Epimerisation of catechins in green tea infusions

Tea catechins undergo many chemical changes such as oxidation and epimerisation during the course of the manufacturing and brewing processes. In the production of green tea, the oxidation is limited by inactivating the enzymic activity in freshly picked tea leaves by firing or steaming. As a result, epimerisation of the catechins is thought to be one of the most important reactions in the manufacture of green tea. The epimerisation of catechins in green tea infusions using both purified water and tap water at different temperatures has been investigated by HPLC and is reported in the present paper. Individual catechins can undergo epimerisation at high temperatures; however, in green tea infusions, the predominant change appears to be epimerisation from the epistructure to the nonepistructure. It has been found that this epimerisation takes place more easily in tap water than in purified water. The complexity of the ions present in the tap water together with the difference in pH between tap and purified water are thought to be the main factors influencing this observation. In the infusion brewed with tap water, the catechins are easily epimerised and then rapidly degraded. Stability studies on catechins in green tea strong infusions, which were prepared with a mixture of ethanol and purified water, in the ratio of tea leaves to solvent 1:5 (w/v), have shown that epimerisation can be observed at 40°C after a few days storage. Therefore, it is thought that not only temperature, but also heating time influences the epimerisation of catechins in green tea infusions.