A Grape-Derived Solvent for the Recovery of Phenolic Compounds from Food Waste By-Products Using Ultrasonic-Assisted and Overnight Extraction

This present study aimed to investigate the recovery of polyphenols from red onion peel (OPP) and pomace of Merlot (MWP) and Syrah (SWP) grape varieties, using a common biphasic solvent (70/30 ethanol/water) and a new biphasic and eco-friendly solvent that has been developed in our laboratory (ethanol of grape origin). Moreover, overnight and ultrasonic-assisted extractions were carried out to investigate whether the extraction method could affect the obtained results. Results showed that 70% ethanol achieved a significantly (<i>p</i> < 0.05) higher yield in total phenolic content (TPC) and total flavonoid content (TFC), while the extracts with ethanol of grape origin exhibited considerably higher antioxidant activity as evidenced by the DPPH and complementary by FRAP assays. The overnight and ultrasonic-assisted extraction played a significant (<i>p</i> < 0.05) role in achieving better extraction of bioactive components such as phenolic compounds. Color parameters were also determined, showing that the presence of yellow, blue and red color tones depend on the extraction method and solvent, due to the different compositional characteristics of pigments, mainly anthocyanins. In addition, physicochemical parameters such as pH and total soluble solids (TSSs) of the extracts were also measured. Finally, the composition of ethanol of grape origin was characterized by means of Ultraviolet-Visible (UV-Vis) and Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy, which confirmed the presence of ethanol and acetic acid. This study brings new results regarding the prospect of using new solvents for the recovery of bioactive compounds from agricultural by-products, and the development direction of scientific research or industrial production, based on ultrasonic-assisted and overnight extraction methods.