Effect of mass transfer on the removal of caffeine from green tea by supercritical carbon dioxide

To remove caffeine from green tea, supercritical CO₂ (SC-CO₂) extraction using 95% (v/v) ethanol as a modifier was carried out on a laboratory scale in the ranges of 150-300 bar and 50-80 °C. The extraction yield of caffeine and catechins including epigallocatechin gallate (EGCG) increased with an increase in temperature at a constant pressure, and also increased with increasing pressure at a fixed temperature. When the CO₂ mass flow rate increased, the total extraction yield of caffeine and catechins also increased, but the extraction efficiency of CO₂, which was determined by the amount of the solutes extracted per amount of CO₂ used, decreased, possibly due to the negligible effect of external mass transfer resistance around green tea particles and the reduced contact time for SC-CO₂ and green tea. The reduction of green tea particle size by grinding also resulted in the enhanced extraction of caffeine and catechins, which indicates the larger particle size yielded the slower extraction rate. These results gave rise to the conclusion that internal mass transfer resistance is predominant over the external mass transfer resistance in the extraction of green tea by SC-CO₂ like other herbaceous materials. In addition to the extraction of caffeine, the substantial amount of catechins was also found to be extracted during the decaffeination processes.