Optimization of the Microwave-Assisted Extraction of Simple Phenolic Compounds from Grape Skins and Seeds

A method for the extraction of phenolic compounds from grape seeds and skins using microwave-assisted extraction (MAE) was developed. Optimization of the effects of the extraction parameters in terms of the results of extraction was obtained using the response surface methodology. The parameters studied were extraction solvent (methanol, ethanol, acetone, and water), percentage of methanol in water, quantity of sample in relation to volume of extraction solvent (solid:liquid, 10-50 mg mL(-1)), power (100-500 W), magnetic stirring speed (0-100%), and extraction time (5-20 min). Finally, the repeatability and the intermediate precision of the method were determined. The best conditions proved to be: 65% methanol in water as an optimum extraction solvent; 0.5 g of grape skin or seed in a volume of 25 mL; a power of 500 W with the maximum stirring speed (100%); and an extraction time of 5 min. The phenolic compounds proved to be stable in the optimized extraction conditions. The resulting repeatability and the intermediate precision of the optimized method showed a relative standard deviation below 7%. The new method applied on Napoleon grape allowed for the determination of catechin (453.2 (mg kg(-1))), epicatechin (306.3 mg kg(-1)), caftaric acid (22.37 mg caffeic acid equivalents kg(-1)), dihydrokaempferol-glycoside (11.13 mg kaempferol equivalents kg(-1)), quercetin (18.28 mg kg(-1)), quercetin-3-glucoside (20.09 mg quercetin equivalents kg(-1)), and kaempferol-3-glucoside (11.10 mg kaempferol equivalents kg(-1)).