Comparative Evaluation of Extraction Methods for Optimising Phenolics and Antioxidant Activity in Tribulus terrestris

The medicinal plant Tribulus terrestris L. (TT) contains antioxidant and pharmacologically active phenolic compounds. However, extraction procedures can greatly affect extract yield, composition and biological activity. This study evaluated how conventional and advanced extraction methods affected TT extracts' phenolic content and antioxidant activity. TT was extracted using maceration, Soxhlet, ultrasound-assisted extraction (UAE) and microwave-assisted extraction (MAE) with ethanol-water solvent systems applied for different extraction times. The extracts were assessed for total phenolic content (TPC), total phenolic acid content (TPAC) and antioxidant capacity through 2,2-diphenyl-1-picrylhydrazyl (DPPH), cupric ion reducing antioxidant capacity (CUPRAC) and ferric reducing antioxidant power (FRAP) assays. Phenolic acid composition was analysed by HPLC. TPC values ranged between 27.66 and 37.98 mg GAE/g and TPAC between 8.273 and 24.337 mg CAE/g. Antioxidant activity (IC₅₀) varied from 0.1132 to 0.1904 mg/mL, with UAE (80:20, 10 min) demonstrating the highest DPPH activity. CUPRAC and FRAP values extended from 97.462 to 308.769 mg TE/g and 8.917-17.291 mg TE/g, respectively, with UAE consistently yielding the most active extracts. HPLC detected eight phenolic acids, with protocatechuic acid (2.794 mg/g extract) being the most abundant, followed by p-coumaric, chlorogenic and caffeic acids. The extraction method affected the TT extracts' phenolic content and antioxidant capability. The UAE produced the highest phenolic-rich and bioactive extract. To maximise T. terrestris' phytochemical and nutraceutical potential, extraction procedures need to be optimised.