Abstract
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).
Highlights
Different techniques have been employed for the final determination of these phenolic compounds in the extracts from grapes, including spectrophotometry, high-performance liquid chromatography (HPLC), high-performance liquid chromatography coupled with mass spectrometry (HPLC-MS), and ultra-high-performance liquid chromatography (UHPLC) [29]
It has previously been found that a large proportion of the phenolic compounds are stable at up to temperatures of 100 ◦ C in the extraction conditions applied in this study [30]
The working temperature was fixed at this value to minimize the extraction time necessary for recovery of the phenolic compounds
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Due to the interest in their use as nutraceuticals in other foods, industrial methods have been developed for the recovery and extraction of the phenolic compounds present in grapes [15,16]. Regarding extraction methods based on MAE, the effects of several extraction variables, including solvent, temperature, time, and solvent to sample ratio, on the recoveries of individual anthocyanins from grape skins have previously been evaluated [22]. Different techniques have been employed for the final determination of these phenolic compounds in the extracts from grapes, including spectrophotometry, high-performance liquid chromatography (HPLC), high-performance liquid chromatography coupled with mass spectrometry (HPLC-MS), and ultra-high-performance liquid chromatography (UHPLC) [29]. The aim of this work is to develop an analytical method allowing for the determination of phenolics in grape skins and seeds. Agronomy 2021, 11, 1527 among the extraction conditions on the level of recovery of specific phenolic compounds, allowing for a faster optimization
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