Optimization of the conditions for ultrasound-assisted extraction of phenolic compounds from Opuntia ficus-indica [L.] Mill. flowers and comparison with conventional procedures

Opuntia ficus-indica [L.] Mill. (OFI) flower has been studied less than OFI fruit and cladode for the bioactive compounds occurrence. Also, ultrasound-assisted extraction (UAE) technique has been applied less than conventional techniques to extract bioactive compounds, generally. In this work, UAE was developed to extract phenolic compounds from OFI flowers, collected from wild population plants in the department of Bejaia (Algeria). Single-factor experiments and response surface methodology were designed to study the effect of ethanol concentration (20-100%), extraction time (10-60 min), and extraction temperature (30-70 ?C) on the total phenolic content (TPC). Optimal UAE conditions were 36% ethanol, an extraction time of 60 min, and a temperature of 53 ?C, which produced an extraction of 24.4 +/- 0.82 mg of total phenolics expressed as mg Gallic Acid Equivalent (GAE)/g Dry Weight (DW). The bioactive compounds extraction efficiency was statistically higher using UAE than using conventional extraction methods (maceration and Soxhlet extraction). The Liquid Chromatography Time-of-Flight Mass Spectrometry (LC/MS-TOF) analysis of the optimized UAE extract detected mostly the occurrence of phenolic acids (piscidic and eucomic acids) and isorahmnetin derivatives. Interestingly, piscidic and eucomic acids have been identified and quantified for the first time in OFI flower extract. The antioxidant activity assessed with different assays were 13.8 +/- 0.27 mu mol Trolox Equivalent (TE)/g DW for DPPH, 4078 +/- 80.3 mu mol TE/g DW for Ferric Reducing Antioxidant Power (FRAP), 3091 +/- 139 mu mol TE/ g DW for total antioxidant activity (TAA) by phosphomolybdate assay, 80.8 +/- 4.17 mu mol TE/g DW for Trolox Equivalent Antioxidant Capacity (TEAC) by ABTS assay. The UAE process deeply enhanced the yield and significantly reduced the extraction time in comparison with conventional methods (maceration and Soxhlet extraction), confirming that OFI flower is a valuable source of bioactive compounds that can be exploited in some industries.