Formation and Characterization of (−)-Epigallocatechin Gallate-Caffeic Acid Adduct and Their Quantitative Changes during the Processing of Green Tea

Epigallocatechin (EGCG) is the predominant polyphenol compound of green tea. It is also reactive during processing and can form new catechin derivatives by reacting with other metabolites. In this study, two novel EGCG derivatives (EGCG-CAs) formed by conjugating caffeic acid (CA) were identified in green tea. Through the stimulated reaction, the formation of EGCG-CAs was systematically studied. The highest yield of EGCG-CAs was achieved when the initial mass ratio of EGCG to caffeic acid was 1:1 at 120 °C for 90 min. Quantitative analysis on tea samples undergoing processing revealed that EGCG-CAs were mainly formed at the fixation stage. Their contents were increased significantly after final drying. Based on sensory evaluation, it was indicated that S-configuration of 2‴ showed a higher threshold than that of R-configuration, suggesting that the methylene configuration of the caffeic acid moiety in EGCG-CA may be associated with the bitter taste threshold.