Simultaneous Ultrasound-Assisted Emulsification−Extraction of Polar and Nonpolar Compounds from Solid Plant Samples

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Abstract
A new approach to solid sample preparation for the simultaneous isolation of polar and nonpolar compounds using a microemulsion as leaching medium is proposed. Methanol/water (dispersed-phase)−hexane (continuous-phase) emulsions formed in the presence of ultrasound and a solid sample allow polar and nonpolar compounds to be transferred to the dispersed and continuous phase, respectively. The efficiency of this dual sample preparation approach was assessed in the characterization of natural products of variable hardness including acorns, grape seeds, and alperujo (a residue of olive oil production). The time needed for quantitative extraction of the target fractions (phenol compounds and fatty acids) is 9 min for acorns and alperujo and 20 min for grape seeds; the longer time needed for grape seeds can be attributed to higher matrix hardness. Such good performance can be ascribed to the ultrasound-enhanced formation of methanol/water microdroplets 1−15 μm in size, which act as solid−liquid microextractors spanning a large surface area. The presence of the sample was found to greatly improve emulsion stability, which can be ascribed to the amphiphilic nature of the fatty acids in the samples. Following leaching and separation of the two phases by centrifugation, the polar and nonpolar fractions were analyzed by HPLC-diode array detection and GC/MS, respectively. The proposed approach provides extraction efficiency similar to the Folch method (reference method for fat extraction, 4.5 h) in a shorter time and extraction efficiency equal to or higher than the stirring-based method (reference method for phenol compounds extraction, 24 h).