Combined effect of pH and temperature on the stability and antioxidant capacity of epigallocatechin gallate (EGCG) in aqueous system

The stability of (−)-epigallocatechin gallate (EGCG) in an aqueous system with a pH range of 2.2–8.0 was investigated at temperatures ranging from 25 to 165 °C. Within these ranges, the stability of EGCG decreased with increasing pH and temperature. The lowest (1.06 × 10−7 s−1) and highest (8.83 × 10−2 s−1) degradation rate constants were obtained at pH 3 and 25 °C, and pH 8 and 135 °C, respectively. Epimerization rate constants between EGCG and (−)-gallocatechin gallate (GCG) both increased with increasing temperature and pH. However, the rate of epimerization from EGCG to GCG was found to always be higher than that of GCG to EGCG. To further evaluate the effects of pH and temperature on catechins, antioxidant capacity analysis was conducted on thermally processed EGCG solutions using DPPH and ABTS assays. Changes in the total antioxidant capacity of samples after thermal treatments under various pH values were found to be insignificant.