Color and chemical stability of tea polyphenol (−)-epigallocatechin-3-gallate in solution and solid states

(−)-Epigallocatechin-3-gallate (EGCG), the main component with the highest biological activity in green tea, has been extensively studied for its activities and degradation behaviors in infusions and solutions. However, neither the characterization of its degradation products in solutions, nor its color degradation and stability in the solid state are well documented. Therefore, in this present study, the color and chemical stability of EGCG in solutions with various concentrations ranging from 0.05 mg/mL to 30 mg/mL and in the solid state following storage at 0% and 85% relative humidity (RH), 80 °C were investigated. Color parameters were monitored over time, and the chemical stability was determined using high performance liquid chromatography (HPLC)–diode array detection (DAD) analysis and HPLC–mass spectrometry (MS). Mathematical models for color degradation of EGCG in both solution and solid states were established. The chemical stability of EGCG was found to be affected by its physical state, concentration of reactants, oxygen levels, pH, and RH conditions. Epimerization and oxidation were found to be the two major reactions causing the degradation of EGCG in the solution and solid state, respectively. Major degradation products, catechin monomers and oxidation dimers, were identified. Cleavage products, gallic acid and catechin monomers, were also found. These results further elaborated the degradation mechanisms of EGCG in solutions of varying concentrations and revealed the chemical changes resulting in color degradation of EGCG in the solid state.