Cellulose-Rich Polysaccharide Extracts with Gel-Forming Potential and Improved Antioxidant Properties from Stem (Vitis vinifera L.) By-Products: Ultrasound-Assisted Aqueous Extraction and Characterization

The valorization of wine by-products aligns with circular bioeconomy principles. This study investigates the ultrasound-assisted aqueous extraction (UAE) of bioactive compounds and cell wall polysaccharides from Syrah grape stems (Vitis vinifera L.) to produce polysaccharide extracts with the intrinsic potential to form cellulose-rich gels with enhanced antioxidant properties. Extractions were performed at three temperatures (10, 20, and 50 °C) and three ultrasonic power densities (120, 206, and 337 W/L), and compared to conventional extraction (CE, 200 rpm). The results demonstrated that UAE significantly accelerated the extraction kinetics for total phenolics (TP), flavonols, and antioxidant capacity (ABTS, FRAP), achieving up to a 3.1-fold increase in TP yield at 20 °C. Notably, UAE at 337 W/L and 20 °C produced antioxidant levels equivalent to those obtained by CE at 50 °C, enabling high efficiency at lower, compound-preserving temperatures. Carbohydrate analysis revealed that the extracts were inherently “cellulose-rich” (glucose ~49–52 mol%), with co-extracted pectins and hemicelluloses constituting a composite polysaccharide matrix with inherent gel-forming capacity, as evidenced by its composition. While total polysaccharide yield was maximized at 10 °C, UAE’s primary effect was the facilitation of extraction and potential structural modification of polymers rather than increasing bulk yield. The process reduced extraction times by 3- to over 6-fold to achieve equivalent bioactive yields compared to CE. This work establishes UAE with water as a process aligned with green chemistry principles, an efficient strategy for the integrated, one-step recovery of antioxidant phenolics and gel-forming polysaccharides from grape stems, transforming this underutilized residue into a multifunctional extract precursor for cellulose-rich hydrogels suitable for food and pharmaceutical applications.