Background & aims: Extraction of major phenolic compounds from agricultural crop residues is important for the development of value-added products. Almond (Prunus amygdalus) is one of the most indigenous fruit crops that is grown in large quantities in Iran. Almond hulls are considered as by-product which are used only for animal feed. The aim of this study was to compare the phenolic compounds extraction yield from almond hulls using ultrasound and percolation methods. Experimental: An optimization procedure using central composite design (CCD) with 4 factors (solvent, frequency, time and temperature) at 5 levels was used in order to investigate the effect of these parameters on the extraction. Finally, the effect of solid to solvent ratio on phenolic compounds extraction was evaluated and compared with soxhlet method. Results: The results indicated that the optimum extraction conditions is found to be 86% ethanol, 50 °C , 75 min and 163 Hz for temperature, time and frequency, respectively. In addition, the extraction of phenolic compounds was evaluated in different ratios of solid to solvent (10: 1, 20: 1, 30: 1, 40: 1(w / w) at the optimum conditions and antioxidant activity of the extracts was measured by DPPH method. The maximum extraction of phenolic compounds was obtained in the ratio of 40: 1(w / w) with the amount of 1134.19± 0.23 mg/g of dry matter. The maximum free radical scavenging activity of the extract was 87.42% for ultrasound extraction which was significantly (α<0.05) different from percolation method. Recommended applications/industries: Application of new techniques including ultrasound in the extraction of phenolic compounds can be an effective method not only to increase the extraction yield of phenolic compounds but also due to the shorter extraction time, less damage is done to these compounds.
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