Improving Thermal and Light Stability of Black Grape Anthocyanins Using Cobalt Complexation

Anthocyanins (ANCs) are water-soluble pigments with antioxidant properties, offering potential as alternatives to synthetic food colorants. However, their stability is compromised by factors such as pH, temperature, and light exposure. Previous research demonstrated improved pH stability in black grape ANCs through cobalt ion (Co⁺²) complexation. This study investigates the effects of Co⁺² complexation on the thermal and light stability of black grape ANCs extracted from pomace. These ANCs were purified, complexed with Co⁺² (ANC-Co⁺²), and diluted in a pH 4.5 buffer solution. Purified ANCs were also diluted in a pH 3.5 buffer solution. Both ANCs and ANC-Co⁺² were heated to 40°C, 60°C, and 80°C for 7 h to assess thermal stability. To evaluate light stability, ANCs and ANC-Co⁺² were stored separately under ultraviolet (UV) light and daylight for one week. Thermal stability results revealed that complexation could significantly (P≤0.05) extend ANC half-life by more than threefold and reduce temperature sensitivity by approximately 50%. Thermal degradation of ANCs was endothermic, nonspontaneous, and more structured in the transition state, as indicated by thermodynamic parameters (activation enthalpy (ΔH)>0, free energy of inactivation (ΔG)>0, and activation entropy (ΔS)<0). Light stability tests revealed that complexation significantly (P≤0.05) extended ANC half-life by over tenfold and twentyfold under UV and daylight exposure, respectively. Therefore, Co⁺² complexation represents an effective technique to enhance the thermal and light stability of grape ANCs, making them more suitable for use as food colorants.