Optimization of Ultrasonic-Assisted Extraction of Antioxidant Compounds from Starfruit (Averroha carambola L) Leaves

Extracts of starfruit leaves (SL) were used with a response surface methodology (RSM) to optimize experimental conditions for ultrasonic-assisted extraction (UAE). A 3(3-1) fractional factorial design (extraction-time X-ET, pulse-cycle X-PC and sonication-amplitude X-SA) was employed for UAE, and SL extracts were characterized by total soluble polyphenols (TSP) content and antioxidant capacity (1,1diphenyl- 2-picryl-hydrazyl [DPPH], 2,20-azino-bis-(3-ethylbenzothiazoline-6sulfonic acid) [ABTS] and ferric-ion reducing antioxidant power [FRAP]). SL extracts showed TSP values between 329.71 and 383.30 mg GAE/g. The best conditions to obtain the highest values for DPPH, ABTS and FRAP activity were 690.18 (X-ET = 15 min, X-PC = 0.7 s, X-SA = 100%), 250.59 (X-ET = 45 min, X-PC = 1 s, X-SA = 40%) and 355.12 (X-ET = 30 min, X-PC = 0.4 s, X-SA = 70%) mmol TE/g, respectively. The experimental data was also adjusted in a polynomial quadratic model. The optimal conditions for different variables were determined as X-ET = 22.2 and 45 min, X-PC = 0.4, 0.8 and 1 s and XSA between 55 and 100%. The predicted conditions can be useful in further analysis to obtain and improve a good extraction of antioxidants and TSP in SL using UAE.