Sustainable management of wine-derived leftovers: Enhanced extraction of antioxidants and production of levulinic acid by microwave-assisted processing

Within the framework of circular economy and process intensification, this work aimed to develop a two-way, microwave (MW)-assisted valorisation protocol for winemaking waste, namely grape stalk (GS), grape marc (GM) and exhausted grape marc (EGM). The first step evaluates the recovery of biologically active compounds (BACs). In this context, MW-assisted extraction (MAE) (200 W, 100 ºC, 2 h) demonstrated to be an intensified approach for processing EGM, enhancing the total antioxidant capacity (TAC) by up to 436% (TAC) compared to conventional soaking. Various polyphenols including flavonols (quercetin), flavanols (catequin, epicatechin), anthocyanins (procyanidins (B1-B7)), as well as glycosylated structures (i.e., kaempferol 3-O-glucoside), were detected by LC-MS analysis in this matrix. Simultaneously, the conversion to the bio-based chemical levulinic acid (LevA) was carried out. Considering the environmental factor involved in the dimension of sustainability, environmentally friendly practices were adopted, based on an aqueous single-phase without organic solvent, reusable catalysts as p-toluenesulfonic acid (p-TsA), and short reaction times (20 min). LevA yield reached 33.09% molar from the extractives-free (EF)-EGM matrix, surpassing the 22.31% obtained from the untreated one. Nonetheless, GS exhibited higher efficiency, yielding up to 34.75 and 59.83% from the untreated- and EF-GS matrices, respectively.