Epimerisation of tea polyphenols in tea drinks

The present study was carried out to quantify green tea epicatechin (GTE) derivatives and to investigate the origin of epicatechin epimers present in 18 selected canned or bottled tea drinks. The major GTEs present in tea are (−)‐epigallocatechin gallate (EGCG), (−)‐epigallocatechin (EGC), (−)‐epicatechin gallate (ECG) and (−)‐epicatechin (EC). HPLC analysis showed that the content of total GTEs was lower (16.4–268.3 mg l−1) in the canned and bottled tea drinks than in tea traditionally prepared as a beverage in a cup or teapot (3–5 g l−1). The major finding was that they contained higher levels of epicatechin epimers, namely (−)‐gallocatechin gallate (GCG), (−)‐gallocatechin (GC), catechin gallate (CG) and (−)‐catechin (C), than of GTEs, ranging from 7.6 to 331.8 mg l−1. To investigate the origin of these epimers, GTEs were extracted from longjing green tea and autoclaved at various temperatures for 10–60 min. It was found that at least 50% of GTEs were epimerised to their corresponding epimers when autoclaved at 120 °C for 20 min. It is concluded that epicatechin epimers in tea drinks are not originally present in green tea leaf but are instead derived from thermal conversion of GTEs.