Microwave-assisted extraction of non-coloured phenolic compounds from grape cultivars

摘要

A new microwave-assisted extraction (MAE) method was developed for the fast analysis of non-coloured phenolic content in grapes. The stability of 25 phenolic compounds under the microwave extraction conditions was assessed to define the optimal temperature. Several variables were evaluated to study their influence on the extraction process, including microwave power, stirring, extraction temperature, extraction time, sample weight and extraction volume. The final extraction conditions were 3.0 g extracted with 10 mL of solvent (50 methanol in water at pH 2) at 70 degrees C, a system power of 750 W and a stirring of 50. The extraction time was set at 3 min (together with a 2 min pre-heating step). Repeatability and reproducibility were also evaluated, and the resulting relative standard deviation (RSD) values (n = 5) were lower than 10 for all phenolic compounds analysed. Finally, the new method was successfully applied to 80 grape samples (including wine and table grapes). Subsequently, the results were compared to those obtained by means of ultrasound-assisted extraction (UAE). Similar extraction yields were obtained for non-coloured phenolic compounds under the optimized conditions. However, MAE proved to be slightly more efficient than UAE in the extraction of flavonols, also allowing the simultaneous treatment of various samples.