermodynamic data and thermal stability

The intermolecular copigmentation of cyanidin-3-O-glucoside (C3G) with three colorless phenolic compounds and its thermal stability were investigated. The influence of the copigment (ferulic acid, dopamine, or (+)-catechin), the pigment-to-copigment molar ratio (1 : 1, 1 : 10, and 1 : 100), the pH value (pH 3–7), and the temperature (20, 30, 40, and 50 °C) on the copigmentation effect, stoichiometric ratio (n), the equilibrium constant (K), and thermodynamic parameters (ΔG°, ΔH°, and ΔS°) were obtained. The strongest immediate copigmentation reactions were observed at pH 3 and 1 : 100 molar ratio, being significantly higher with (+)-catechin, followed by ferulic acid and then dopamine. The greatest hyperchromic shift was found in the complex of C3G/(+)-catechin, which was reasonable for its n and K values of 0.7 and 17.1, respectively. Moreover, the reactions were favored at low temperature. The thermodynamic data indicated that all these complexes were formed by spontaneous exothermic reactions. The C3G/(+)-catechin complex at the molar ratio 1 : 100 exhibited the greatest thermodynamic properties, with negative values of ΔG° (−6.92 kJ/mol), ΔH° (−38.7 kJ/mol), and ΔS° (−108.0 J/K/mol). The C3G/(+)-catechin complex exhibited a low Ea value (78 J/mol), indicating greater thermal stability against temperature change compared with those with the other phenolic pigments. In addition, copigmentation of anthocyanins in mulberry juice with (+)-catechin significantly reduced the loss of anthocyanins during pasteurization (80 °C for 15 min).