A study was conducted to optimize the extraction conditions for total phenolic contents (TPC) of star fruit residues using response surface methodology (RSM). By using Design Expert (Version 6.0.10, Stat-Ease Inc., Minneapolis) statistical software, a five-level, three-factor central composite rotatable design (CCRD) was employed to investigate the effects of three independent variables including solvent concentration, X1 (40-80%), extraction temperature, X2 (25-55°C), and extraction time, X3 (90-270 min) on total phenolic content. These independent variables were coded at three levels and
their natural values chosen according to preliminary experimental results. In single factor experiments, 60% acetone, 180 minutes extraction time and 40°C extraction temperature were set as center points due to their highest TPC value, which were 2366.71, 2436.03, and 2510.95 mg GAE/100 g dry weight (DW) of star fruit residues, respectively. The results showed that the acetone concentration was statistically the most significant factor (p<0.01) and the optimal extraction conditions obtained were: acetone concentration, 65.34%; extraction temperature, 43.18°C; and extraction time, 233.51 min. Under the above-mentioned conditions, the experimental TPC was 965.65 ± 30.87 mg GAE/100 g DW, which was well matched with the predicted value, 965.52 mg GAE/100 g DW.