Optimization of subcritical water extraction of flavanols from green tea leaves

The subcritical-water extraction (SWE) of six kinds of flavanols from green tea leaves and the effect of extraction conditions were investigated by varying the temperature and time. The maximum yield of total flavanols, 71.36 ± 4.23 mg/g green tea leaves (mean ± SD), was obtained under extraction temperature/time conditions of 150 °C/5 min. The efficiency of SWE for total flavanols was slightly higher than that of the conventional extraction solvents such as methanol and ethanol. The extraction of flavanols via SWE was specifically adequate for epimer structures such as catechin, catechin gallate, and gallocatechin gallate due to the epimerization of epicatechins. The extraction efficiency of epimers was increased at temperatures up to 170 °C, whereas that of epicatechins was decreased. Thus, most epicatechins were converted to epimers during SWE, leading to some flavanol destruction at high temperatures, except when a short extraction time of 5 min was used.